I Finally Read "A New Kind of Science"

This book made me think new thoughts; this is rare, so I am posting about it.
If you read nothing else in the post, read the end.

"A New Kind of Science" is a 13-year-old book preceded, and regrettably often prejudged, by its reputation. Many of the criticisms that have come to define the work are valid, so let's get that part out in the open. The book can be read in its entirety here; it is enormous, both physically (~1,200 pages) and in scope, which has led to a limited and specialized readership lodging many legitimate, though mostly technical, complaints. To make matters worse, Wolfram comes across as rather smug and boastful, taking for granted the revolutionary impact of his work, staking claims to originality that are often incorrect, and failing to adequately cite his ideological predecessors in the main body of the text. These are legitimate concerns and egregious omissions to be sure.

However, the book itself was written to be accessible to anyone with basic knowledge of math/science, and I feel it gains so much for being simplistic and frankly written in this way. Furthermore, the biggest ideas in the book, even if not completely original or 100% convincing as-is, are still as beautiful and important as they are currently underappreciated. Even if he cannot claim unique ownership of them all, Wolfram has done an heroic job explicating these ideas for the lay reader and his book has vastly increased their popular visibility. But I fear that many may be missing out on a thoroughly enjoyable, philosophically insightful book simply on the basis of some  overreaching claims and some rather technical flaws; I get the sinking feeling it's going the way of Atlas Shrugged—a big book that's cool to dismiss without ever having read.

I'm not going to get into the specific criticisms; suffice it to say that there are issues with the book, though Wolfram has gone to some trouble to defend his positions. But this notorious reception, coupled the fact that the book weighs in at almost 6 pounds, had kept me and surely many others from ever giving it a proper chance. This post is not meant to be an apology, and neither is it intended to be a formal book review. Rather, I am going to show you several things that I took away from my reading of it that I feel very grateful for, regardless of the extent to which they represent any sort of paradigm shift, or even anything new to human inquiry. Lots of these ideas were very new to me, and they were presented so well that I feel I have ultimately gained a new perspective on many issues, including life itself, which I have been trying sedulously for years to better understand.

To attempt to write a general review this book would be quite difficult, for its arguments depend so much upon pictures (of which there are more than 1000!), careful explanations, and repeated examples to build up intuition for how very simple rules can produce complex behavior, and how this fact plausibly accounts for many phenomena in the natural and physical world (indeed, perhaps the universe itself). Wolfram uses this intuition to convey compelling explanations of space and time, experience and causation, thinking, randomness, free will, evolution, the Second Law of thermodynamics, incompleteness and inconsistency in axiom systems, and much more. I will be talking about most of the non-math/physicsy stuff in this post, because most of it's beyond my ken.

This will make a little more sense later on

To begin with, the first 5 to 8 chapters are nothing if not eye-opening; they could and should be read by all high-schoolers who are interested in science, and unless you are a scientist yourself I guarantee you will gain many new insights into some fundamental issues. Some of the physics (ch. 9) got a little heavy for me, but this may well be the most interesting part of the book for many. The final chapter (12) was what did it for me personally.

In this long last chapter, the main thesis of the book is driven home. It is as follows: the best (and indeed, perhaps the only) way to understand many systems in nature (and indeed, perhaps nature itself) is to think in terms of simple programs instead of mathematical equations; that is, to view processes in nature as performing rule-based computations. Simple computer programs can explain, or at least mimic, natural phenomena that have so far eluded mathematical models such as differential equations; Wolfram argues that nature is ultimately inexplicable by these traditional methods.

He demonstrates how very simple programs can produce complexity and randomness; he argues that because simple programs must be ubiquitous in the natural world, they are responsible for the complexity and randomness we observe in natural systems. He shows how idealized model programs like cellular automata can mimic in shockingly exact detail the behavior of phenomena which science has only tenuously been able to describe: crystal growth (e.g., snowflakes), fluid turbulence, the path of a fracture when materials break, biological development, plant structures, pigmentation patterns...thus indicating that such simple processes likely underlie much of what we observe.

Indeed, he makes a case (originally postulated by Konrad Zuse) that the universe itself is fundamentally rule based, and essentially one big ongoing computation of which everything is a part. It gets a little hairy, but in chapter 9 Wolfram discusses how the concept of causal networks can be used to explain how space, time, elementary particles, motion, gravity, relativity, and quantum phenomena all arise. Indeed, he argues that causal networks can represent everything that can be observed, and that all is defined in terms of their connections. This is predicated on the belief that there are no continuous values in nature; that is to say, that nature is fundamentally discrete. There were a lot of intriguing ideas here, but I cannot go into them all right now. There does seem to be a reasonable case to be made for some kind of of digital physics. I am way out of my league here though, so I'll stop. Check out that wikipedia article!


Cellular automata and other related easy-to-follow rule-based systems are used to demonstrate, or at least to hint at, most of these claims. If you haven't seen these before, check out that link: it takes you to Wolfram's own one-page summary of how these things work. In fact, I'm going to cut-and-paste most of it below. But here's a brief description: imagine of a row of cells that can be either black or white. You start with some initial combination of black and/or white cells in this row; to get the next row, you apply a set of rules to the original cells which tells you what color cells in the next row should be based on the colors of the original cells above. The rules that determine the color of a new cell are based on the colors of three cells: the cell immediately above it and the cells to the immediate right and left of the one above it. Thus, the color of any given cell is affected only by itself and its immediate neighbors. Simple enough, but those neighbors are in turn governed by their neighbors, and those neighbors by their neighbors, etc, so that the whole thing ends up being highly interconnected.  When you repeatedly apply a given rule and step back to observe the collective behavior of all the cells, large-scale patterns can emerge. You often get simple repetitive behavior (like the top picture below) or nested patterns (second picture below). However, sometimes you find behavior that is random, or some mixture of random noise with moving structures (last picture below).

Look at the picture just above; the left side shows certain regularities, but the right side exhibits random behavior (and has indeed been used for practical random number generators and encryption purposes). How might one predict the state of this system after it has evolved for a given number of time-steps? (This is an important "exercise left to the reader" so think about it before reading on).

The 'take-home' here is that sometimes simple rules lead to behavior that is complex and random, and the lack of regularities in these systems defy any short description using mathematical formulas. The only way to know how that sucker right there is going to behave in 1,000,000,000 steps is to run it and find out. 

If you like looking at pictures like this one, you should definitely check out the book. I read it digitally but I ordered a physical copy as soon as I finished because man, what a terrific coffee-table book this thing makes!

Computational universality

Now here's where things got really interesting for me. Unless you have studied computer science (which I honestly really haven't), you might be surprised to find out that certain combinations of rules, like those shown above, can result in systems that are capable of performing any possible computation and emulating any other system or computer program (which I honestly kind of was). Indeed, the computer you are reading this on right now has this capability. Hell, your microwave probably has this capability; given enough memory, it could run any program or calculate any function provided the function is able to be computed at all.

This idea, called universal computation, was developed by Alan Turing in the 1930s: a system is said to be "universal" or "Turing complete" if it is able to perform any computation. If a system is universal, it must be able to emulate any other system, and it must be able to produce behavior that is as complex as that of any other system; knowing that a system is universal implies that the system can produce behavior that is arbitrarily complex.

When studying the 256 rule-sets that generate the elementary cellular automata, Wolfram and his assistant Matthew Cook showed that a couple of them (rule 110 and relatives) could be made to perform any computation; that is, a couple of these extremely simple systems, among the most basic conceivable types of programs, were shown to be universal.

