The dynamics of evoked and ongoing activity in the behaving monkey

What happens in primary sensory areas in the absence of sensory input (eg, primary visual cortex when eyes are closed)? Previous findings from voltage sensitive dye imaging (VSDI) experiments done on anesthetized cats (Grinvald et al, 1988; Arieli et al, 1995; Arieli et al, 1996; Tsodyks et al, 1999; Kenet et al, 2003) indicated that activity in the visual cortex depends not only on the nature of visual inputs but also on the state of the cortex at the time of stimulation. Furthermore, patterns that looked like orientation columnsmaps appeared spontaneously. Do those spontaneous cortical states appear also during the awake state and have any functional significance? We combined simultaneous VSDI with electrophysiological recordings of local field potentials (LFP) and multiunit activities. During ongoing activity sessions, the VSDI revealed coherent spatio-temporal activity over the primary visual area. We found that in comparison to the ongoing activity in the anesthetized preparation, the dynamics of the ongoing activity in the awake monkey is much faster, and the coherence-length is much smaller. We also detected in the LFPs short episodes of high-energy oscillations in the ∼30 Hz range. Those short episodes were not detected in the evoked sessions, in contrast with the situation reported for anesthetized cat (Gray and Singer 1989). During the evoked sessions, cortical columns with similar orientation preference were phase coherent. We observed a clear phase shift for the orthogonal orientation columns in V1. We report here that the averaged coherence over space between the VSD signals and single unit activity was significantly higher than between VSD-VSD signals and VSD-LFP signals. The cortical evoked response showed maximal coherence between