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Abstract:
With a recently developed neuronal circuit model of the
prefrontal cortex (Tanaka 1997: Soc Neurosci Abstr), we have
investigated the roles of horizontal connectivity and local
inhibition in the formation of visuospatial memory fields of
prefrontal cortical neurons. The model circuit is multilayered and
contains many modules or columns. The pyramidal cells in the
superficial layers (PS cells) are assumed to make extensive
horizontal connections that are stronger between iso-directional
columns. A directionally modulated transient input to the PS cells
triggers the autonomic dynamics of the circuit to form memory
fields of the cortical cells in the delay period. This simulates
the neuronal activity observed from monkey prefrontal cortex
performing an oculomotor delayed-response task (Funahashi et al.
1989: J Neurophysiol). Our simulation showed that the widths of the
memory fields increase when the horizontal connections become more
extensive. However, the local inhibition affected the widths of the
memory fields. Like in the visual cortex, the local inhibition
contributes to shape the memory field. That is, insufficient
inhibition caused flattening of the memory fields, having no
directional information. For excessive inhibition, on the contrary,
the activity of the cortical neurons were suppressed globally and
the memory fields were shrunk. Further increasing the inhibition
nullified the memory fields. Our simulations suggest that changing
the horizontal connectivity alters the widths of the memory fields
and the local inhibition contributes to shape the memory
fields.
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