Wednesday, October 03, 2007

Part 1 - The PFC: anatomy and the network memory theory

The PreFrontal Cortex (PFC) is widely implicated in many 'high-level' cognitive functions, from working memory to planning, and others. It is frequently decomposed into a number of subregions, each supposedly specialised in some way or another for particular tasks. It is however difficult to establish these claims for certain given that current neuroimaging techniques do not have adequate spatial and temporal resolution to examine the real-time workings of these very small subregions of the PFC. All that can be done is to collect a wide variety of evidence from various methodologies in support of the theory. This is what Fuster does in this paper (ref at end): neuroanatomical, neuroimaging, single-cell recordings and behavioural studies are all reviewed in support of his theory of PFC functioning. His view is that the PFC essentially performs the function of temporally integrating information in order to achieve behavioural goals. This view was covered briefly covered in a previous post, but a number of details were missing from that short paper, which this paper attempts to address.

Fuster considers the PFC as being at the apex of the motor hierarchy in the Network Memory theory. As such, the supposed widely distributed network nature of the cortex in general is of great importance (quote "Any hypothetical modularity of the PFC is functionally meaningless if taken out of wide-ranging networks that extend far beyond the confines of any given prefrontal area."), and that is where the paper starts - the placement of the PFC in the cortex, with regard to neural connectivity. A summary of these connections by means of a quote (text in italics added): "The PFC is connected with the brainstem, the thalamus, the basal ganglia, and the limbic system (hypothalamus in particular). Much of that connectivity with subcortical structures is reciprocal. Especially well organized topologically are the connections between the PFC and the thalamus. The prefrontal connections with the mediodorsal thalamic nucleus have been used as a criterion for identifying the PFC in a wide variety of species." In terms of cortical connectivity, the PFC has connections to the cortices of association, but not with the primary sensory or motor cortices - which given the Network Memory theory's placement of the PFC at the top of a hierarchy, makes sense.

The PFC may be divisible into three main sub-regions: medial, orbital, and lateral. Each of these is 'connected' to itself and to each other. Damage to each of these three sub-regions typically produce distinct behavioural deficits. Damage to the orbital PFC (such as that sustained by the famous Phineas Gage) results in behaviour which is impulsive, an inability to inhibit instinctual behaviours, and typically exhibit a severe attention disorder (inability to withstand distraction). Damage to the medial part of the PFC generally result loss of spontaneity, and difficulty in the initiation of movements and speech. However, it is damage to the lateral PFC which produce the most characteristic cognitive deficit (according to Fuster). For humans, the most notable deficit is in the ability to plan and carry out sequences of actions which manifests itself in a difficulty in both speech and behaviour, and the difficulty to initiate and perform sequences in an orderly manner. Fuster points to this in particular to be a strong indication that the lateral PFC "plays a crucial role in the organization and execution of behavior, speech, and reasoning."

After reviewing the basics of the Network Memory theory (which may be seen in this post), the paper then goes on to describe in broad terms how the PFC fits in, and what more precisely its role is. The low levels of the motor hierarchy are located in the primary motor cortex, the so-called phyletic motor memory, which defines movements by muscle actions. Higher up in the motor hierarchy, these may be grouped, such that movements are defined by goals and/or trajectories. These higher levels are located in the PFC, and include, for instance, "elementary linguistic structures". The lateral PFC in particular "appears to harbour networks representing schemas, plans, and concepts of action." However, as sequences become over-learned and automatic, their representations 'move' towards lower executive levels: i.e. as the 'novelty' of an action decreases (or becomes more routine), less planning is required, and so the representation may 'drop down the hierarchy' (Fuster mentions that they may be organised entirely in sub-cortical structures). Despite this, if ambiguities in the action remains, then the involvement of the PFC remains necessary, in order to "integrate events in the temporal domain". A note is made that despite these functions being at the top of the hierarchy, they are not necessarily organised hierarchically: they are to some extent at least organised heterarchically. This is an important point, as it is an effort to acknowledge that the PFC does not perform top-down serial processing, as is often implied by a strict hierarchical structure.

This paper review continues tomorrow with a closer look at the 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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