Spatio-Temporal Analysis of Synchronous Oscillatory Activities in Visual System

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Several studies have shown that cells in the visual cortex (A17) of the cat display stimulus-evoked oscillatory firings (30-60Hz) and synchronization among multiple cells. Previously we reported that the cells in the lateral geniculate nucleus (LGN) also showed stimulus evoked synchronous oscillatory firings of higher frequency range (60-100Hz). The difference in their frequency ranges suggested that the oscillatory activity in the cortex was not likely to derive from that in the LGN. On the other hand, Gray et al. reported a transient nature of the synchronization of the oscillatory local field potentials in the cortex, that is, the transition between asynchronous and synchronous states could occur within a trial duration. For further comparison of the synchronization between the two areas, we investigated whether such temporal modulation (non-stationarity) exists also in the -synchronization in the LGN. Since the traditional method of the cross-correlogram loses any information on the non-stationarity by the temporal averaging ovei-the trial duration, we adopted the unitary event (UE) analysis to trace the dynamics of the spike synchrony. We found that also in the LGN, a reasonable portion of synchronous activities was non-stationary even under the presentation of the stationary spot stimulus. The modulation of the number of the UEs had apparently different time scale (less than a few hundred milliseconds) from the rate modulations of the two cells. This result may suggest that the synchrony plays a different functional role from the firing rate in the visual information processing. Furthermore, the unitary event analysis was applied to the spike trains of individual trials. We found a rather large variability and transiency in both the magnitude and the duration of the unitary events over different trials.