Figure 2. Spatial summation for broadband and stimulus-locked responses.

(a, b) For moving bar experiments, each stimulus position was associated with a short time window (1 s for S1, S2, S3; 0.5 s for S4). The Fourier transform was computed within each window, indicated in the schematics as t1, t2, t3, and t4. The power of the broadband response (a) or the amplitude of the stimulus-locked response (b) was then calculated and concatenated into a time series. (c) Time series are shown for the broadband response to wide bar apertures (red) and narrow bar apertures (black) for an electrode located on the V1/V2 boundary in S2. The bar aperture made 8 sweeps across the visual field: 2 horizontal, 2 vertical, and 4 diagonal, indicated by the black arrows in the circular apertures. The diagonal sweeps included blank periods (white background)[6]. The mean response during the blank periods was subtracted to render a meaningful baseline level of 0 μV². Each sweep of the bar elicits a time series peak, except the last diagonal because the stimulus is blanked when it would cross the electrode’s receptive field (lower left visual quadrant). The inset bar graph shows the mean +/− SEM of the highest response during each of the 8 sweeps for the narrow (black) and wide (red) bar apertures, averaged across 15 visually responsive electrodes in V1, V2 and V3 in S1, S2 and S3. See supplementary methods ‘Channel Selection’ and Table S1 for details on criteria used to select these 15 channels. The peak power is about the same for the two bar aperture widths. The fact that the power does not increase much for the wide bar is not a ceiling effect, as evidence by the fact that the response increase further when the stimulus contrast increases (Figure S2). (d) Same as (c), but using the stimulus-locked response instead of the broadband response. The stimulus-locked peaks are higher for the wide bar aperture than the narrow bar aperture, both in the example time series and in the mean across channels (inset).