LFP oscillations provide a time reference for excess spike synchrony among V1 neurons

Abstract

When inspecting visual scenes, primates perform on aver-age four saccades per second, implying that the processingof image components may be accomplished in less than200 ms. Individual neurons thereby contribute only witha few spikes for these complex computations, suggestingthat information is encoded not only in the firing rate butalso in the timing of spikes. Recently, local field potential(LFP) oscillations have been suggested to modulate thetiming of single spikes and thus act as a mechanism real-izing the temporal coding of neural information. Wehypothesized that such mechanism should be apparent inthe neuronal activity of V1 during natural visual behavior.Therefore we measured simultaneously neuronal activi-ties and eye movements while monkeys freely viewimages of natural scenes [1]. In a series of analyses on therecordings in this experiment, we have revealed importantproperties of the neural activity in the primary visual cor-tex during free viewing, summarized as follows: (1) theneuronal firing rate starts to increase about 50 ms after fix-ation-onset, reaches a peak value at about 70 ms and thendecays slowly [1] (Figure 1a); (2) the LFP shows oscilla-tory activity in the beta-frequency band during the fixa-tion periods, and these oscillations are locked to the onsetof saccades [2] (Figure 1b); (3) the timing of spikes ismodulated by these oscillations, i.e. spikes prefer a spe-cific phase of the LFP oscillation (phase locking) [2]; (4)the first visually evoked spikes occurring about 70 ms afterfixation-onset show a much stronger phase locking thanall other spikes [3], and (5) spike synchrony across neu-rons occurs in excess as revealed by the Unitary Event