Brain mechanisms associated with top-down processes in perception.

Perception arises through an interaction between sensory input and prior knowledge. We propose that at least two brain areas are required for such an interaction: the 'site' where analysis of afferent signals occurs and the 'source' which applies the relevant prior knowledge. In the human brain, functional imaging studies have demonstrated that selective attention modifies activity in early visual processing areas specific to the attended feature. Early processing areas are also modified when prior knowledge permits a percept to emerge from an otherwise meaningless stimulus. Sources of this modification have been identified in parietal cortex and in prefrontal cortex. Modification of early processing areas also occurs on the basis of prior knowledge about the predicted sensory effects of the subject's own actions. Activity associated with mental imagery resembles that associated with response preparation (for motor imagery) and selective attention (for sensory imagery) suggesting that mental imagery reflects the effects of prior knowledge on sensory processing areas in the absence of sensory input. Damage to sensory processing areas can lead to a form of sensory hallucination which seems to arise from the interaction of prior knowledge with random sensory activity. In contrast, hallucinations associated with schizophrenia may arise from a failure of prior knowledge about motor intentions to modify activity in relevant sensory areas. When functioning normally, this mechanism permits us to distinguish our own actions from those of independent agents in the outside world. Failure to make this distinction correctly may account for the strong association between hallucinations and paranoid delusions in schizophrenia; the patient not only hears voices, but attributes (usually hostile) intentions to these voices.