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Abstract
Neocortical assemblies produce complex activity patterns both in response to sensory
stimuli and spontaneously without sensory input. To investigate the structure of these
patterns, we recorded from populations of 40-100 neurons in auditory and somatosensory
cortices of anesthetized and awake rats using silicon microelectrodes. Population
spike time patterns were broadly conserved across multiple sensory stimuli and spontaneous
events. Although individual neurons showed timing variations between stimuli, these
were not sufficient to disturb a generally conserved sequential organization observed
at the population level, lasting for approximately 100 ms with spiking reliability
decaying progressively after event onset. Preserved constraints were also seen in
population firing rate vectors, with vectors evoked by individual stimuli occupying
subspaces of a larger but still constrained space outlined by the set of spontaneous
events. These results suggest that population spike patterns are drawn from a limited
"vocabulary," sampled widely by spontaneous events but more narrowly by sensory responses.