Turning on and off with excitation: The role of spike-timing asynchrony and synchrony in sustained neural activity

Delay-related sustained activity in the prefrontal cortex of primates, a ne urological analogue of working memory, has been proposed to arise from syna ptic interactions in local cortical circuits. The implication is that memor ies are coded by spatially localized foci of sustained activity. We investi gate the mechanisms by which sustained foci are initiated, maintained, and extinguished by excitation in networks of Hodgkin-Huxley neurons coupled wi th biophysical spatially structured synaptic connections. For networks with a balance between excitation and inhibition, a localized transient stimulu s robustly initiates a localized focus of activity. The activity is then ma intained by recurrent excitatory AMPA-like synapses. We find that to mainta in the focus, the firing must be asynchronous. Consequently, inducing trans ient synchrony through an excitatory stimulus extinguishes the sustained ac tivity. Such a monosynaptic excitatory turn-off mechanism is compatible wit h the working memory being wiped clean by an efferent copy of the motor com mand. The activity that codes working memories may be structured so that th e motor command is both the read-out and a direct clearing signal. We show examples of data that is compatible with our theory.