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 declarative memories. However, to unravel the mechanisms how the brain read out the stored memory, it is essential to investigate the network mechanisms of memory signal processing.
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 visual object presentation and during memory retrieval. We found that, during visual stimulation, neuronal signals flowed from the layers Ⅳ to Ⅱ/Ⅲ, and then to Ⅴ/Ⅵ. During memory retrieval, in contrast, the direction of signal flows reversed: from the layers Ⅴ/Ⅵ to Ⅱ/Ⅲ. This finding suggests that our brain is implemented with interlaminar networks that can flexibly change the direction of signal flows depending on the cognitive demands. (Science 331: 1443-1447, 2011) .
Figure legend 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.
*Department of Physiology, The University of Tokyo School of Medicine