Perceived Diagonals in Grids and Lattices

Diagonal lines are seen running between the intersections in repetitive grid patterns, but not in lattices of dots. We show that these lines, which are of reverse contrast to the grid, cannot be explained by global Fourier components in the pattern, or by low-pass orientation-specific filtering. The lines depend upon local diagonal components at the intersection, and we suggest that the output of local detectors are combined by collector units receiving their input from different visual field positions. Relevant neurophysiological evidence is discussed, and the model is extended to subjective contours and the Fraser twisted cord.