TEMPORAL COMPRESSION OF SPIKE PATTERNS DURING REACTIVATION IN THE RAT MEDIAL PREFRONTAL CORTEX

D.R. Euston*; B.L. McNaughton

NSMA, Univ Arizona, Tucson, AZ, USA

Spiking patterns of hippocampal neurons during a task replay, or "reactivate", during subsequent sleep. During slow-wave sleep, replay occurs predominantly during hippocampal sharp waves, which span 50-100 ms. Spike-train cross-correlations suggest that replay is temporally compressed by up to 10 times relative to behavior (Skaggs, McNaughton, Wilson, and Barnes, 1996); however, the temporal patterns manifested in sleep may appear shorter simply because they are truncated by the sharp-wave window. In the present study, reactivation was studied in the medial prefrontal cortex. A rat was trained to run to a sequence of locations around the perimeter of a 1.5 m arena using medial forebrain bundle stimulation as reward. The rat learned the task well, typically completing 100 cycles of the 8-element sequence in a 50 minute period and producing highly stereotyped trajectories. Each behavioral session was sandwiched between two 20-30 minute sleep periods. 40-80 cells per session were simultaneously recorded from either the dorsal anterior cingulate (ACd) or the prelimbic cortex (PL). Reactivation was robust. Specifically, the cell-pair correlations during sleep accounted for a up to 35 percent of the variability in the behavior correlations, even after accounting for the correlations during sleep preceding behavior. Owing to the repetitiveness of the task, the spike train cross-correlograms during behavior showed well-defined structure (patterns of peaks and troughs) spanning roughly 20 seconds. In many cases, this structure was preserved during subsequent sleep, but at a much compressed timescale. Spike temporal patterns were thus preserved, but replay occurred at an accelerated pace. In data from 16 sessions, temporal compression varied between 4 and 7 times. Similarities between maze and sleep cross-correlations extended for roughly 800 ms in the sleep timescale, and spanned several seconds in the maze time-frame.

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