Rule 110 from a single black cell (16 steps; see 250 steps below)

In general, this itself is not new knowledge; von Neumann was the first to show that a cellular automaton could be a universal computer, and it was known that other simple devices could support universal computation. However, this was the simplest instantiation of universality yet discovered, and Wolfram uses this to argue that the phenomenon must indeed be quite more widespread than originally thought, and indeed very common in nature. While most basic sets of rules generate very simple behavior (like the first and second rules pictured above), past a certain threshold you get universality, where a system can emulate any other system by setting up the appropriate initial conditions, like rule 110:

How universality can actually be achieved with cellular automata in practice is described with great clarity in the book, but it would be too complicated to get into here. Pretty neat though! Wolfram goes on to show how universality is instantiated in Turing machines, cellular automata, register machines, and substitution systems, by showing how each one can be made to emulate the others by setting up appropriate initial conditions, despite great differences in their underlying structure.

"It implies that from a computational point of view a very wide variety of systems, with very different underlying structures, are at some level fundamentally equivalent...every single one of these systems is ultimately capable of exactly the same kinds of computations."

Any kind of system that is universal can perform the same computations; as soon as one gets past the threshold for universality, that's all. Things can't get more complex. It doesn't matter how complex the underlying rules are; one universal system is equivalent to any other, and adding more to its rules cannot have any fundamental effect. He goes on to say,

"...my general expectation is that more or less any system whose behavior is not somehow fundamentally repetitive or nested will in the end turn out to be universal."

This and related research led Wolfram to postulate his "new law of nature", the Principle of Computational Equivalence: that since universal computation means that one system can emulate any other system, all computing processes are equivalent in sophistication, and this universality is the upper limit on computational sophistication.

"No system could be constructed in our universe that is capable of more complex computations than any other universal system; no system can carry out computations that are more sophisticated than those carried out by a Turing machine or cellular automaton."

Another way of stating this is that there is a fundamental equivalence between many different kinds of processes, and that all processes which are not obviously simple can be viewed as computations of equivalent sophistication, whether they are man-made or spontaneously occurring. When we think of computations, we typically think of carrying out a series of rule-based steps to achieve a purpose, but computation in fact much broader, and as Wolfram would argue, all-encompassing. Thus, as in cellular automata, the process of any system evolving is itself a computation, even if its only function is to generate the behavior of the system. Thus, all processes in nature can be thought of as computations; the only difference is that "the rules such processes follow are defined not by some computer program that we as humans construct but rather by the basic laws of nature."

Wolfram goes on to suggest that any instance of complex behavior we observe is produced by a universal system.

"I suspect that in almost any case where we have seen complex behavior... it will eventualy be possible to show that there is universality. And indeed... I believe that in general there is a close connection between universality and the appearance of complex behavior."

"Essentially any piece of complex behavior that we see corresponds to a kind of lump of computation that is at some level equivalent."

He argues that this is why some things appear complex to us, while other things yield patterns or regularities that we can perceive, or which can be described by some some formal mathematical analysis:

"If one studies systems in nature it is inevitable that both the
evolution of the systems themselves and the methods of perception and analysis used to study them must be processes based on natural laws. But at least in the recent history of science it has normally been assumed that the evolution of typical systems in nature is somehow much less sophisticated a process than perception and analysis.

Yet what the Principle of Computational Equivalence now asserts is that this is not the case, and that once a rather low threshold has been reached, any real system must exhibit essentially the same level of computational sophistication. So this means that observers will tend to be computationally equivalent to the systems they observe— with the inevitable consequence that they will consider the behavior of such systems complex."

Thus, the reason things like turbulence in fluids or any other random-seeming phenomena appear complex to us is that we are computationally equivalent to these things. To really understand the implications of this idea, we need bring in the closely related idea of irreducibility.

Computational irreducibility

Wolfram claims that the main concern of science has been to find ways of predicting natural phenomena, so as to have some control/understanding of them. Instead of having to specify at each step how, say, a planet orbits a star, it is far better to derive a mathematical formula or model that allows you to determine the outcome of such systems with a minimum of computational effort. Sometimes, you can even find definite underlying rules for such systems which make prediction just a matter of applying these rules.

However, there are many common systems for which no traditional mathematical formulas have been found which can easily describe their behavior. And just because you know the underlying rules, there is often no way to know for sure how the system will ultimately behave, and it can take an irreducible amount of computation to actually do this. Imagine how you would try to predict the row of black and white cells after the rule-110 cellular automaton had run for, say, a trillion steps. There is simply no way to do this besides carrying out the full computation; no way to reduce the amount of computational effort that this would require. Thus,

"Whenever computational irreducibility exists in a system it means that in effect there can be no way to predict how the system will behave except by going through almost as many steps of computation as the evolution of the system itself.

...what leads to the phenomenon of computational irreducibility is that there is in fact always a fundamental competition between systems used to make predictions and systems whose behavior one tries to predict.

For if meaningful general predictions are to be possible, it must at some level be the case that the system making the predictions be able to outrun the system it is trying to predict. But for this to happen the system making the predictions must be able to perform more sophisticated computations than the system it is trying to predict."

This is because the system you are trying to predict and the methods you are using to make predictions are computationally equivalent; thus for many systems there is no general way to shortcut their process of evolution, and their behavior is therefore computationally irreducible. Unfortunately, there are many common systems whose behavior cannot ultimately be determined at all except for through direct simulation, and thus don't appear to yield to any mathematical short description. Wolfram argues that almost any universal system is irreducible, because nothing can systematically outrun a universal system. He gives the following thought experiment:

"For consider trying to outrun the evolution of a universal system. Since such a system can emulate any system, it can in particular emulate any system that is trying to outrun it. And from this it follows that nothing can systematically outrun the universal system. For any system that could would in effect also have to be able to outrun itself."

Since universality should be relatively common in natural systems, so too will computational irreducibility, making it impossible to predict the behavior of these systems. He argues that traditional science has always relied on computational irreducibility, and that "its whole idea of using mathematical formulas to describe behavior makes sense only when the behavior is computationally reducible. This seems to impose stark limits on traditional scientific inquiry, for it implies that it is impossible to find theories that will perfectly describe a complex system's behavior without arbitrarily much computational effort.

Free Will and Determinism

The section of the book uses the idea of computational irreducibility to demystify of the age-old problem of free will in a way I find quite satisfying, even beautiful. Humans, and indeed most other animals, seem to behave in ways that are free from obvious laws. We make minute-to-minute decisions about how to act that that do not seem fundamentally predictable.

Wolfram argues that this is because our behavior is computationally irreducible; the only way to work out how such a system will behave, or to predict its behavior, is to perform the computation. This lets us have our materialist/mechanistic cake and eat it: we can admit that our behavior essentially follows a set of underlying rules with our autonomy intact, because our rules produce complexities that are irreducible and hence unpredictable.

We know that animals as living systems follow many basic underlying rules— genes are expressed, enzymes catalyze biochemical pathways, cells divide—but we have also seen how even very basic rule-sets result in universality, complexity, and computational irreducibility of the system.

"This, I believe, that is the ultimate origin of the apparent freedom of human will. For even though all the components of our brains presumably follow definite laws, I strongly suspect that their overall behavior corresponds to an irreducible computation whose outcome can never in effect be found by reasonable laws."

The main criterion for freedom in a system seems to be that we cannot predict its behavior. For if we could, then the behavior of the system would thus be predetermined. Wolfram muses,

"For as we have seen many times in this book even systems with quite simple and definite underlying rules can produce behavior so complex that it seems free of obvious rules. And the crucial point is that this happens just through the intrinsic evolution of the system—without the need for any additional input from outside or from any sort of explicit source of randomness.

And I believe that it is this kind of intrinsic process—that we now know occurs in a vast range of systems—that is primarily responsible for the apparent freedom in the operation of our brains.

But this is not to say that everything that goes on in our brains has an intrinsic origin. Indeed, as a practical matter what usually seems to happen is that we receive external input that leads to some train of thought which continues for a while, but then dies out until we get more input. And often the actual form of this train of thought is influenced by memory we have developed from inputs in the past—making it not necessarily repeatable even with exactly the same input.

