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, m ainly because none of the neuropsychological tests used for their asse ssment is sufficiently specific for frontal dysfunction. In animals, t he delayed reaction paradigm is considered to be a specific marker of the function of dorsolateral region of the prefrontal cortex. It seeme d of interest, therefore, to attempt to apply this paradigm to patient s with recent and limited cortical lesion of vascular origin. The perf ormance of patients with dorsolateral prefrontal lesion (n = 10) was c ompared to that of patients with post-central lesion (n = 10) and cont rol 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 task s coupled with a Delayed Reversal task in which the patient had to dis cover the rule by himself in the absence of explicit cues (internally driven tasks). Patients with prefrontal lesion showed a specific defic it in the Delayed Response task, the emergence of a stereotyped behavi our in the Delayed Alternation task and an inability to deduce and to transfer rules (non-alternation and reversal), mainly because of diffi culty in abandoning previous behaviours. Our study demonstrates that t he prefrontal cortex plays a role in behavioural adaptation to challen ging new situations by inhibiting not only ongoing elaborated programm es but also the emergence of previously established automatic programm es. The respective role of the prefrontal cortex and the basal ganglia in these two levels of behavioural organization is discussed.