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Abstract:
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 the amplitude of ongoing activity (primarily synaptic potentials) is large, suggesting that it may play an
important role in cortical processing by affecting the evoked activity and therefore the final behavior itself.
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. Several questions have been
recently addressed: what are the spatial-temporal characterizations of the ongoing activity in early visual areas
of the behaving monkey? Does it affect the evoked activity? How is it related to the functional architecture?
We investigated the cortical activity in the primary visual cortex of a behaving monkey during both evoked
and ongoing conditions. We combined simultaneous VSDI with electrophysiological recordings of the local
field potential (LFP) single and multi unit activities. In the evoked condition, the monkey was trained to fixate
for 10s while presented with a full field moving grating, whereas, during the ongoing condition, the monkey
was required to sit quietly in a totally dark room. We found that the VSD signals in both conditions are often
highly similar to the LFP, just like in the anesthetized cat. The similarity between the VSD signals and LFP
was highest within the α (9-14 Hz) frequency band. For the awake monkey, the ratio between amplitude of
ongoing and evoked activity was much smaller (~1/6) than what was found in the anesthetized cats. However,
extensive spike triggered averaging (STA) of the VSD signals revealed coherent spontaneous activity also in
the awake primate. Some cells exhibited coherent activity with large assemblies in both area V1 and V2.
Cortical states related to orientation representations, if any had a short life time and short coherence length,
much smaller than those found in the anesthetized cats. We also detected in the LFPs short episodes of
high-energy oscillations in the ~30Hz range. Those short episodes were not detected in the evoked sessions. In
contrast with the Situation reported for anesthetized cat (Gray and Singer). 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. The cortical evoked response showed maximal coherence between
VSD-VSD, VSD-LFP and VSD-Spike rate within three distinct frequency bands, 4Hz, 9-14Hz 16-19Hz.
These results suggest that ongoing activity is richer in awake animal may play multiple functional role in the
awake primate as well.
