Effect of spatial frequency on collinear facilitation.

Contrast sensitivity for a Gabor target can be increased by a factor of two when identical patches are separated by about three wavelengths (lambda) and positioned collinearly (Polat and Sagi, 1993, 1994a, 1994b). The facilitation effect was found for a wide range of spatial frequencies but was tested with well-experienced observers. Since practice modifies the range of lateral interactions, in this study naive observers were tested in order to document the initial stage of collinear facilitation. Surprisingly, we found that facilitation is maximal for the high spatial frequencies and minimal for the low spatial frequencies. We also found that when experienced observers were tested, facilitation at the low spatial frequencies was evident, suggesting that the initially reduced facilitation was due to inefficient lateral interactions. We suggest that the absence of facilitation for low spatial frequencies is due to the slow propagation velocity of the remote input, resulting in a mismatch between the flanker's input and the target's integration time.