BRAIN MECHANISMS ASSOCIATED WITH TOP-DOWN PROCESSES IN PERCEPTION

Perception arises through an interaction between sensory input and pri or knowledge. We propose that at least two brain areas are required fo r such an interaction: the 'site' where analysis of afferent signals o ccurs and the 'source' which applies the relevant prior knowledge. In the human brain, functional imaging studies have demonstrated that sel ective attention modifies activity in early visual processing areas sp ecific to the attended feature. Early processing areas are also modifi ed when prior knowledge permits a percept to emerge from an otherwise meaningless stimulus. Sources of this modification have been identifie d in parietal cortex and prefrontal cortex. Modification of early proc essing areas also occurs on the basis of prior knowledge about the pre dicted sensory effects of the subject's own actions. Activity associat ed with mental imagery resembles that associated with response prepara tion (for motor imagery) and selective attention (for sensory imagery) suggesting that mental imagery reflects the effects of prior knowledg e on sensory processing areas in the absence of sensory input. Damage to sensory processing areas can lead to a form of sensory hallucinatio n which seems to arise from the interaction of prior knowledge with ra ndom sensory activity. In contrast, hallucinations associated with sch izophrenia may arise from a failure of prior knowledge about motor int entions to modify activity in relevant sensory areas. When functioning normally, this mechanism permits us to distinguish our own actions fr om those of independent agents in the outside world. Failure to make t his distinction correctly may account for the strong association betwe en hallucinations and paranoid delusions in schizophrenia; the patient not only hears voices, but attributes (usually hostile) intentions to these voices.