But it seems likely that the individual steps in each train of thought follow quite definite underlying rules. And the crucial point is then that I suspect that the computation performed by applying these rules is often sophisticated enough to be computationally irreducible—with the result that it must intrinsically produce behavior that seems to us free of obvious laws."

Intelligence in the Universe

Wolfram has a wonderful section about intelligence in the universe, but this post is quickly becoming quite long so I will stick to my highlights. Definitely check it out if what I say here interests you.

Here he poignantly discusses how "intelligence" and "life" are difficult to define, and how many features of commonly given definitions of intelligence (learning and memory, communication, adaptation to complex situations, handling abstraction) and life (spontaneous movement/response to stimuli, self-organization from disorganized material, reproduction) are in fact present in much simple systems that we would not describe as intelligent or alive.

"And in fact I expect that in the end the only way we would unquestionably view a system as being an example of life is if we found that it shared many specific details with life on Earth."

Discussing extraterrestrial intelligence, he introduced me to an idea that is probably a well-known science fiction trope, but one that genuinely surprised me. He talks about how earth is bombarded with radio signals from around our galaxy and beyond, but that these signals seem to be completely random noise, and thus they are assumed to be just side effects of some physical process. But, he notices, this very lack of regularities in the signal could actually be a sign of some kind of extraterrestrial intelligence: "For any such regularity represents in a sense a redundancy or inefficiency that can be removed by the sender and receiver both using appropriate data compression." If this doesn't make sense to you, then you will probably also enjoy his section on data compression and reducibility. The whole book is really worth taking the time to read!

An Incredible Ending

I'm going to quote the last few paragraphs of the book in full, because they are extremely beautiful to me and there is no way I could do them justice. If you read nothing else in this blog post, read this. Feeling the full intensity of its impact/import really depends on one having read the previous like, 800 pages, and have understood the main arguments in them, so if you are planning to read the book in its entirety you might save this part until then for greatest effect. Still, if this is the only thing you ever read by Stephen Wolfram, I think it should be this. It is a good stylistic representation of the book (the short sentences, the lucid writing, the hubris) and it is the ultimate statement of the work's conclusions. Fair warning: much of it is going to sound absolutely outrageous if you haven't read the book, and especially if you haven't read the parts of this post about universality and computational reducibility. In fact, even still it sounds kind of preposterous!

But having been preoccupied with these questions about life for many years now, this passage resonated with me deeply and immediately and I am still reeling from it. Though I am not completely convinced (though are we ever, of anything?), the ideas summarized herein constitute, at least for me personally, a singularly compelling theory of existence, of nature, of life... of everything. Granted, I am taking a lot on faith for now, but I know I will have occasion to return to these thoughts time and time again as they percolate across my lifetime; indeed, it is largely for this reason that I took the time to write this post. Well, here it is; as elsewhere in the quoted material, any emphasis is mine:

*********************************************************************
"It would be most satisfying if science were to prove that we as humans are in some fundamental way special, and above everything else in the universe. But if one looks at the history of science many of its greatest advances have come precisely from identifying ways in which we are not special—for this is what allows science to make ever more general statements about the universe and the things in it.

Four centuries ago we learned for example that our planet does not lie at a special position in the universe. A century and a half ago we learned that there was nothing very special about the origin of our species. And over the past century we have learned that there is nothing special about our various physical, chemical and other constituents.

Yet in Western thought there is still a strong belief that there must be something fundamentally special about us. And nowadays the most common assumption is that it must have to do with the level of intelligence or complexity that we exhibit. But building on what I have discovered in this book, the Principle of Computational Equivalence now makes the fairly dramatic statement that even in these ways there is nothing fundamentally special about us.

For if one thinks in computational terms the issue is essentially whether we somehow show a specially high level of computational sophistication. Yet the Principle of Computational Equivalence asserts that almost any system whose behavior is not obviously simple will tend to be exactly equivalent in its computational sophistication.

So this means that there is in the end no difference between the level of computational sophistication that is achieved by humans and by all sorts of other systems in nature and elsewhere. For my discoveries imply that whether the underlying system is a human brain, a turbulent fluid, or a cellular automaton, the behavior it exhibits will correspond to a computation of equivalent sophistication.

And while from the point of view of modern intellectual thinking this may come as quite a shock, it is perhaps not so surprising at the level of everyday experience. For there are certainly many systems in nature whose behavior is complex enough that we often describe it in human terms. And indeed in early human thinking it is very common to encounter the idea of animism: that systems with complex behavior in nature must be driven by the same kind of essential spirit as humans.

But for thousands of years this has been seen as naive and counter to progress in science. Yet now essentially this idea—viewed in computational terms through the discoveries in this book—emerges as crucial. For as I discussed earlier in this chapter, it is the computational equivalence of us as observers to the systems in nature that we observe that makes these systems seem to us so complex and unpredictable.

And while in the past it was often assumed that such complexity must somehow be special to systems in nature, what my discoveries and the Principle of Computational Equivalence now show is that in fact it is vastly more general. For what we have seen in this book is that even when their underlying rules are almost as simple as possible, abstract systems like cellular automata can achieve exactly the same level of computational sophistication as anything else.

It is perhaps a little humbling to discover that we as humans are in effect computationally no more capable than cellular automata with very simple rules. But the Principle of Computational Equivalence also implies that the same is ultimately true of our whole universe.

So while science has often made it seem that we as humans are somehow insignificant compared to the universe, the Principle of Computational Equivalence now shows that in a certain sense we are at the same level as it is. For the principle implies that what goes on inside us can ultimately achieve just the same level of computational sophistication as our whole universe.

But while science has in the past shown that in many ways there is nothing special about us as humans, the very success of science has tended to give us the idea that with our intelligence we are in some way above the universe. Yet now the Principle of Computational Equivalence implies that the computational sophistication of our intelligence should in a sense be shared by many parts of our universe—an idea that perhaps seems more familiar from religion than science.

Particularly with all the successes of science, there has been a great desire to capture the essence of the human condition in abstract scientific terms. And this has become all the more relevant as its replication with technology begins to seem realistic. But what the Principle of Computational Equivalence suggests is that abstract descriptions will never ultimately distinguish us from all sorts of other systems in nature and elsewhere. And what this means is that in a sense there can be no abstract basic science of the human condition—only something that involves all sorts of specific details of humans and their history.

So while we might have imagined that science would eventually show us how to rise above all our human details what we now see is that in fact these details are in effect the only important thing about us.

And indeed at some level it is the Principle of Computational Equivalence that allows these details to be significant. For this is what leads to the phenomenon of computational irreducibility. And this in turn is in effect what allows history to be significant—and what implies that something irreducible can be achieved by the evolution of a system.

Looking at the progress of science over the course of history one might assume that it would only be a matter of time before everything would somehow be predicted by science. But the Principle of Computational Equivalence—and the phenomenon of computational irreducibility—now shows that this will never happen.

There will always be details that can be reduced further—and that will allow science to continue to show progress. But we now know that there are some fundamental boundaries to science and knowledge.

And indeed in the end the Principle of Computational Equivalence encapsulates both the ultimate power and the ultimate weakness of science. For it implies that all the wonders of our universe can in effect be captured by simple rules, yet it shows that there can be no way to know all the consequences of these rules, except in effect just to watch and see how they unfold."

********************************************************************

Laudato Si Pope-pourri

It's not everyday that I want to read something a Pope wrote. In fact, today, June 19, 2015 was the only day I have ever wanted that, if memory serves. Today I read Papa Francesca's second encyclical, entitled "Laudato Si -- On Care For Our Common Home". It was a unique experience to navigate to w2.vatican.va, download an essay the Pope just wrote about environmentalism, and then read it on my computer.

