How disorders of supervisory processing relate to the overall organisation of mind

 

Tim Shallice

Emeritus Professor

University College London

 

The lecture will be concerned with what one can learn about the organisation of supervisory or executive processes from studies of the cognitive problems of patients with frontal lobe lesions. But what type of basic process can studies of lesion patients uncover? When cognitive neuropsychology began in the late 60s and 70s the emphasis was on individual patients showing strong or classical dissociations between their performance on different tasks. This was held to provide evidence on the functions of specific damaged subsystems. However in the 90s it became clear that some neuropsychological disorders could be explained in terms of damage to connectionist networks. This led to some theorists, such as Patterson & Plaut to argue against the utility of studying individual patients in detail and theoretically to reject the concept of functional subsystems in favour of connectionist networks composed of sets of ‘hidden units’. And this approach resonates with the emphasis on networks rather than individual subsystems coming from resting-state fMRI. I shall take a half-way house position arguing for the use of a range of neuropsychological methodologies and theoretically for regions of cortex having their own specific modifications of basic cortical distributed processing. I will illustrate this by considering disorders of writing.

The main part of the lecture will be concerned with using this perspective to understand the disorders arising from damage to different regions of prefrontal cortex. Before the break, I will briefly discuss disorders of medial frontal cortex in terms of ‘energisation’ processes and of frontopolar ones in terms of their role in setting up and realising intentions. However, the emphasis will be on the contrasting roles of the left and right lateral prefrontal cortices. Often, a superficial interpretation of fMRI findings suggests that functions are bilaterally organised. But lesion data suggests a much greater degree of lateralisation of function. Moreover, this contrast in function is not adequately captured by the difference between processing of verbal versus visuo-spatial material. In addition, I see the left as having specific roles in the construction of hierarchical representations and in certain type problem-solving in small well-structured problem-spaces. The right lateral region will be held to have key roles in active monitoring and, following Goel, in problem-solving in ill-structured problem-spaces. The left lateral prefrontal cortex will be held – following Hagoort – to have a microstructure that facilitates ‘unification’. In addition it will be speculated that the left lateral prefrontal cortex has a greater degree of internal inhibition between candidate solutions than the right leading to winner-takes-all rather than parallel processing.