In Search of Memory

A few weeks ago, I read Eric Kandel’s In Search of Memory. It was different from the books that I normally post about here, as it is an autobiography with the emergence of modern neurobiology woven in, rather than an argument for this or that perspective. But I am a neurobiologist at heart and an aspiring scientist at the very beginning of my career, so I couldn’t resist:

memory

Eric Kandel won the nobel prize in the year 2000 for his pioneering work on the molecular mechanisms of memory formation and storage. In this book, Kandel lays down the path of his life, professional and otherwise, from his earliest days in Vienna, just before World War II, up through his acceptance of the nobel prize in Stockholm, just a few years ago. His book will be of interest to biologists, philosophers, psychologists and laymen alike. The material is presented in the order that it was first discovered and assumes no prior knowledge, leaving no bars to entry for this exciting journey. All the same, weathered experimentalists will surely enjoy the ride that is the birth of this new science, from single-cell recordings in hippocampal cells, to the neural networks of Aplysia, to the beginnings of the differentiation of  the neural substrates for unconscious vs conscious information processing.

And a fun fact for those who are, like me, still trying to break into this field: What was Eric Kandel, nobel laureate in biology, studying in his Junior year of college? None other than Northern European History. He didn’t set foot in a lab until medical school, when he was entranced by the promises of psychoanalytic theory. I think we’ll be ok.

Finally, for those of you who wonder what I am doing when I am not reading or writing about consciousness (or wonder why I post so scarcely now!), I am now excitedly spending the majority of my time in the BRAIN Lab at Washington University in St. Louis on a summer research fellowship studying up on neurogenetics (i.e., how genetic variation influences how our brains respond to our environment and modulates risk for psychopathology). Check us out!

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Memory and Personal Identity

Today in metaphysics I had to write up an impromptu response to the question: “Is memory important to Personal Identity?” I only had thirty minutes to write and not much time to prepare, so it’s a little rough, but I am nonetheless satisfied with what resulted, so I’ve reprinted it here:

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Memory is not vitally important to personal identity. This is not to say that it is not useful to our everyday determination of “who’s who,” more on that later, but we can show that it is possible to maintain personal identity without maintaining memory.

Consider the case of Tom, or case 1. Tom is about to be tortured. But the torturer, being a slightly nice guy, proposes the following: before torturing Tom, he will wipe all of Tom’s memories. Should this make Tom feel any better? “Of course not,” I would expect Tom to reply, “I’ll still be tortured, I just won’t remember that it’s still me who was tortured!” So we are not making the situation any better—we are actually making it worse! Not only are we going to torture Tom, we are also going to turn him into an amnesiac! If this case is persuasive, then we have shown that memory is not necessary for personal identity.

But I think we can go one step further and show that it is not sufficient either. Consider case 2: Exactly the same as before, except this time we will not merely erase Tom’s memories, but instead transfer them to another body, say, Jane’s body. Now, who would Tom, pre-memory transfer, want us to torture, Tom’s body or Jane’s body? I think that, thinking only of himself, he should want us to torture Jane’s body, and here’s why: this case is no different from the first. If removal of memory is not enough to remove personal identity, as the case 1 seems to show, then how could implantation of memory create a person? If it were able to do so, then we would have two “Toms” at the end of the procedure: one amnesiac Tom in Tom’s body and one “normal” Tom in Jane’s body—but this seems to be obviously mistaken, Tom can only be in one place at a time! So which Tom is illusory? Well, if we stick with our judgment for case 1, then it seems we have to say that the “Tom” in Jane’s body is the illusory one. It’s not really Tom, Jane just thinks that she is Tom. Tom is still in Tom’s original body. If all of the preceding is true, then we have shown that memory is neither necessary nor sufficient for personal identity.

