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Abstract:
We have simulated the brain mechanisms involved in semantic
processing tasks requiring recalled semantic features. The model is
consistent with the anatomy and physiology of the component brain
circuits. A projection from posterior perceptual areas to
prefrontal cortex serves to represent the stimulus input to working
memory. A projection from working memory to semantic memory excites
semantic features associated with the input stimuli. We assume that
the cortical areas involved in the task are organized as cortical
columns. Oscillations in corticothalamic loops sustain activity of
prefrontal cortex neurons for the duration of the task and subserve
the working memory function. The semantic representation of a
concept is encoded as a constellation of features over cortical
columns. We use phase information to bind concepts excited in
working memory with their component semantic features
(Singer,Ann.Rev.Phys.,55:349-374,1993). Groups of neurons
representing concepts in prefrontal cortex oscillate in distinct
phase in a mechanism similar to that proposed by Lisman &
Idiart (Science,267:1512-1515,1995). Through excitatory
projections, semantic memory neurons are driven to fire
synchronously and in phase with the neurons that excite them. A
process of phase comparison is used to select task appropriate
responses. Our results are in agreement with that of Sarnthein et
al. (PNAS,95:7092-7096,1988) who found synchronization between
prefrontal and posterior association cortex during human working
memory tasks. The properties of the model are in agreement with a
number of other of observable behavioral and neural
phenomena.
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