What pattern the eye sees best.

The visibility of gratings improves with increasing stimulus area. This effect is usually interpreted as being due to physiological summation within the extent of the largest spatial filter and due to probability summation between the outputs of linear, independent filters beyond that range. It is generally assumed that this improvement is isotropic to the patch configuration. In contrast, the existence of long-range facilitation that is configuration-specific suggests that the visibility of a local contrast is dependent on the spatial configuration of the stimuli. We measured contrast thresholds for circular and elongated Gabor patches with a static carrier. The patch envelope orientation was either the same as the bar orientation (collinear) or orthogonal to it. Contrast sensitivity was highest for elongated configurations that were collinear with the grating bars, and reached maximal efficiency at a length of about four grating cycles (eight bar widths), but a width of only one cycle.