At this point, it would be prudent to evaluate just what it is that we are saying, and just what we are not saying. The preceding argument aims to show that memory is not important from a metaphysical standpoint—but this says nothing about the epistemic standpoint. In real life, memory is often all that we have to go on for determining personal identity. How do I know that I am the same person as I was last week? Because I have the memories of what I did last week! If we agree with the preceding argument, though, it would seem as if we were contradicting ourselves. We cannot determine our own identity based solely on our memories. Well, okay, maybe we can’t, for all we know, some mad scientist  implanted some false memories into my brain while I slept, and I am not who I think I am, this is not outside the realm of possibility. But it seems pretty unlikely—so, inferring to the best explanation, that barring unusual circumstances memory goes hand-in-hand with personal identity, I conclude that I am most likely the same person as my memory tells me I was last week. It’s not certain, but it is very likely. It’s important to note here, though, that these are all epistemic worries. They tell us nothing about the metaphysics of personal identity. I can use memory as a good “indicator” of personal identity, so in that sense it is very important to our conception of personal identity, but that does not mean that the two are inextricably linked. As an analogy, if I hear a dog bark, I usually infer  that a dog is nearby, but for all I know someone is merely playing a recording of a dog’s bark and there is, in fact, no dog nearby. Personal identity refers to the dog, and the recording of a bark is the memory of Tom in Jane’s body.

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There’s a technical point about memory that I did not have time to address in my original response, but which I want to bring up here. In order for something to really be a memory, it has to stand in a causal relation to the event that it recalls—that is, the event that it recalls has to itself be what caused the memory to exist. So in this sense, I probably have many things that I would call “memories” in my head that are not truly memories. Perhaps I am misremembering something, or perhaps I have heard a story of my childhood so many times that, even though unbeknownst to me my own memory of the event is gone, I have recreated the scene in sufficient detail for me to be able to picture it vividly. On this definition of memory, Jane never really had memories of Tom’s life. They felt like memories to Jane, but since they were not caused by the events in Tom’s life, but rather by the torturer’s memory implantation, they are not, in this strict sense, true memories. If you were to adopt this more nuanced view of memory, then a memory theory of personal identity may be more plausible. Unfortunately though, you can still show, as per case 1, that memory is not necessary for personal identity.

On the “Grandmother Cell”

In trying to pin down how the brain stores memories, ideas, and concepts, many theories have been proposed. The most plausible so far is that discrete patterns of activation correspond to distinct memories, or mental states. This view can be further specified through the theory of embodied cognition, which, at its most basic, states that mental processing is aided by the areas of the brain that are normally responsible for movement or perception. To illustrate this, consider the following: When you are walking down a street, your motor cortex, in concert with a number of brainstem regions, functions to produce the walking motion that allows you to travel to your destination. As you see, hear, smell, and feel your surroundings, your primary sensory cortices are at work, helping to integrate all of this information into your experienced picture of the world. Now, what would we see in your brain if we asked you merely to imagine yourself walking down this same street? As it turns out, we would see activation in many of the same areas we did when you were actually experiencing the original event.

Through this lens, then, it is relatively straightforward to explain these sorts of memories or mental processes. Your brain does not necessarily store the information explicitly, but it could rather be said that it merely re-induces the processing patterns that were co-occurrent with the original experienced event, resulting in the same sorts of images and feelings being constructed by the sensory and motor cortices, but this time the output is available only to the mind’s eye.

Now, to the Grandmother Cell Hypothesis. This hypothesis asserts that single neurons can respond to relatively complex ideas or concepts, such as your own grandmother. In your brain, then, we would find a single neuron that fires only when you think about your grandmother. Therefore, the view proposes, this neuron is solely responsible for your knowledge or recognition of your grandmother—which flies in the face of the distributed view that I started the article with. Seemingly, these views are highly incompatible with each other. This is where, I think, a lesson from Damasio can be highly instructive.

Recall the idea of Convergence-Divergence Zones (CDZs, henceforth, per Damasio). In short, a CDZ is an area in the brain that records co-occurrent inputs, or patterns of activation. Then, at some point in the future, backward projections from this zone re-activate the upstream neurons that caused the original inputs, thus re-inducing the original pattern of activation, and restoring whatever mental state was associated with it. Suppose the Grandmother Cell is a very low-level version of a CDZ: It records the co-occurrent patterns of neural activation that are associated with your grandmother, and then re-induces them when you recall your grandmother later on. In this way, the two originally opposing views can be used in conjunction to present a more unified picture. It would not be said that the concept of your grandmother is stored by your grandmother cell, but the data needed to induce—perhaps in your sensory and motor cortices, as above—the concept of your grandmother in your mental space could be stored by such a simple system. A grandmother cell, then, represents the lowest-level coding of an idea, while the corresponding induced pattern of neural activity represents the highest-level biological coding of an idea.