Our capacity for memorizing and remembering things or facts (declarative memory), is so fundamental for us that without this capacity, we can hardly lead a day-to-day living — making a breakfast, reading a newspaper or finding our way home. Accumulating evidence suggest that declarative memory is implemented by the neuronal network embedded within the temporal cortex. Recent studies have identified single neurons coding for the content of declarative memories. However, to unravel the mechanisms how the brain read out the stored memory, it is essential to investigate how the memory signals are processed in a neuronal network.

In the present study, we simultaneously recorded neuronal activities from all the cortical layers in the temporal cortex of monkeys performing a cognitive memory task. Then, we identified the direction of signal flow across cortical layers during the presentation of a visual object and during the memory retrieval. We found that, during visual stimulation, neuronal signals flowed from the layers IV to II/III, and from the layers II/III to V/VI. During memory retrieval, in contrast, the direction of signal flows reversed: from the layers V/VI to II/III. These findings suggest that our brain is implemented with interlaminar networks that can flexibly change the direction of signal flows depending on the cognitive demands.

Program member
Yasushi Miyashita (Department of Physiology, Graduate School of Medicine)

Cerebral cortex can be anatomically divided into six layers (layer I–VI). (Right) Information flows across cortical layers identified in the present study. During the acquisition of visual stimulus, signals flowed from layer IV via II/III to V/VI (arrow in green). During memory retrieval, signals flowed in the opposite direction: from layers V/VI to II/III.