Top-down processing mediated by interareal synchronization

Perception and cortical responses are not only driven "bottom-up" by the ex ternal stimulus but are altered by internal constraints such as expectancy or the current behavioral goal. To investigate neurophysiological mechanism s of such top-down effects, we analyzed the temporal interactions of neuron s on different levels of the cortical hierarchy during perception of stimul i with varying behavioral significance. We found that interareal interactio ns in a middle-frequency range (theta and alpha :4-12 Hz) strongly depend o n the associated behavior, with a phase relationship and a layer specificit y indicating a top-down-directed interaction. For novel unexpected stimuli, presumably processed in a feed-forward fashion, no such interactions occur red but high-frequency interactions (gamma, 20-100 Hz) were observed. Thus corticocortical synchronization reflects the internal state of the animal a nd may mediate top-down processes.