Top-down signal from prefrontal cortex in executive control of memory retrieval

Knowledge or experience is voluntarily recalled from memory by reactivation of the neural representations in the cerebral association cortex(1-4). In inferior temporal cortex, which serves as the storehouse of visual long-ter m memory(5-8), activation of mnemonic engrams through electric stimulation results in imagery recall in humans(9), and neurons can be dynamically acti vated by the necessity for memory recall in monkeys(10,11). Neuropsychologi cal studies(12) and previous split-brain experiments(13) predicted that pre frontal cortex exerts executive control upon inferior temporal cortex in me mory retrieval; however, no neuronal correlate of this process has ever bee n detected. Here we show evidence of the top-down signal from prefrontal co rtex. In the absence of bottom-up visual inputs, single inferior temporal n eurons were activated by the top-down signal, which conveyed information on semantic categorization imposed by visual stimulus-stimulus association. B ehavioural performance was severely impaired with loss of the top-down sign al. Control experiments confirmed that the signal was transmitted not throu gh a subcortical but through a frontotemporal cortical pathway. Thus, feedb ack projections from prefrontal cortex to the posterior association cortex( 2,3,14) appear to serve the executive control of voluntary recall.