Thursday, October 04, 2007

Part 2 - The PFC: a temporal organiser

There have been a number of studies in the past (including Endel Tulving, who proposed the view of separate semantic and episodic memory, and the associated HERA model), particularly neuroimaging based, which have observed two things in particular: firstly that the encoding of new memories activates the left PFC more than the right PFC, and secondly that the reverse activation profile is seen when memory is retrieved. This indicates the sort of specialisation that Fuster moves away from. An argument is thus provided to counter this: " is not altogether clear that the asymmetric activations of the two prefrontal cortices in encoding and retrieval are attributable to their differential involvement in these two cognitive operations. The apparent functional dissociation of right and left PFC may be a product of the subtractive method if the material utilized to test the two operations carries a different semantic load... in both encoding and retrieval tasks, the executive memory networks of lateral PFC may be activated inasmuch as the tasks consist of temporally integrative acts based on internal or external contingencies." In summary, the lateral PFC's most important function is the temporal organisation of behaviour.

Whereas routine behaviours sequences may be made up of chained behaviours, where one action leads to the next, there is no temporal contingency required - i.e. no planning. However, when ambiguity or uncertainty is introduced, such temporal planning ("cross-temporal contingencies") is required. Hence the role of the PFC (in delay tasks, for example, where there is at least one ambiguity that needs to be resolved) in temporal integration. The figure below, taken directly from the paper (figure 4 in paper), gives examples of these two events: in the top case, a routine sequence of behaviours occurs, and in the bottom, a complex or novel sequence. In this latter case, any given act is contingent on the goal, the plan, and other acts - this is the hypothesised role of the lateral PFC: the temporal organisation of behaviour.

Given this role for the lateral PFC, working memory is proposed as the first "temporal integrative function" of the PFC which has been verified in primates using electrophysiology. These experiments, which used delayed response tasks (and occurred in the early '70s), have identified four properties of neural firing related to spatial memory (thus also working memory):
(1) it was absent after stimuli that did not call for prospective action;
(2) it was absent in the mere expectation of reward;
(3) it was correlated with the accuracy of the animal's performance; and
(4) it could be diminished or aborted by distracting stimuli occurring in the delay period.

These properties strongly indicate "retention of the memorandum" (that which is to be remembered), which is dependant on the future need for action. "In sum, considerable evidence from several methodologies supports a prospective role of the PFC, in addition to its retrospective role of working memory. Ingvar (1985) dubbed that prospective role the 'memory of the future'". The retention of the memorandum could then allow the activation of the necessary motor functions (so called motor attention or preparatory set), thus priming the system as a whole for the execution of the required actions: “A temporally symmetric and complementary function of preparatory set, or 'motor attention' would activate the network's motor components and thus prime executive systems for the anticipated action.”

The final part of this paper review will be posted tomorrow, on more specific cortical mechanisms involved in this proposed function of the PFC...

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

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