Properties of spontaneous population activity in rat visual cortex. (A) Spontaneous state alternations of the ECoG between ECoG-synchronized and ECoG-desynchronized states under urethane anesthesia. During synchronized states, the ECoG exhibits a large-amplitude slow-wave pattern, whereas during desynchronized states the ECoG shows a low-amplitude fast-wave pattern. (B) Bimodal distribution of ECoG amplitude RMS values under urethane anesthesia. Data are from ~8 h of spontaneous ECoG recording. (C) ECoG power spectrograms of the two urethane states. Note the higher power in the slow frequency range (< 3 Hz) in the synchronized state as compared to the desynchronized state, whereas the reverse holds true for frequencies faster than 3 Hz. Also note a distinct spectral peak at around 4 Hz in the desynchronized state. (D) Histogram showing the durations of each state (pooled data from three animals). While on average, the length of each cycle is on the order of < 15 min, episodes of the urethane synchronized state can last up to 50 min, especially during deep anesthesia. (E) Averaged CSD plots along the depth of the cortex, triggered off of spontaneous activity patterns (see Methods). Note that despite differences in amplitude, laminar CSD profiles are qualitatively similar across states, with net densities of current flow being highest in supragranular (SG) layers (Sync: AVRECSG/AVRECtotal = 0.684 ± 0.044; Desync: AVRECSG/AVRECtotal = 0.698 ± 0.022, ± SEM). Furthermore, no prominent granular (G) or infragranular (IG) sink-source pairs are apparent in profiles of either state. These findings suggest that spontaneous population activity is, to a large degree, governed by similar anatomical network components in synchronized and desynchronized states.