Abstract
What happens in primary sensory areas when sensory input is shut down (e.g. primary visual cortex when eyes are closed)? Previous findings from Voltage Sensitive Dye Imaging (VSDI) experiments done on anesthetized cats (Grinvald et al., 1991; 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 columns maps appeared spontaneously. Do those spontaneous cortical states have any functional significance, and what that might be? (Ferster D., 1996; Ringach D.L., 2003) As a natural step we went on to investigate whether those states also occur in the brains of animals that are awake. VSDI was recently implemented also on the awake monkey (Slovin et al., 2002; Seidemann et al., 2002) allowing monitoring of activity from the same patch of cortex for up to a year. In addition, spatio-temporal pattern of cortical activity can be obtained without signal averaging. We investigated the cortical activity in the primary visual cortex of behaving monkeys during both evoked and ongoing conditions. We combined simultaneous VSDI with electrophysiological recordings of local field potentials (LFP) and multi unit activities. In the evoked conditions, the monkeys were trained to fixate for 10s while presented with a full field moving grating, whereas, during the ongoing conditions, the monkeys were trained to sit quietly in a totally dark room while their eyes were covered. We have detected coherent spatio-temporal activity during ongoing activity over the primary visual areas. This ongoing spatio-temporal activity shows its highest correlation to the local-field potentials at the range of the alpha frequency band (9-14 Hz). 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 have also observed inter-area differences in coherent activity between area V1 and V2. Those results strengthen the hypothesis that the ongoing activity plays an important role in cortical function rather then being epiphenomena of the anesthetized state.
