This effect lasted up to several minutes, was N-methyl-D-aspartate (NMDA) receptor-dependent, and was observed in both somatosensory and auditory cortices. The phenomenon was similar to what we observed in auditory cortex of awake, passively listening

animals. These data suggest that the formation and reverberation of sensory-evoked patterns may partake in learning-related phenomena in multiple neocortical regions of anesthetized animals, which may provide a convenient model for the study of memory mechanisms in the brain. We first investigated changes in spontaneous activity patterns induced by sensory stimulation by recording activity from neuronal populations in primary somatosensory cortex (S1). Under urethane anesthesia (Figure 1A), brain activity

showed a synchronized state with characteristic slow wave oscillations (Steriade et al., 1993), in which generalized bursts of population activity (UP states) were interspersed with CP-690550 concentration periods of neuronal silence (DOWN states) (Figure 1C, bottom). UP states were accompanied by negative deflections of the local field potential (LFP) (Figure 1C, top), indicative of synchronized synaptic inputs. Urethane promotes a condition Gefitinib order of behavioral unconsciousness that closely mimics the full spectrum of natural sleep (Clement et al., 2008), although the duration of DOWN states is reported to be shorter in natural sleep (Johnson et al., 2010) as compared to anesthetized conditions. Injection of amphetamine rapidly changed the brain state; within a few minutes after injection, cortical

activity transitioned to a strongly desynchronized state, which lasted for at least 30 min (Figures 1B and 1D). Tactile stimulation did not change either synchronized or desynchronized brain states (Figures 1A and 1B, shaded area). Surprisingly, the average stimulus-triggered responses in S1 were very similar in synchronized and desynchronized states, despite Procainamide large differences in spontaneous neuronal activity among these states (Figures 1E and 1F). To investigate fine-scale temporal changes in spontaneous neuronal activity induced by sensory stimulation, we first calculated the relative latency of each neuron. This reflects its timing in relation to other neurons based on cross-correlogram analysis (see Experimental Procedures; Figure 2A). Figure 2B shows cross-correlograms of 32 neurons from a representative experiment, sorted by latency during the stimulation period after amphetamine injection (middle panel). Consistent with previous results from auditory and visual cortex (Jermakowicz et al., 2009 and Luczak et al., 2009), neurons showed similar temporal patterns during spontaneous and stimulus-evoked conditions. For example, neurons that were firing earlier than other neurons during stimulation also tended to fire earlier than other neurons during spontaneous activity before or after tactile stimulation (Figure 2B, top and bottom panel, respectively).