Dual oriented mechanisms in contour interaction

Foveal visual acuity is impaired by the presence of contours that surround the target (contour interaction (CI)). In order to determine the roles of various surrounding contours, visual acuities for a square C were measured in the presence of four and two flanking bars. Bars of different orientations relative to the gap of the C produced different CI patterns. The CI produced by a pair of bars was alleviated by adding another pair of bars of an orthogonal orientation. These results indicate that CI is mediated by not one, but at least two orientation selective mechanisms that actively inhibit each other.