The visibility of gratings improves with increasing stimulus area. This effect is usually interpreted as being due to probability summation between the outputs of linear, independent spatial filters, although non-linear spatial summation can have similar effects. In order to distinguish between probabilistic and physiological summation models, we measured contrast thresholds using the Visual Evoked Potential (VEP). Our previous work suggests that spatial summation in the VEP is nonlinear and that it occurs preferentially for collinear configurations. Traditional probability summation models predict that areal summation will improve threshold independent of stimulus configuration. Contrast thresholds were derived from VEP contrast response functions for either circular or elongated Gabor patches with aspect ratios up to 6:1. The carrier orientation was either the same as the patch envelope orientation (collinear) or orthogonal to it. Response amplitudes were larger and contrast sensitivity was higher for collinear configurations. The results are consistent with nonlinear, configuration dependent summation that is more extensive along the axis of orientation.
Polat, U., & Norcia, A. M. (1998). Elongated physiological summation pools in the human visual cortex. Vision Research, 38(23), 3735–3741. https://doi.org/10.1016/S0042-6989(97)00426-4