Cognition, behavior and the frontal lobes.

Our knowledge of how the brain functions has markedly improved in recent years, but this improvement has largely been constrained to simpler levels of processing such as those involved in sensory or motor systems. In contrast, the neural bases of higher functions such as consciousness, will, long-term planning, complex problemsolving, etc., are still under investigation, although everyone agrees that the frontal lobes (FL) play an important role in such processes. The importance of understanding these neural bases of higher functions becomes even more evident in the study of aging, since a decrease of frontal lobe perfusion is reported during aging, and because this age-related frontal lobe dysfunction has been proposed to explain part of the cognitive disorders (attention disorders, decreased mental flexibility and abstraction) and behavioral changes (mainly apathy or indifference) associated with aging. In order to understand the contribution of the FL in cognition and behavior, it may be important to first consider the FL, at a most global level, as one unit dedicated to every aspects of voluntary action, whatever the output (cognitive, behavioral or in the realm of social interactions). All these aspects – that include anticipation of the goal, selection of the appropriate piece of information, monitoring of information within the working memory buffer, execution of the plan, validation of its pertinence – are mainly organized within the prefrontal cortex (PFC) in closed relationships with the premotor and primary motor cortices. Indeed, whereas the latter structures are oriented toward the execution and motor preparation of response, the PFC can be seen as a multimodal associative region that integrates many parameters in order to elaborate and adjust adaptive behaviors (for review, see Goldman-Rakic, 1987; Fuster, 1997). According to Miller and Cohen (2001), the main function of the PFC is to exert a top-down control on other brain areas that allows the most adaptive and integrated response to occur. In this view, the PFC constitutes an interface between afferent sensory signals (it is the only region of the neocortex that receives messages from both the external word and internal milieu) and the output system. It allows behavioral adaptation by creating the temporal buffer needed for the preparation of the most adequate pattern of responses. By elaborating new patterns of behavior, the PFC de facto inhibits the automatic activation of reactive responses triggered by external stimuli. The difficulties encountered by patients with prefrontal lesions to prevent reflexive or purely externally-driven behavior are represented by a tendency to involuntarily grasp objects presented in front of them, to