Monday, October 08, 2007

Hammers and Distributed Memory

In “The Feeling of what Happens” (1999), Antonio Damasio describes, among many other things, the distributed nature of memory. The following quote describes an example of how the concept of an object (in this case a hammer) may be represented in the brain (from page 220 of the book):

The brain forms memories in a highly distributed manner. Take, for instance, the memory of a hammer. There is no single place of our brain where we will find an entry with the word hammer followed by a neat dictionary definition of what a hammer is. Instead, as current evidence suggests, there are a number of records in our brain that correspond to different aspects of our past interaction with hammers: their shape, the typical movement with which we use them, the hand shape and the hand motion required to manipulate the hammer, the result of the action, the word that designates it in whatever many languages we know. These records are dormant, dispositional, and implicit, and they are based on separate neural sites located in separate high-order cortices. The separation is imposed by the design of the brain and by the physical nature of our environment. Appreciating the shape of a hammer visually is different from appreciating its shape by touch; the pattern we use to move the hammer cannot be stored in the same cortex that stores the pattern of its movement as we see it; the phonemes with which we make the word hammer cannot be stored in the same place, either. The spatial separation of the records poses no problem, as it turns out, because when all the records are made explicit in image form they are exhibited in only a few sites and are coordinated in time in such a fashion that all the recorded components appear seamlessly integrated.

If I give you the word hammer and ask you to tell me what hammer means, you come up with a workable definition of the thing, without any difficulty, in no time at all. One basis for the definition is the rapid deployment of a number of explicit mental patterns concerning these varied aspects. Although the memory of separate aspects of our interaction with hammers are kept in separate parts of the brain, in dormant fashion, those different parts are coordinated in terms of their circuitries such that the dormant and implicit records can be turned into explicit albeit sketchy images, rapidly and in close temporal proximity. The availability of all those images allows us, in turn, to create a verbal description of the entity and that serves as a base for the definition.

This 'story' of the recall of an abstract concept (by which I mean something which is not explicitly tied to a particular sensory experience) describes a situation where memory, as is generally thought of (recalling objects and events), is fully distributed throughout the brain, and can not be localised in a particular region. This concept has had much supporting empirical evidence found in recent years, including that I've reviewed in last weeks series of posts (which I believe was first proposed by Lashley in the early 20th century).


alfredo said...

this kind of ideas are corrleative with the global working space of Baars (1988), in his book homonime.
brain would be a kind og thaetre of counciusness, where spacialized prossesors works and united for the task of recall, por instance.

Paul said...

Based on my understanding of the global workspace theory (GWT), I'm not sure how consistant it is with the proposal in the quote. Damasio suggests that the coordination of distributed representations is an emergent process through sensory experience of the real world (i.e. no 'coordinating module') - whereas the GWT has a structural requirement for a workspace, and a number of (essentially independant?) functional modules operating in parallel where it is the module which 'shouts loudest' which attains the centralised workspace, rather than the cooperative/integrative process indicated in the quote.

Having said that, I'm not intimately familiar with all the details, so there may well be a way of reconciling these two points of view - perhaps by viewing the GWT as an abstract functional model rather than an architecture?

derekjames said...

Ah, the local/distributed representation issue. What about studies like the one carried out by Quian Quiroga et al. in 2005 that found localized activation in response to celebrities such as Bill Clinton or Jennifer Aniston, whether or not the subject was looking at a photograph, line drawing, or other representation?

Paul said...

Thanks for your comment Derek, and indeed alfredo earlier.

I'm afraid I'm not familiar with that study, and am no expert on neuroimaging (so may very well have to be corrected on a number of the following points), but does not the method of imaging impact on what information is actually viewed? For example, using the subtraction of a baseline, what you see in activation diagrams is that activation which is in addition to that of the baseline. Similarly, if one subtracted the brain image of someone looking at a picture of one face from that of another face, the result would not necessarily represent unique characteristics of the representation of the second face, merely that differences exist in the neural representations. So the common activations when looking at different visual representations of two different faces (from your example) maybe it show more the commonalities between the different representations of a single face than it does for the differences between the different faces? Sorry, that was incredibly long winded... :-)

I assume that this is also related to the 'grandmother cell' argument, where it is proposed that for each object that we have experience of, there is a cell which fires when it is detected (i.e. object recognition) - and which is usually viewed as a thought experiment?

derekjames said...

The study I mentioned wasn't fMRI, it was single-cell recording in epilepsy patients who had already had intrusive surgery and had agreed to participate in this study.

Here's the actual paper:

And yes, in effect they found "grandmother cells", which fire selectively only in the presence of a particular person, independent of representation. Like many of the controversies in cognition (nature/nurture, prototype/exemplar categorization, etc.) the actual answer is mostly likely a complex blend of the extreme positions.

In other words, I think memory is probably localized in some aspects and cases and distributed in others.