I read it more as a curiosity than anything else, and though I found much to disagree with, I was in strong accord with two of the Pope's main points as I have understood them: (1) that every living thing is valuable and all of nature is interdependent; and (2) that rampant consumerism/individualism is a soul-sucking trap that perverts nature, prizes inequity, ruins the environment, and leads millions of people to lead ultimately meaningless lives. These ideas are neither new nor original, and so I don't want to spend too much time on this post. However, I do want to share with you some of the passages I found most interesting in case you never get to read it. The Pope seems like a pretty decent human being!

Adumbrating the issues to be addressed, the Pope tells us:

"I will point to the intimate relationship between the poor and the fragility of the planet, the conviction that everything in the world is connected, the critique of new paradigms and forms of power derived from technology, the call to seek other ways of understanding the economy and progress, the value proper to each creature, the human meaning of ecology, the need for forthright and honest debate, the serious responsibility of international and local policy, the throwaway culture and the proposal of a new lifestyle" (13).

Quoting Patriarch Bartholomew, the Pope urges us "to look for solutions not only in technology but in a change of humanity; otherwise we would be dealing merely with symptoms. [Bartholomew] asks us to replace consumption with sacrifice, greed with generosity, wastefulness with a spirit of sharing..." (2).

The Pope looks at technology with a far more jaundiced eye than I do, but he may have a couple points here worthy of our consideration:

"Furthermore, when media and the digital world become omnipresent, their influence can stop people from learning how to live wisely, to think deeply and to love generously. In this context, the great sages of the past run the risk of going unheard amid the noise and distractions of an information overload. Efforts need to be made to help these media become sources of new cultural progress for humanity and not a threat to our deepest riches. 

True wisdom, as the fruit of self-examination, dialogue and generous encounter between persons, is not acquired by a mere accumulation of data which eventually leads to overload and confusion, a sort of mental pollution. Real relationships with others, with all the challenges they entail, now tend to be replaced by a type of internet communication which enables us to choose or eliminate relationships at whim, thus giving rise to a new type of contrived emotion which has more to do with devices and displays than with other people and with nature. Today’s media do enable us to communicate and to share our knowledge and affections. Yet at times they also shield us from direct contact with the pain, the fears and the joys of others and the complexity of their personal experiences. For this reason, we should be concerned that, alongside the exciting possibilities offered by these media, a deep and melancholic dissatisfaction with interpersonal relations, or a harmful sense of isolation, can also arise." (31-32)

Throughout, he constantly questions the modern conception of "progress" held by many in the developed world:

There is a tendency to believe that every increase in power means an increase of 'progress' itself, an advance in security, usefulness, welfare and vigour, an assimilation of new values into the stream of culture, as if reality, goodness and truth automatically flow from technological and economic power as such. The fact is that 'contemporary man has not been trained to use power well', because our immense technological development has not been accompanied by a development in human responsibility, values and conscience." (76)

Later in the encyclical the Pope expands on this theme more forcefully: "we need to grow in the conviction that a decrease in the pace of production and consumption can at times give rise to another form of progress and development. Put simply, it is a matter of redefining our notion of progress" (139). I'm about to quote the Pope at length, but it is well worth reading:

"Environmental protection cannot be assured solely on the basis of financial calculations of costs and benefits. The environment is one of those goods that cannot be adequately safeguarded or promoted by market forces. Once more, we need to reject a magical conception of the market, which would suggest that problems can be solved simply by an increase in the profits of companies or individuals. Is it realistic to hope that those who are obsessed with maximizing profits will stop to reflect on the environmental damage which they will leave behind for future generations? Where profits alone count, there can be no thinking about the rhythms of nature, its phases of decay and regeneration, or the complexity of ecosystems which may be gravely upset by human intervention. Moreover, biodiversity is considered at most a deposit of economic resources available for exploitation, with no serious thought for the real value of things, their significance for persons and cultures, or the concerns and needs of the poor" (138).

"A technological and economic development which does not leave in its wake a better world and an integrally higher quality of life cannot be considered progress. Frequently, in fact, people’s quality of life actually diminishes – by the deterioration of the environment, the low quality of food or the depletion of resources – in the midst of economic growth. In this context, talk of sustainable growth usually becomes a way of distracting attention and offering excuses. It absorbs the language and values of ecology into the categories of finance and technocracy, and the social and environmental responsibility of businesses often gets reduced to a series of marketing and image-enhancing measures" (141).

"The principle of the maximization of profits, frequently isolated from other considerations, reflects a misunderstanding of the very concept of the economy. As long as production is increased, little concern is given to whether it is at the cost of future resources or the health of the environment; as long as the clearing of a forest increases production, no one calculates the losses entailed in the desertification of the land, the harm done to biodiversity or the increased pollution. In a word, businesses profit by calculating and paying only a fraction of the costs involved. Yet only when “the economic and social costs of using up shared environmental resources are recognized with  transparency and fully borne by those who incur them, not by other peoples or future generations”, can those actions be considered ethical" (143).

"Many things have to change course, but it is we human beings above all who need to change. We lack an awareness of our common origin, of our mutual belonging, and of a future to be shared with everyone. This basic awareness would enable the development of new convictions, attitudes and forms of life. A great cultural, spiritual and educational challenge stands before us, and it will demand that we set out on the long path of renewal" (144).

I have expressed similar concerns about our ultimately destructive vision of progress myself, though I tend to be far more critical of consumerism and far more hopeful about technological advance. Still, "the economy accepts every advance in technology with a view to profit, without concern for its potentially negative impact on human beings" (76). Further, the Pope argues that

"We have to accept that technological products are not neutral, for they create a framework which ends up conditioning lifestyles and shaping social possibilities along the lines dictated by the interests of certain powerful groups. Decisions which may seem purely instrumental are in reality decisions about the kind of society we want to build. The idea of promoting a different cultural paradigm and employing technology as a mere instrument is nowadays inconceivable. The technological paradigm has become so dominant that it would be difficult to do without its resources and even more difficult to utilize them without being dominated by their internal logic. It has become countercultural to choose a lifestyle whose goals are even partly independent of technology, of its costs and its power to globalize and make us all the same." (pg 76)

 Along these same lines, he continues:

"A science which would offer solutions to the great issues would necessarily have to take into account the data generated by other fields of knowledge, including philosophy and social ethics; but this is a difficult habit to acquire today. Nor are there genuine ethical horizons to which one can appeal. Life gradually becomes a surrender to situations conditioned by technology, itself viewed as the principal key to the meaning of existence. In the concrete situation confronting us, there are a number of symptoms which point to what is wrong, such as environmental degradation, anxiety, a loss of the purpose of life and of community living" (pg 82)

Talking politics, the Pope criticizes the short-sightedness that is built into our systems of governance:

"A politics concerned with immediate results, supported by consumerist sectors of the population, is driven to produce short-term growth. In response to electoral interests, governments are reluctant to upset the public with measures which could affect the level of consumption or create risks for foreign investment. The myopia of power politics delays the inclusion of a far-sighted environmental agenda within the overall agenda of governments" (130).
...
A healthy politics is sorely needed, capable of reforming and coordinating institutions, promoting best practices and overcoming undue pressure and bureaucratic inertia. It should be added, though, that even the best mechanisms can break down when there are no worthy goals and values, or a genuine and profound humanism to serve as the basis of a noble and generous society" (132).

Though I am picking and choosing the passages which stood out to me, it should be clear that one common thread running through the Pope's message is that consumerism is at the heart of all that is wrong with society today:

"This paradigm leads people to believe that they are free as long as they have the supposed freedom to consume. But those really free are the minority who wield economic and financial power. Amid this confusion, postmodern humanity has not yet achieved a new self-awareness capable of offering guidance and direction, and this lack of identity is a source of anxiety. We have too many means and only a few insubstantial ends.
...
Obsession with a  consumerist lifestyle, above all when few people are capable of maintaining it, can only lead to violence and mutual destruction." (149-150)

He urges us to be mindful of whom we give our money, and to not give our money to those whose practices we cannot support in good conscience: "[there is a] great need for a sense of social responsibility on the part of consumers. Purchasing is always a moral–and not simply economic–act” (150). He reminds of of "the moral imperative of assessing the impact of our every action and personal decision on the world around us. If we can overcome individualism, we will truly be able to develop a different lifestyle and bring about significant changes in society" (151).
  
