DELAYED-RESPONSE TASKS AND PREFRONTAL LESIONS IN MAN - EVIDENCE FOR SELF-GENERATED PATTERNS OF BEHAVIOR WITH POOR ENVIRONMENTAL MODULATION

The functions of the frontal lobes in humans are still under debate, mainly because none of the neuropsychological tests used for their assessment is sufficiently specific for frontal dysfunction. In animals, the delayed reaction paradigm is considered to be a specific marker of the function of dorsolateral region of the prefrontal cortex. It seemed of interest, therefore, to attempt to apply this paradigm to patients with recent and limited cortical lesion of vascular origin. The performance of patients with dorsolateral prefrontal lesion (n = 10) was compared to that of patients with post-central lesion (n = 10) and control subjects (n = 24), in four experiments: a Delayed Response task in which the correct answer was previously indicated by an explicit cue (externally guided task); Delayed Alternation and Non-Alternation tasks coupled with a Delayed Reversal task in which the patient had to discover the rule by himself in the absence of explicit cues (internally driven tasks). Patients with prefrontal lesion showed a specific deficit in the Delayed Response task, the emergence of a stereotyped behaviour in the Delayed Alternation task and an inability to deduce and to transfer rules (non-alternation and reversal), mainly because of difficulty in abandoning previous behaviours. Our study demonstrates that the prefrontal cortex plays a role in behavioural adaptation to challenging new situations by inhibiting not only ongoing elaborated programmes but also the emergence of previously established automatic programmes. The respective role of the prefrontal cortex and the basal ganglia in these two levels of behavioural organization is discussed.