Local functional state differences between rat cortical columns.

Surface evoked potentials (SEPs) during auditory clicks and whisker twitches are usually larger during quiet sleep (QS) over waking and REM sleep. However, SEP amplitudes from single trials fluctuate periodically between high and low values regardless of sleep-wake cycle. To test the hypothesis that state-independent fluctuations represent local functional sleep-like states of individual cortical columns, we examined single trial SEP amplitudes from multiple cortical locations across sleep-wake cycles. Bilateral stimuli produced SEP amplitude fluctuations in each hemisphere that usually covaried (r = 0.4), but with frequent hemispheric differences. Two neighboring whiskers, twitched simultaneously on the same side, produced highly correlated SEPs in neighboring cortical columns (r = 0.9) with frequent divergences. We found 50% more disparity during QS over waking, indicating that the differences did not result from recording noise or stimulus inconsistency. Local SEP fluctuations also followed local differences in the delta wave signal during QS (r = 0.4), suggesting that similar mechanisms may modulate the SEP. The duration of the localized sleep-like (high SEP amplitude) state was dependent on the duration of prior wake-like (low SEP amplitude) state (r = 0.5), suggesting a use dependence of prior functional state period. Since SEP indicators fluctuated independently from whole animal sleep state, and were frequently different between hemispheres and nearby cortical columns, these data support the theory that sleep-like functional states may be localized to brain regions at least as small as cortical columns.