The Pope, as you might expect, also does not support measures to control population growth. I disagree with this, but he makes a defensible point:

"To blame population growth instead of extreme and selective consumerism on the part of some, is one way of refusing to face the issues. It is an attempt to legitimize the present model of distribution where a minority believes that it has the right to consume in a way which can never be universalized, since the planet could not even contain the waste products of such consumption" (29).

But he doesn't just criticize greed, consumption, and individualism without offering ways to address it. He discusses how "ecological education" needs to start at a very young age. The family environment plays a crucial role because

"In the family we first learn how to show love and respect for life; we are taught the proper use of things, order and cleanliness, respect for the local ecosystem and care for all creatures. In the family we receive an integral education, which enables us to grow harmoniously in personal maturity. In the family we learn to ask without demanding, to say “thank you” as an expression of genuine gratitude for what we have been given, to control our aggression and greed, and to ask forgiveness when we have caused harm. These simple gestures of heartfelt courtesy help to create a culture of shared life and respect for our surroundings" (155).

His vision of an alternative lifestyle is as powerful as it is simple, and it still holds together just as well if you cut out all the religious stuff:

"Christian spirituality proposes an alternative understanding of the quality of life, and encourages a prophetic and contemplative lifestyle, one capable of deep enjoyment free of the obsession with consumption. We need to take up an ancient lesson, found in different religious traditions and also in the Bible. It is the conviction that “less is more”. A constant flood of new consumer goods can baffle the heart and prevent us from cherishing each thing and each moment. To be serenely present to each reality, however small it may be, opens us to much greater horizons of understanding and personal fulfilment. Christian spirituality proposes a growth marked by moderation and the capacity to be happy with little. It is a return to that simplicity which allows us to stop and appreciate the small things, to be grateful for the opportunities which life affords us, to be spiritually detached from what we possess, and not to succumb to sadness for what we lack. This implies avoiding the dynamic of dominion and the mere accumulation of pleasures.

Such sobriety, when lived freely and consciously, is liberating. It is not a lesser life or one lived with less intensity. On the contrary, it is a way of living life to the full. In reality, those who enjoy more and live better each moment are those who have given up dipping here and there, always on the look-out for what they do not have. They experience what it means to appreciate each person and each thing, learning familiarity with the simplest things and how to enjoy them. So they are able to shed unsatisfied needs, reducing their obsessiveness and weariness. Even living on little, they can live a lot, above all when they cultivate other pleasures and find satisfaction in fraternal encounters, in service, in developing their gifts, in music and art, in contact with nature, in prayer. Happiness means knowing how to limit some needs which only diminish us, and being open to the many different possibilities which life can offer.

Sobriety and humility were not favourably regarded in the last century. And yet, when there is a general breakdown in the exercise of a certain virtue in personal and social life, it ends up causing a number of imbalances, including environmental ones. That is why it is no longer enough to speak only of the integrity of ecosystems. We have to dare to speak of the integrity of human life, of the need to promote and unify all the great values. Once we lose our humility, and become enthralled with the possibility of limitless mastery over everything, we inevitably end up harming society and the environment" (159-163).

Another theme that cut through the encyclical was his message of empathy, of how important it is to "see each human being as a subject who can never be reduced to the status of an object" (59), and how "all creatures are connected, each must be cherished with love and respect, for all of us as living creatures are dependent on one another" (29). "All of us are linked by unseen bonds and together form a kind of universal family, a sublime communion which fills us with a sacred, affectionate and humble respect" (64). Every act of cruelty towards any creature is contrary to human dignity. We can hardly consider ourselves to be fully loving if we disregard any aspect of reality" (67). "The earth is essentially a shared inheritance, whose fruits are meant to benefit everyone" (67).

"We fail to see that some [people] are mired in desperate and degrading poverty, with no way out, while others have not the faintest idea of what to do with their possessions, vainly showing off their supposed superiority and leaving behind them so much waste which, if it were the case everywhere, would destroy the planet. In practice, we continue to tolerate that some consider themselves more human than others, as if they had been born with greater rights" (65).

Also resonant with me is his repeated emphasis on ecology and how everything coexists in a delicate web of interrelationships:

"Ecology studies the relationship between living organisms and the environment in which they develop. This necessarily entails reflection and debate about the conditions required for the life and survival of society, and the honesty needed to question certain models of development, production and consumption. It cannot be emphasized enough how everything is interconnected. Time and space are not independent of one another, and not even atoms or subatomic particles can be considered in isolation. Just as the different aspects of the planet – physical, chemical and biological – are interrelated, so too living species are part of a network which we will never fully explore and understand. A good part of our genetic code is shared by many living beings. It follows that the fragmentation of knowledge and the isolation of bits of information can actually become a form of ignorance, unless they are integrated into a broader vision of reality" (95).

The Pope said much, much more in his message; I've merely highlighted the stuff I found most compelling. If any of it sounds interesting, you might check it out! He ends with two prayers, one of which I've reproduced below. I'm not religious in the slightest, but if I was, this is a prayer I could bow my head to:

A prayer for our earth:

All-powerful God,
you are present in the whole universe
and in the smallest of your creatures.
You embrace with your tenderness all that exists.
Pour out upon us the power of your love,
that we may protect life and beauty.
Fill us with peace, that we may live
as brothers and sisters, harming no one.
O God of the poor,
help us to rescue the abandoned
and forgotten of this earth,
so precious in your eyes.
Bring healing to our lives,
that we may protect the world and not prey on it,
that we may sow beauty,
not pollution and destruction.
Touch the hearts
of those who look only for gain
at the expense of the poor and the earth.
Teach us to discover the worth of each thing,
to be filled with awe and contemplation,
to recognize that we are profoundly united
with every creature
as we journey towards your infinite light.
We thank you for being with us each day.
Encourage us, we pray, in our struggle
for justice, love and peace.

Forgetting as "Interference" during Consolidation and Retrieval

"With one singular exception, time’s arrow is straight. Unidirectionality of time is one of nature’s most fundamental laws. It has relentlessly governed all happenings in the universe... Galaxies and stars are born and they die, living creatures are young before they grow old, causes always precede effects, there is no return to yesterday, and so on and on. Time’s flow is irreversible.
The singular exception is provided by the human ability to remember past happenings. When one thinks today about what one did yesterday, time’s arrow is bent into a loop. The rememberer has mentally traveled back into her past and thus violated the law of the irreversibility of the flow of time. She has not accomplished the feat in physical reality, of course, but rather in the reality of the mind, which, as everyone knows, is at least as important for human beings as is the physical reality."

-Endel Tulving

Forgetting is probably the scariest thing I can think of short of death itself (indeed, perhaps even scarier). The two phenomena seem to have a lot in common if you think about it! Both are inexorable, incompletely understood, terrifying natural destructions of the self. Both, also, seem to be necessary side-effects of the very processes that enable life and our ability to remember it. Life is the constant (and ultimately losing?) struggle of processes creating order against the prevailing thermodynamic tendency toward disorder (entropy), made possible by a continual flux of energy from the environment. Memory, too, is a way of imposing temporary order on fleeting perceptions, allowing our sensory experience to temporarily persist in the form of impressions which, if attended to and encoded, become memories... at least, until we forget them.

