Tuesday, October 10, 2006

Cortical Dynamics of Working Memory

Notes on a lecture given by Joaquin Fuster (2006), at the Almaden Institute Conference on Cognitive Computing - http://video.google.com/videoplay?docid=-3002336180397686566&q=almaden+cognitive+computing.

This fascinating lecture provides of summary of Prof. Fusters work on cortical dynamics, in this case focusing on working memory. He defines working memory at the start as the active (online) retention of information for a prospective action to solve a problem or to reach a goal (with the emphasis on prospective action). This is what I consider to be a fairly generic definition of what the purpose of working memory is - although naturally, over the course of my research, the systems/dynamics underlying this statement vary quite significantly. From this basic definition, two further qualifications are presented: Firstly, the actively held information is unique for the present context, but is inseparable from past context - in essence, it is long-term memory updated for prospective use. Secondly, working memory and long-term memory share much of the same cortical structure. This distinction I will dwell on
for a little. The implication of this statement is first that long-term and working memory are functionally distinct systems; secondly, significant common ground between the two memory systems exists, showing that while they may be functionally separable, they are not neurally separable. If I may mention the 'classic' working memory (WM) theory of Baddeley and Hitch, one can see that the functional separability is very clearly defined (due to it being based on behavioural studies) in Baddeley's WM, whereas the second point seems to have been largely neglected, until recently at least (with the introduction of the episodic buffer). On a final note, Prof. Fuster notes that working memory may also be described as attention towrds an internal cue - an interesting note in itself.

One thing that I am still slightly bewildered by, and something which I would be glad of comments on, is the difference between working memory on the one hand, and short-term memory on the other. The way I currently understand it, short-term memory may be seen as somewhat of a special case of working memory. Together with the basic sensory stores (the persistence of sensory 'representations' for a short period of time after the occurrence of the stimulus), working memory replaced the Atkinson and Shriffrin view of a short-term memory store. Corrections and other points of view gladly accepted on this point.

Points of note in the presentation (and approximate time in minutes):

- 20mins: when describing the overlapping cortical hierarchies of the frontal lobe and posterior sensory regions, Fuster makes reference to the fact that Semantic memory is a result of the abstraction over time of individual experienced instances (episodic memory). Given that Endel Tulving defined episodic memory as a subset of semantic memory, there seems to be a large discrepancy between the now classical view of the structure of declarative memory, and the theories presented by Prof. Fuster. On the other hand, more recent evidence has suggested that episodic and semantic memory essentially work in parallel (see here - http://www.cus.cam.ac.uk/~jss30/pubs/Graham2000%20Neuropsygia.pdf).

- 31mins: a description of the Cortical Perception-Action Cycle - a fundamental principle of neural operation.

- 48mins: The description of constraints on the modelling of the memory network. Two types of constraint are described - structural and dynamic terms. In structural terms, the model would have to have a network structure, would have to deal in a relational code (resultant from the fact that memories are defined by associations, at all levels), it must be both hierarchical and heterarchical (in other words, the interaction of multiple hierarchies in parallel - the overlapping cortical hierarchies), and finally it must be capable of plasticity (the networks must not be static, they must be able to change). In Dynamical terms, the memory must be content addressable (be addressable by content or by association), it must accomodate variability (be stochastic), the system must be capable of reentry (that is, it must be able to update its own 'knowledge', or long-term memory - hence plasticity), the obvious necessity for parallel processing (he gives the example of perceptual processing), although having said that, there is a necessity for serial processing when it comes to conscious attention (the Global Workspace Theory of Bernard Baars springs to mind here...), and finally, the model must be able to accomodate category, in both perception and action (another spring to mind: Krichmar and Edelman's brain based device work http://cercor.oxfordjournals.org/cgi/content/abstract/12/8/818). Although Prof. Fuster notes that these are his personal views on constraints, in my humble opinion, they deserve at least acknowledgement when it comes to such model building, due to his vast experience and research credentials.

- Animations of brain activations of patients performing delayed matching to sample tasks (the basic working memory task for humans) in various modalities: visual (54mins), spatial (56mins), verbal/auditory (57mins). This are particularly impressive, and display beautifully what he said previously concerning the overlapping cortical networks - I must recommend them.

- 70mins: a question regarding the relationship between perception and memory - essentially the answer described that perception is formed by memory, thus aligning with the theories of active perception, or expectation-driven perception/attention.

As a final note, I would just like to mention his conclusions. Firstly, that memories are defined by associations at all levels. Secondly, that hierachical networks for perceptual memory reside in posterior regions, whereas executive memory networks reside in the frontal cortex. Thirdly, that the prefrontal cortex (at the top of the perception/action cycle hierarchy) mediates cross temporal contingencies (as eloquently argued in his first book "The Prefrontal Cortex"). And finally, working memory is maintained by recurrent activity between the prefrontal cortex and the perceptual posterior regions - as shown in those animations.

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