Figure 4.

Different contributions of contrast responses and surface responses. A, VSD activation pattern evoked by a 2 × 2° square contour stimulus (inset, red dot marks the fixation point), presented 1.5 left of the fixation point and 2.5° below for 300 ms, averaged 60–100 ms (n = 11 trials, large blood vessels are colored gray). Blue and red regions (area 0.46 mm²) depict the ROIs used to measure edge and center activation, respectively. B, Average activation evoked by the stimulus in A at the edges (blue) and center (red) ROIs, and at the center region of the response evoked by a large surface stimulus (orange), centered at the same location as A (size 8 × 8°, n = 21 trials). Significance of >0 activation measured by Wilcoxon signed rank test (***p < 0.001). C, VSD pattern evoked by a 2 × 2° square surface stimulus (inset) presented at (−1.5 and −2.5°) for 300 ms, averaged 60–100 ms (n = 20 trials, large blood vessels are colored gray). Red region depicts ROI used to measure center activation, same one as in A. D, Comparison of spatial profiles of the responses shown in A (cyan curve) and C (blue curve, shading marks ±1 SEM). Paths spanned the top-left corner, center, and bottom-right corner of both responses (1.2 × 13.6 mm). Curves smoothed with a Gaussian (σ = 0.17 mm) for visualization purposes. Center activation in response to the filled square is clearly present and much higher than that evoked by the square contour.