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Abstract:
In the experimental protocol of a working memory task using a
monkey as a subject, tuned activity of the prefrontal cortical
neurons that is sustained during the delay period is a neuronal
substrate of the working memory. However, how the prefrontal cortex
performs such mnemonic processes is not clear. This study addresses
this question by means of computer simulations of the dynamics of a
model prefrontal cortical circuit. The model assumes that the
pyramidal cells receive two types of intracortical inhibitions,
'parallel and anti-parallel inhibitions,' in accordance with recent
experimental findings. The parallel and anti-parallel refer to the
relationship between the preferred directions of the presynaptic
interneurons and the postsynaptic pyramidal cells. The simulations
showed that well-tuned sustained activity is formed and maintained
in the model prefrontal cortical circuit when both types of the
intracortical inhibitions are appropriately regulated. Under
inappropriate conditions, on the other hand, the cortical neurons
exhibited either transient activity or high-frequency epileptic
activity. The following three factors are suggested to be crucial
for the formation and maintenance of spatial working memory: the
cortical amplification of the activity due to the excitatory
closed-loop circuitry, the suppression of excessive excitation by
the parallel inhibition, and the sharpening of the activity profile
by the anti-parallel inhibition.
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