Spike triggered analysis of direction selective simple and complex V1 cells.

<p><b>A.</b> The Spike Triggered Average (STA) for a simple cell (<i>left</i>) and a complex cell (<i>right</i>), shown as “<i>x</i>-<i>t</i>” plots: the <i>horizontal axis</i> corresponds to the spatial location perpendicular to a bar oriented in the neuron’s preferred direction, and the <i>vertical axis</i> indicates the time preceding a spike (10–140 ms). The grayscale value represents the magnitude of the STRF, indicating how strongly an increased/decreased luminance value at a spatiotemporal location increases or decreases the probability to spike. The tilt in the simple cell STA (<i>left</i>) demonstrates that it is direction selective, but the STA cannot capture the direction selectivity of the complex cell (<i>right</i>). <b>B.</b> V1 DS complex cells are sensitive to large numbers of motion features but the specific number of significant STC filters strongly depends on the amount of available data. With increasing data, more and more filters are detected (lines correspond to 8423, 28435, and 117097 spikes, respectively). However, the filters added (both “excitatory” and “suppressive”) with more data do not resemble those first detected, having higher spatial frequencies and less spatial localization (suppressive filters for smaller amounts of data are present but not shown). Note that the suppressive filters (<i>bottom row, right</i>) have opposite direction selectivity from the excitatory filters (left). The thin frame indicates the filters with the largest positive and negative eigenvalues, corresponding to the <i>red dots</i> in (C). <b>C.</b> The corresponding eigenvalue spectra of the spike-triggered covariance (STC) matrices estimated from 6% and 100% of the data indicate two vs. eight significant filters (<i>red circles</i>), respectively. <b>D.</b> The profiles of temporal power along each pixel position for the three STRFs indicated in <b>B</b> (last row) are diverse, ranging from relatively localized and unimodal to spatially extensive and multimodal.