Just imagine "you" without your memories, without the impressions of your past experiences... there's really no such thing! Without memories of the past serving to verify our continued existence, we lose the concept of time altogether and are consigned to exist forever in the present moment. Less existentially dramatic, imagine everyday cases of forgetting like losing access to knowledge you worked hard to acquire, skills you've trained painstakingly to master. Needless to say, I have a morbid fascination with forgetting (and remembering, which we will see is part and parcel). Like those latterday immortality-seekers, in my own way I too care deeply about such preservation... not of life per se (though I do think I support these efforts), but of my memories of my own life as-lived; unadulterated access to my past experiences as-encoded. I'm not asking for superhuman mnemonic powers or an eidetic memory; no, I humbly desire continued access to all the ideas I've had and the things I've tried consciously to remember. It's scary when you realize you don't know what you no-longer know... because you've necessarily forgotten about all the things you've forgotten, all that you once knew. At the same time, even in the face of its many foibles, human memory is an astounding innovation! An apparatus shaped by natural forces to serve only a few basic needs (food/water, shelter/safety, mates...) is currently being used to perform all sorts of amazing feats in a world it simply was not evolved for. But more about memory later; this post is about forgetting.

Forgetting occurs in at least two ways. One is active around the time of initial memory formation (encoding), and the other is active as you recall information from memory (retrieval). I'll give you a foretaste of both phenomena, to be savored at length in due time. The first type of forgetting has to do with the fact that recently formed memories take time to fully consolidate, and during this time they are vulnerable to interference by other mental activity, which can weaken and even eliminate incipient memories! We will talk about how sleep and alcohol actually improve memory for previously learned material by reducing the amount mental activity in the intervening period, which would otherwise interfere with the consolidation process (e.g. Gais, Lucas, & Born, 2006; Bruce & Pihl, 1997; Lamberty et al., 1990). We will also talk about how neuroscience bears this out; recently induced long-term potentiation (LTP) - the neural mechanism thought to underlie memory formation - is inhibited by subsequent LTP, even if the tasks are unrelated (Xu et al., 1998; Abraham, 2003). This damaging effect of new learning has on prior learning becomes less and less pronounced as the delay between new and prior learning is increased. This is super fascinating, so stick around; we'll save this best bit for last.

The second way forgetting can occur is through memory retrieval; that is, remembering one thing may necessarily make it more difficult to remember other things. When you go to retrieve a memory, something has cued you to search for that memory. If many different memories are associated with the same cue, then they "compete" for access to conscious awareness during the memory search; the more of these cue-to-memory associations there are, the more difficult it will be to recall the desired information. Furthermore, increasing the strength of one cue-to-memory association necessarily weakens the associations of all other memories to that cue: this is why it is hard to remember all of the things in a given category, like the wives of Henry VIII. As you recall each additional wife, the relative strengths of the others in memory are thereby diminished, and thus the remaining wives become harder to recall. "Ah, I can never remember that last one!" Selectively retrieving an item from memory, though bolstering future recall for that item, harms our later recall of similar items. As we will see, this negative effect of retrieval may even extend beyond related items; much evidence exists to suggest that retrieval of information is impaired by the previous retrieval of other, unrelated items. This can be viewed as a basic consequence of increasing non-specific mental activity during the retention interval, when presumably the original memories for the to-be-remembered material are still in a state of consolidation. Incredibly, recent research suggests retrieving an well-remembered item from memory may put that item in a state of "reconsolidation" thus re-opening even well-remembered, durable memories to the interfering effects of mental activity (Dudai, 2004). That is, recently activated memories may be just as vulnerable to vitiation by subsequent mental activity as recently formed memories are, though this is still the subject of much debate.

Since these processes of encoding and retrieval are going on literally all of the time in each and every sober, waking one of us, so too is forgetting. I have now given you a relatively complete overview; next, we look into both retrieval-based and encoding-based forgetting phenomena in greater detail. In this post, I rely heavily on two review papers (Anderson & Neely, 1996; Wixted, 2004) both for ideas and sources; I highly recommend reading them both in full, especially if this post leaves you with lingering questions.

"Interference" during Memory Retrieval

An old, solid theory of forgetting was based on various, often related memories interfering with each other. The topic of interference generated loads of research from 1900-1970 and led to many important discoveries in the study of human memory. Though it is no longer alone completely satisfactory as a unified theory of forgetting, it is extremely robust in many respects and it continues to shed light on our understanding of cognition.

In this context, "interference" is the impaired ability to remember information that is similar to other information in memory. It is a theory of forgetting, which itself has several proposed mechanisms. Interference occurs when newly acquired information impairs our ability to retrieve previously stored memories (retroactive interference), or conversely, when what you already know makes it harder to learn something new (proactive interference). It happens when new learning crowds out old learning, or vice versa. Imagine you get a new telephone number; at first, when people ask you for your number, you may give them your old one by accident (proactive interference). However, after a year's experience using your new phone number, you may find your old number difficult to recall (retroactive interference).

But this is not the usual way! Typically, the more you know about something, the more readily you will be able to associate incoming information with stuff you've already stored in memory; indeed, high levels of domain-specific knowledge can enable otherwise low-aptitude people to perform at the same level as their high-aptitude counterparts (Walker, 1987). By virtue of these additional associations and interconnections, the new memory will be more durable and accessible via multiple pathways. And I've talked before about the testing effect: the fact that retrieving a given memory makes that memory easier to retrieve in the future. So what gives?

The trouble starts when the same retrieval cues are related to multiple items in memory. Retrieval cues can be attributes of the target memory or just incidental contextual factors that were present at the time of encoding. For example, when you park your car at the store, aspects of your parking experience are encoded into a mental representation of the event (store, time of day, the fact that you drove a car, the type of car you drove, your internal state...). To the extent that your other parking experiences are similar, they will also contain these characteristics. If these serve as the primary cues which you use to recall your car's location, other memories sharing these features will also be evoked.

Interference increases with the number of competing memories associated with a given cue or set of cues; thus, going from retrieval cue to target memory depends not only on the strength of cue-target association, but also on whether the cue is related to other items in memory.

Early research into these phenomena was done using the something called the A-B, A-D paradigm. Participants would be instructed to learn random pairs of words (A-B), such as dog-boat, desk-rock, etc. Once they had learned these associations, they would be given another list (A-D) where the first word in each pair was the same, but the word it was paired with was different (dog-sky, desk-egg). Various methods of testing show that memory performance on the first list (A-B) suffers when second list of responses (A-D) must be learned, presumably because people show mutually incompatible responses to the common cue "A" (McGeoch, 1942).

Observations such as this led to the ratio rule: the probability that you recall B given cue A is equal to the strength of the A-B association relative to the strengths of all other associations involving cue A. That is,

P(B|A) = strength(AB) / [strength(AB)+strength(AD)+...]

Further findings emerged from a paradigm known as Part-Set Cuing. Slamecka (1968) had people study six words from each of five different categories (e.g., types of trees, birds, etc) and tested them later by giving them each category and having them recall the 6 items that went with each. Crucially, the experimental group was given a couple of items from each category as cues (they were cued with part of each set) to help the recall the remaining items, while the control group was given no cues. It turned out that, when you count the number of non-cued target items recalled in both groups, the people who received the cues performed significantly worse than those who got no cues. Roediger (1973) showed that this deficit in recall for remaining items increases as a function of the number of part-set cues given. The crucial factor is that the cues and the targets have a common retrieval cue: the category. However, while part-set cuing hinders recall of remaining items in the set, it has no effect on people's ability to recognize those remaining items in a list of distractors.

Dewey Rundus (1973) claimed that part-set cuing induces "retrieval competition" between cued items and non-cued target items. In terms of the ratio rule described above, presentation of non-target items strengthens the association of these items to their category cue, which reduces the relative strength of target items. Like, if the category was fruits and you had been given 6 fruits to remember, then providing you with the cue 'orange' at the time of recall strengthens the fruit-orange association while weakening the association of the 5 others, thus making them more difficult to recall.

So increasing the strength of one cue-item association necessarily weakens the association of all other items with that cue. As if that wasn't bad enough, it turns out that a shared cue may not even be required for such "retrieval induced forgetting". Many studies have demonstrated Output Interference, the finding that recall of a target item is impaired by previous retrieval of other items regardless of whether or not the target item shares cues with those retrieved items! This observation is extremely important and has helped to shape recent theory about forgetting.

