Fig. 4.

Summation for other types of contrast patterns. A trivial explanation for subadditivity is that the partial apertures contain luminance edges at the boundary between the contrast pattern and the gray background, whereas the full aperture does not. To test this explanation, we measured summation using the original pattern (Noise), a band-pass pattern that lacks the previously described luminance edges (Band-Pass Noise), patterns for which the previously described luminance edges are present in both the partial and full apertures (Checkerboard, Bars), and patterns that include a spatial gap such that the same luminance edges are present in the partial and full apertures (Checkerboard with Spatial Gap, Bars with Spatial Gap). The median summation ratios in V1, V2, and V3 are shown [error bars indicate standard error across general linear model (GLM) bootstraps]. In all cases, subadditive summation occurs, arguing against the edge explanation. There appears to be some variation in summation ratio across stimulus types; accounting for these dependencies on stimulus type is a direction for future research. (For additional experiments ruling out the edge explanation, see Supporting Fig. G.)