A COMPUTATIONAL APPROACH TO PREFRONTAL CORTEX, COGNITIVE CONTROL AND SCHIZOPHRENIA - RECENT DEVELOPMENTS AND CURRENT CHALLENGES

In this chapter we consider the mechanisms involved in cognitive contr ol - from both a computational and a neurobiological perspective - and how these might be impaired in schizophrenia. By 'control', we mean t he ability of the cognitive system to flexibly adapt its behaviour to the demands of particular tasks, favouring the processing of task-rele vant information over other sources of competing information, and medi ating task-relevant behaviour over habitual, or otherwise prepotent re sponses. There is a large body of evidence to suggest that the prefron tal cortex (PFC) plays a critical role in cognitive control. In previo us work, we have used a computational framework to understand and deve lop explicit models of this function of PFC, and its impairment in sch izophrenia. This work has lead to the hypothesis that PFC houses a mec hanism for representing and maintaining context information. We have d emonstrated that this mechanism can account for the behavioural inhibi tion and active memory functions commonly ascribed to PFC, and for hum an performance in simple attention, language and memory tasks that dra w upon these functions for cognitive control. Furthermore, we have use d our models to simulate detailed patterns of cognitive deficit observ ed in schizophrenia, an illness associated with marked disturbances in cognitive control, and well established deficits of PFC. Here, we rev iew results of recent empirical studies that test predictions made by our models regarding schizophrenic performance in tasks designed speci fically to probe the processing of context. These results showed selec tive schizophrenic deficits in tasks conditions that placed the greate st demands on memory and inhibition, both of which we have argued rely on the processing of context. Furthermore, we observed predicted patt erns of deterioration in first episode vs multi-episode patients. We a lso discuss recent developments in our computational work, that have l ed to refinements of the models that allow us to simulate more detaile d aspects of task performance, such as reaction time data and manipula tions of task parameters such as interstimulus delay. These refined mo dels make several provocative new predictions, including conditions in which schizophrenics and control subjects are expected to show simila r reaction time performance, and we provide preliminary data in suppor t of these predictions. These successes notwithstanding, our theory of PFC function and its impairment in schizophrenia is still in an early stage of development. We conclude by presenting some of the challenge s to the theory in its current form, and new directions that we have b egun to take to meet these challenges. In particular, we focus on refi nements concerning the mechanisms underlying active maintenance of rep resentations within PFC, and the characteristics of these representati ons that allow them to support the flexibility of cognitive control ex hibited by normal human behaviour. Taken in tote, we believe that this work illustrates the value of a computational approach for understand ing the mechanisms responsible for cognitive control, at both the neur al and psychological levels, and the specific manner in which they bre ak down in schizophrenia.