In one early study, AD Smith (1971) had people study 7 items from 7 unrelated categories; the final test was simply cuing people with the category names in different orders. He found that the average number of items recalled dropped significantly with each additional category, from 70%  for the first to 45% for the seventh. The same thing was found when examining paired associates (Roediger & Schmidt, 1980): participants studied 20 A-B pairs and were then given A-____ as the test cue and asked to recall the target (B). Performance was found to decrease as a function of previous retrievals; across 5 sequential blocks (of 4 questions each), there was a systematic decline in average correctly recalled (.85, .83, .80, .76, and .73). The decline in successful future retrieval caused by prior retrieval, then, does not depend on the category. The findings hold for recognition tests, too. Smith repeated the previous experiment but gave a test that listed 7 items alongside 7 unrelated distractors for each category, requiring subjects to pick the 7 that appeared on the initial study list, very similar decreases were observed. The finding appears to be quite robust (Ratcliff, Clark, & Shiffrin, 1990).

Other studies have shown that both general output interference and cue/category-specific retrieval competition operate separately and simultaneously (Neely, Schmidt, & Roediger, 1983). Even controlling for the passage of time (and thus any residual effects of working memory), output interference persists. As we will see, this distinction between cue-dependent forgetting and forgetting that results from nonspecific mental activity is important because it implies (at least) two different processes.

The Paradox of Interference

Any cognitive act that involves representations stored in memory requires the process of retrieval. If retrieval itself is a source of interference, then accessing what we already know contributes to forgetting, independent of any new learning (Roediger, 1974). If semantic memory (memory for facts) was as susceptible to interference as episodic memory, then this would lead to the paradox of interference (Smith, Adams, & Schorr, 1978). That is, as an expert learned more and more facts about a specific topic/subject/category, he or she should develop more and more difficulty in remembering any one of them (i.e., by the ratio rule). But thankfully, this does not seem to happen. Why?

John Anderson (1974) performed an interesting study where people were asked to study various "facts" of the basic form "a person is in the place", for instance, "a hippie is in the park" or "a lawyer is in the church". Then, subjects were given test items where they had to verify whether or not certain statements were true (eg, "a hippie is in the school"). The crucial finding was that the greater the number of facts learned about a person or location, the longer it took for subjects to verify a statement about that person or location.

Assuming, as we have been, that facts are stored in memory as a network of associations, we can imagine that cue like "a hippie is in the park" causes memory nodes "hippie"and "park" to become activated, and that this activation spreads to other nodes that they are linked to in memory. If activation from one node intersects with the activation from another node (i.e., if they are associatively linked), a person would say "true". Under these assumptions, the speed at which activation spreads down an associative link coming from a node is thought to be slower the greater the number of other associative links there are fanning out from that same node. Thus, the more different facts a person learns about someone (hippie, lawyer, etc) the longer the verification times become.

This "fan effect" is not just an artifact of using these questionable pseudo-facts. In a follow-up study addressing these concerns, students studied true facts about famous people (eg, "Teddy Kennedy is a liberal senator") alongside up to 4 fantasy facts about them. Even when people knew they were being tested only on the true facts, a fan effect occurred as the number of fantasy facts learned about the famous person increased (Lewis & Anderson, 1976).

Two studies provided helped to resolve this paradox (McCloskey & Bigler, 1980; Reder & Anderson, 1980), and they both depend on category hierarchies: if memory search can be restricted to subcategories, they found, then only the facts within that subcategory affect search time. For example, an expert on Richard Nixon has many different categories of information stored about him (his foreign policy, his family life, Watergate...). When asked a question about Nixon's wife, the expert can limit the search to the family subcategory; crucially, only facts within this subcategory will produce interference effects. When retrieving information from memory, people first select the relevant subcategory, and the time it takes to do this is affected by the number of irrelevant subcategories BUT NOT by the number of facts within those irrelevant subcategories. Once the relevant category is selected, only the facts within that category show a fan effect.

Thus, interference effects occur in both semantic and episodic memory. But with semantic memory, memories may be more easily compartmentalized into subcategories that allow for a focused search that restricts the source of interference to items within that category.

The new perspective on interference (first hinted at by Roediger, 1974) holds that our tendency to forget is intimately bound to the very mechanisms that allow memory retrieval to occur. It has come to be known as Retrieval-induced Forgetting (RIF), and it has as its counterpart the Testing Effect (aka Retrieval Practice), which is the happier finding that retrieving information from memory enhances future retrieval of the same information, above and beyond simply restudying the information.

A definitive experimental demonstration of both effects can be seen in Goodmon & Anderson (2011), summarized in the graph above. Essentially, participants study pairs of words that consist of a category name and then an item from the category, such as METAL-iron, TREE-birch, FRUIT-orange, METAL-silver, TREE-elm... There are usually 5-10 categories and 5-10 items per category. After studying the initial list, participants are then given a fill-in-the-blank test over some of the items they saw. They might get METAL-i____ and TREE-e____ for example, with the initial letter provided as a cue. After this, there will be items that were practiced (Rp+, such as METAL-iron, TREE-elm), items that were not practiced but related to practiced items by category (Rp-, such as METAL-silver, TREE-birch), and unpracticed, unrelated items (NRp, such as FRUIT-orange).

After this test phase which provides retrieval practice for certain items, participants are given a final test in which they are asked to remember all studied items! The goal is to see how retrieval practice affects participants' recall for unpracticed-related words (Rp-) compared to unpracticed-unrelated words (NRp). The results of these studies follow the same general trend, with one example being given in the graph above. Notice three things: (1) the items people had practiced retrieving (Rp+, like METAL-iron) are remember best, (2) unpracticed items in the same category are remembered worst (Rp-, like METAL-silver), and that unpracticed items from unpracticed categories (NRp, like FRUIT-orange), are in the middle. Thus, in a single experiment, you can demonstrate the beneficial effects of retrieval practice on the practiced items, and the detrimental effects of retrieval practice on items related to the practiced items (that is, retrieval-induced forgetting).

Selectively retrieving items from memory harms our later recall of similar items by some kind of suppression of the latter. This helps to overcome competition by these related associations when we are interested in retrieving one of them in particular, but it has the significant downside of impairing retrieval of those other items were they to again become relevant.

"Interference" during Memory Encoding

In 1924, Jenkins and Dallenbach demonstrated that everyday experiences can interfere with memory. Their study showed that recall of information was better after period of sleep than after an equal period spent awake (controlling for time of day, etc). The findings are robust and have been replicated many times. For example, high school students' ability to remember new vocabulary terms is enhanced when sleep follows within a few hours of learning (Gais, Lucas, & Born, 2006).

In the late 19th century, observations of patients with brain damage leading to retrograde amnesia (memory loss for prior events) revealed that the degree of forgetting was greater for more recently acquired memories than it was for older memories; this came to be known as Ribot's Law in honor of one of it's earliest discoverers. In retrograde amnesia, more recent memories are more likely to be lost than more remote ones. Because of the effects of forgetting occur on a temporal gradient, this phenomenon is appropriately called "temporally graded retrograde amnesia". It can be induced by electroshock therapy and is seen in many neurological disorders including Alzheimer's disease. All this is to suggest that older memories are somehow strengthened against degeneration while newer memories are not. This is consistent with Jost's second law (1897) and the fact that forgetting data is not well fit by an exponential function (that is, a function with a constant decay rate), but fit much better by functions with ever decreasing proportional rates of decay!

This finding of a temporal gradient in interference is very suggestive of the idea that memories consolidate over time. This would imply that retroactive interference would be stronger the closer it is to the original learning; that is, interference should affect newer memories more than it affects older ones.

