Analysis of temporal structure in spike trains of visual cortical area MT
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The temporal structure of neuronal spike trains in the visual cortex can provide
detailed information about the stimulus and about the neuronal implementation of
visual processing. Spike trains recorded from the macaque motion area MT in previous
studies (Newsome et al., 1989a; Britten et al., 1992; Zohary et al., 1994) are
analyzed here in the context of the dynamic random dot stimulus which was used
to evoke them. If the stimulus is incoherent, the spike trains can be highly modulated
and precisely locked in time to the stimulus. In contrast, the coherent motion
stimulus creates little or no temporal modulation and allows us to study patterns
in the spike train that may be intrinsic to the cortical circuitry in area MT. Long
gaps in the spike train evoked by the preferred direction motion stimulus are found,
and they appear to be symmetrical to bursts in the response to the anti-preferred
direction of motion. A novel cross-correlation technique is used to establish that the
gaps are correlated between pairs of neurons. Temporal modulation is also found in
psychophysical experiments using a modified stimulus. A model is made that can
account for the temporal modulation in terms of the computational theory of biological
image motion processing. A frequency domain analysis of the stimulus reveals
that it contains a repeated power spectrum that may account for psychophysical and electrophysiological observations. Some neurons tend to fire bursts of action potentials while others avoid burst
firing. Using numerical and analytical models of spike trains as Poisson processes
with the addition of refractory periods and bursting, we are able to account for peaks
in the power spectrum near 40 Hz without assuming the existence of an underlying
oscillatory signal. A preliminary examination of the local field potential reveals that
stimulus-locked oscillation appears briefly at the beginning of the trial.