Modulation of V1 Spike Response by Temporal Interval of Spatiotemporal Stimulus Sequence
(A) Spatial relation between stimuli in screen coordinates (calibration bar = 1 deg). White cross represents fixation target, and the dashed circle (invisible to the animal) encloses the RF of the recorded neuron determined with a spatial summation test. Gabor stimulus at RF (S2) is at preferrred orientation. S1 was presented at one of four locations, a–d, along the axis orthogonal to that of RF orientation, with a spacing of one RF diameter. All S1 orientations were parallel to S2. There were 44 unique stimulus sequences (4 S1 positions×11 SOAs), plus five single stimulus conditions at each S1 and S2 locations. These 49 stimulus conditions were randomly repeated. (B) Raster and density plots of response to S1 at positions a–d aligned at its onset. Spike density function was derived by convolving spike sequence with an asymmetric kernel function . Y-axis indicates spike density in spikes/s. Note that no S1 alone at positions a-d evoked spike response. (C) Raster and density plots for S2 alone and S1c-S2 sequence stimuli with SOAs of 30 and 50 ms chosen to illustrate response modulation. Trials are aligned at S2 onset. It can be seen that the magnitude of initial and sustained response varied with SOA. (D) An example SOA-time plot compiled from spike density for S1c-S2 sequence stimuli, the first stimulus at positions c and the second stimulus at RF. Y-axis is SOA, determined in 10-ms step. The times of S1 onset for each SOA condition are indicated as small white circles. Data are linearly interpolated across SOA. The S2-alone condition is given above for comparison. Note that the cell’s response varied with SOA. (E) Determination of significant modulation. Spike density curves for S2 alone (black) and S1c-S2 sequence with SOA of 80 ms (green), along with horizotal marks (top) of temporal epochs associated with statistically significant decrease (blue) and increase (red) from S2 alone condition. (F) Time course of significant modulation of spike response by sequence stimuli as shown in E. Spike density following S1–S2 sequence was compared with spike density following S2 for each of temporal epochs of 30 ms with a shift of 5 ms. The temporal epochs with a statistically-significant decrease in spike density as determined with Mann-Whitney U-test are shown in blue bars, and significant increase in red bars, centering on corresponding analysis windows, revealing the magnitude and time course of suppressive and facilitative effects of S1 that depend on S1 position and SOA. The dark symbols represent significant modulation at p<0.01, and the light ones are p<0.05.