 You could test this in the laboratory by having people learn something, and then interfere with this learning at differing intervals by having different groups of people study something else after different amounts of time had elapsed. Amazingly, a laboratory study testing these ideas was conducted in 1900 by Muller and Pilzecker in Germany produced findings absolutely consistent with this hypothesis. Imposing mental exertion earlier in the retention interval resulted in poorer performance when subjects were called upon to remember the original information. And it need not be list-memorization, either! Early research began to uncover temporal gradients when the interfering mental exertion was solving algebra problems (Skaggs, 1925), reading a newspaper (Robinson, 1920), etc. Why would this happen? One leading hypothesis that has received neuroscientific support is that the resources required to consolidate memories are themselves limited, and so any subsequent learning takes away from resources that would have otherwise been used to consolidate the original learning.

As discussed above, an A-B, A-D paired associates learning paradigm is where participants learn a list of items A-B, and then some time later a list of items A-D, and then later are given a cued recall test for their memory of the original list (prompted with A, they have to produce B). The period from original learning of the A-B list until the final test is called the retention interval, and the learning of list A-D produces retroactive interference on participants memories for the A-B list. Later studies using this paradigm to search for an temporal gradient of retroactive interference found an inverted U-shape. Poor recall of A-B was observed if A-D was learned soon after; much better recall of A-B was found if A-D was learned in the middle of the retention interval; poor recall of A-B was observed if A-D was learned right before the cued-recall test. This finding is quite common (Wixted, 2004) and suggestive of both forms of forgetting discussed in this post: interference during consolidation, and interference due to retrieval competition.

Retrograde Facilitation?

If increasing mental exertion during the retention interval results in poorer recall, then reducing mental exertion as much as possible after learning should improve recall! Unfortunately, a waking brain is almost always aflutter with activity... but sleep, alcohol, and benzodiazapenes all reduce this activity, inducing a temporary state of retrograde amnesia and closing the brain (specifically, the hippocampus) to much new input. By limiting this new input, recently formed memories should be protected from the retroactive interference they would otherwise encounter from waking mental activity.

Let's talk about alcohol first, because it's fun. As you may have experienced, alcohol causes a certain degree of anterograde amnesia for materials studied (and events experienced) while under the influence. It is a less widely known that alcohol actually results in improved memory for materials studied just prior to consumption (Bruce & Pihl, 1997; Lamberty et al., 1990; Mann et al. 1984; Parker et al. 1980, 1981). Not that this is a good study strategy... (though who's to say). The prevailing explanation for this finding is that alcohol facilitates recent memories because it prevents the formation of new memories that would otherwise cause retroactive interference (Mueller et al. 1983). The same thing is observed with benzodiazepines (Coenen & Van Luijtelaar, 1997). All of this is entirely consistent with the idea that ordinary forgetting is caused by the retroactive effects of subsequent memory formation that accompanies ordinary mental activity.

No direct tests for a temporal gradient have been done with alcohol or benzos in the retention interval, but some studies a la Muller & Pilzecker (1900) have revealed such an effect for sleep. For example, Ekstrand (1972) used a 24-hour retention interval and had subjects sleep 8 hours either right after the original learning occurred or right before the recall test; people in the immediate-sleep condition recalled 81% of the items, whereas people in the delayed-sleep condition recalled only 66%.

Neural Mechanism

As I've talked about in other posts, long-term potentiation (LTP) in the hippocampus is the leading explanation for how memories are initially formed (Martin et al. 2000).  LTP is a long-lasting enhancement of synaptic transmission  (the "receiving" neuron becomes more sensitive to the "sending" neuron) brought about by high frequency stimulation from sender. Interestingly (!), alcohol and benzodiazepines are both known to block LTP in the hippocampus (Del Cerro et al., 1992; Evans & Viola-McCabe, 1996; Givens & McMahon 1995, Roberto et al. 2002). Furthermore, alcohol does NOT impair the maintenance of hippocampal LTP induced PRIOR to consumption---indeed, consistent with memory findings, it enhances prior LTP!

With respect to sleep-related brain activity, it is known that LTP can be induced during REM sleep but not during non-REM sleep. This may account for the fact that during REM sleep, people are often able to remember mental activity (i.e., dreams), while during non-REM sleep people cannot remember any mental activity taking place (and thus do not experience dreams). Indeed, it has been shown that non-REM sleep protects previously-established memories from interference far better than does REM sleep (Plihal & Born, 1999); further, REM sleep was found to interfere with prior memories just as much as an equal period of intervening wakefulness! All of this is consistent with the observation that, while many prescription antidepressants greatly reduce REM sleep, they are not known to cause memory problems (Vertes & Eastman, 2000). Weirdly, REM sleep does appear to be very important for the consolidation of procedural memories which are non-hippocampus-dependent (Karni et al., 1994).

All this is to suggest that when the demands placed on the hippocampus are reduced, it is better able to coordinate memory consolidation. Cells in the hippocampus which fired together during waking experience were shown to be reactivated together during non-REM sleep in rats (Wilson & McNaughton, 1994). Also, coordinated firing between different areas of the neocortex has been shown to replay itself during quiet, unstimulated wakefulness in monkeys (Hoffman & McNaughton, 2002).

Instead of relying on sleep or alcohol to inhibit LTP, it is much more precise to administer a drug that selectively targets and prevents induction of hippocampal LTP. These drugs, known as NMDA antagonists, prevent LTP and thus prevent learning of hippocampus-dependent tasks. Experiments using these drugs show that, when administered after a learning task or after LTP is artificially induced in the lab by direct neuronal stimulation, they block all subsequent LTP which would otherwise interfere with the original learning; thus, memories for the originally-learned information (or the strength of the artificially induced LTP) are enhanced by taking NMDA antagonists during the retention period. That is, these LTP-blocking drugs produce all the same effects we've seen above, but allow us to draw more specific conclusions about the underlying mechanism.

In one great example of this research, Izquierdo et al. (1999) had rats learn a hippocampus-dependent task and then exposed rats to a novel, stimulating environment either 1 hour or 6 hours later. These researchers observed a temporal gradient: rats forgot more of the original learning when exposed to the novel environment 1 hour after learning, again suggesting that recently established LTP is more vulnerable to disruption by subsequent mental activity than LTP established longer ago. To investigate whether this interference was caused by new LTP associated with exposure to the novel environment, they administered an LTP-blocking drug to a group of rats prior to exposure to the novel environment (1 hour after original learning). These drugs prevented any retroactive interference effects; in this group of rats, memory for the original learning was unimpaired by exposure to the novel environment!

The same findings are observed if you artificially induce LTP and measure it's decay over time. Abraham et al. (2002) induced LTP in the hippocampus of rats using electrical stimulation and these animals were returned to their usual "stimulus poor" home cage environments. In this low-interference environment, LTP decayed very slowly. In the experimental condition, some of the rats were exposed to a complex, stimulating environment (a larger cage, new objects, and other rats). Exposures to this environment caused the originally induced LTP to decay much more rapidly, and this interference was a function of the delay between exposure to the new environment and the induction of LTP.

Thus, regardless of whether original learning is "actual" or artificially induced LTP, subsequent interfering learning (either actual or artificially induced) interferes with the original learning, and this interference is more pronounced the smaller the delay between original and interfering learning. The central message here, and indeed the main argument made by Wixted (2004) from which this post draws shamefully, is this: the hippocampus is extremely important in consolidating newly formed memories and ordinary mental activity (particularly subsequent memory formation) interferes with that process. Thus, biological memory appears to be self-limiting; new memories are created at the expense of partially damaging other memories, especially if these other memories haven't had enough time to consolidate. Even more spooky is the somewhat new idea of reconsolidation: that it is recently activated memories, not just recently formed ones, that are vulnerable to interference (Dudai, 2004). According to this theory, even if a memory has been completely consolidated, reactivation of said memory makes it just as vulnerable to interference as if it were newly formed; thus accessing consolidated memories might simply restart the consolidation process! AHH!