Saturday, October 06, 2007

Part 3 - Areal specificity in the PFC and temporal integration

In this third and final part of my review of this paper by Fuster (reference at the end of this post), the paper turns to look a little more closely at what neural mechanisms are at play in the performance of the central function it is proposed to carry out: temporal integration of information, and more generally, the temporal organisation of behaviour.

The paper then turns to the question of the supposed area specialisations in the cortex: the Network Memory theory predicts that functional subdivisions do not exist, but multiple lesion studies have suggested the opposite, to the extent that some cortical regions are generally accepted to be tied to a particular function. The PreFrontal Cortex (PFC) in particular has been subject to many proposed functional subdivisions, according to functions such as attention, working memory (visual, spatial and verbal), and involvement in episodic and semantic memory. The two points of views thus need to be reconciled.

Fuster addresses this problem by proposing that there is a common underlying process to all of the previously mentioned functions. As has been mentioned, this common process is proposed to be temporal integration. An example of this process in the PFC is given by cells which respond specifically to two temporally separate stimuli which have been associated through experience. These neurons have been demonstrated in monkey studies where the presented stimuli consisted of colours and sounds, so that for a given behavioural goal (which for the monkeys was learning and remembering associations between a colour and a sound, for some reward) cells "...seemed to belong to executive networks that, ..., integrated sounds with colours over time." This example demonstrates a principle with important implications for the concept of working memory, particularly the view of executive functions. Instead of dedicated neural circuitry for executive processes as has been proposed by most psychologically based working memory theories, this example (and other converging evidence) indicates that "...executive working memory seems to be essentially based on the ad hoc activation of executive networks of long-term memory."

From this proposal, it becomes necessary to answer two questions: (1) how are these executive networks activated in a timely fashion for the task at hand; and (2) how is this network (or networks) maintained active for the temporal bridging of separate components?

In order to process perceptual information, some form of modulation is required from the PFC – this being necessary for the context dependant retrieval of information from the posterior cortex (i.e. The perceptual hierarchy of the Network Memory theory). Feedback from these regions would then activate executive representations in the PFC. Together with the influences from sub-cortical regions, and the obvious effect of the external environment on the activation of perceptual and motor memories, this loop may be held responsible for the so called 'monitoring' function of the PFC, where relevant information is activated and made available as and when required. It also addresses the question of 'implementation' of working memory: this mechanism strongly indicates that "[it] is as widely distributed as the long-term memory that supports it." Furthermore, the cooling (known to inhibit brain function in a relatively tightly specified region) of any of these implicated regions in monkeys when performing a behavioural task produces deficits in performance indicative of working memory deficit. This may be explained by viewing such an inhibition as interrupting the 'loops' of recurring activity just mentioned, and thus impairing the monitoring function. In summary then: "Working memory is emerging as a mechanism of temporal integration essentially based on the concurrent and recurrent activation of cell assemblies in long-term memory networks of frontal and posterior cortex."

In the paper conclusion, we are reminded of the evidence from a wide range of disciplines in support of the proposed cardinal function of the PFC being the temporal organisation of behaviour, and temporal integration: anatomical interconnections, and developmental evidence (both in the individual and in evolutionary history). The PFC operates as part of the perception-action cycle, which links the actions of the animal with the environment and its internal neural dynamics.

REF: Fuster (2001), "The Prefrontal Cortex – An update: Time is of the essence", Neuron, vol. 30, pp319-333

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