Thursday, November 16, 2006

Network Memory

In this paper on network memory, Prof. Fuster presents the view that memories are held in the brain not in localised regions, but in distributed networks that span the entire neocortex. There is however some organisation to this: the overlapping cortical hierarchies I mentioned in a previous post.

"The capacity to store information about oneself and ones environment is present throughout the nervous system. Thus, almost all regions of the brain store memory of one kind or another. In primates, the memory of past experience is stored largely in the neocortex - the phylogenetically newest part of the cerebral cortex." These are the first few sentences of the paper, and they describe quite concisely the basis of the paper. He goes on to briefly overview the history of memory research, using it as the basis of the assertion that memories are "essentially associative; the information they contain is defined by neural relationships" (based on Hebb's famous "if they fire together, they wire together" principle of neural connection formation). Through these associative processes cells can become interconnected into functional units of memory - which may represent simple sensory memories (or images). Using this as the basis of complex memories, personal memories must therefore be made up of vast numbers of these relatively simples 'units' of memory interconnected in complex networks

"It is reasonable to assume, as Hayek did, that memory and perception share, to a large extent, the same cortical networks, neurons and connections." Thus, new memories or perceptions are expansions of previously existing ones - additional associations to pre-existing networks. Due to this, any individual neuron, or small group of neurons, may be part of numerous networks and thus memories.If I may make use of another quote which I think is one with important implications (even if it is somewhat fuzzy from certain points of view): "Memory networks are most likely to develop, at least partly, by self-organisation, from the bottom up, that is, from sensory or motor cortical areas towards areas of association. They also probably develop in part from the top down, guided by attention and prior memory stored in the association cortex; here the synchronous convergence would be between new inputs and inputs from old reactivated networks. In any case, networks grow on a substrate made of lateral as well as feedforward and feedback connections." The result of this type of organisation would be hierarchical memories, which is emergent from the interactions of multiple memory units. However, this self organisation does not start from scratch with a newborn: phyletic memory (or memory of the species - that which we are born with) is postulated to determine what the structure of the sensory and motor cortices is at birth - in other words, they already contain the necessary information to begin interacting with the environment from information which is genetically defined. Neural plasticity of these regions (possible remaining to adulthood) allows these memories to be refined before becoming relatively fixed for life. This phyletic memory thus serves as the foundation upon which personal memories may 'grow' - fusing so that there is no way to distinguish between the two.

Perceptual memory is memory acquired through the senses. A hierarchy exists, from "sensorially concrete" (memories of elemental sensations of all modalities) at the bottom of the hierarchy, to "conceptually general" (abstract concepts which ,although originally acquired by sensory experience, has become separable for cognitive operations) at the top. As one rises in the hierarchy, so do the networks representing memories at each level become more distributed and widespread in the cortex. Perceptual memory with this organisation does not however persist independently of motor memory (described below) - there are numerous reciprocal connections and associations between the two hierarchies, which naturally has important implications for sensory-motor integration and working memory.

Motor memory is the representation of motor acts and behaviours, including much, if not all, what has been traditionally defined as procedural memory. Much of the motor memory in the lower levels of the hierarchy is essentially phyletic (i.e. the fulfillment of basic drives, such as defensive reactions), however is conditionable and subject to control from 'higher' levels. The prefrontal cortex is the highest level of the motor hierarchy, which indicates a role not only in the representation of complex actions, but also the operations for their enactment, including working memory.

The next part of the paper is concerned with the dynamics of memory. At any given time, most long-term memory is out of consciousness, presumably equating to the relevant neural networks being relatively inactive. Reactivation occurs through associative processes of recall or recognition, which may be due to either internal or external stimuli. The hippocampus is implicated strongly in this reactivation process. Network dynamics can best be observed using electrical stimulation or neuroimaging. Using the monkey task described in my other post on Fusters cortical hierarchies, the reactivation of networks can be observed in both posterior regions (sensory recognition/recall), and frontal regions (maintenance/planning/action). Please watch the short video clips showing this spread of activation (links from this post)- they really are enlightening. By this view, working memory is active memory, and is thus distinguished by the state of the network, and not the cortical location of activations (although this is dependant on the two distinct cortical hierarchies just reviewed - thereby adding adding a location requirement on working memory).

In the concluding comments, Fuster notes that even though a hierarchical structure is in place, this does not mean that they are rigid and stored in separate cortical domains. Instead, different types of memories are made up of numerous 'elements' distributed across the different levels of the hierarchy (or both hierarchies as the case may be). The final note that the paper makes is concerning working memory and its relationship to long-term memory. From this hierarchy setup, the two are one and the same, although as I noted in the previous paragraph, merely saying that working memory is 'activated' long-term memory isn't sufficient - from my understanding of how working memory would fit within this hierarchical network theory, working memory would be made up of those activated networks across both the perceptual and motor hierarchies (and at multiple levels within those hierarchies) such that some form of coordination (planning?) is required between the two 'types' of memory. In other words, executing a motor action or recognising an object in the visual field in themselves do not require working memory (even though by this theory they would result in activated cortical networks), but identifying an object to be manipulated in some way would require working memory (coordination between the activated networks across both sensory and motor hierarchies).

Apologies for the somewhat disorganised nature of this post. It was written across a couple of periods, and was just an organisation of my thoughts...


Chris Chatham said...

Hi Paul - This is a great post.

I love your characterization of WM - "it is long-term memory updated for prospective use." The part about "heterarchical memory" is interesting too.

I completely agree with your statement that STM (Atkinson & Shiffrin) was largely replaced by the WM model. In some cases, theorists will still refer to STM and WM as distinct, insofar as WM involves an executive component (manipulation of information) but STM need not. So, for example, digit span is more of an STM task (relatively little exec cont. required) but operation span or reading span are more WM tasks. This distinction is unfortunately not always recognized, and many ppl will refer to digit span (myself included, I'm sure) as a WM task.

Paul said...

Hi Chris - thanks for your comment, appreciated as always!

I think many of the problems you referred to concerning labelling of tasks wrt working memory are caused (at least in part) by ambiguity in the definition of working memory itself. I have myself been slightly confused at the distinction between STM and WM. However, I believe this to be because these theorists believe WM to be the 'control structures' (emphasis on 'working'), whereas STM is the memory part - like I said, a case of differing definitions. My view (open to change and correction of course!) is that the Atkinson & Shiffrin view of STM is outdated (linked to how a computer works), and that Wm has subsumed its function - however, that the memory part and the control part cannot be functionally distinguished (hence my interest in the Netowrk memory theory). So your examples of the digit span and the reading span would place different loads on the WM 'system', but that they use essentially the same 'processes'. What do you think?

Chris Chatham said...

I see your point, and am tempted to agree with you because of the general difficulty in ascribing discrete functions to a massively distributed device like the brain. Processes & representations are much more continuous and graded than the clean little boxes in any cognitive psych model of memory. On the other hand, there's a really cool study by Brad Postle where rTMS to superior parietal disrupts maintenance and rTMS to dlPFC disrupts manipulation processes. So there does appear to be some degree of specialization, although I think that study is a special case. Very interesting!