ORIENTATION, MASKING, AND VERNIER ACUITY FOR LINE TARGETS

In an attempt to uncover the properties of the psychophysical spatial mechanisms which optimally respond to the vernier offset between two a butting lines, we investigated the effects of one-dimensional band-lim ited spatial noise masks superimposed with the target, on vernier thre sholds. Unidirectional vernier thresholds were measured in the presenc e of masks varying in orientation, spatial frequency content and lumin ance modulation. Because of the dependence of vernier thresholds on ta rget visibility, the effects of these masks on target detection thresh olds were also measured. In accordance with the results of Findlay [(1 973) Nature, 241, 135-137] but contrary to an hypothesis that the dire ction of the vernier offset is mediated by the differential output of spatial filters of a single orientation, our results reveal a bimodal orientation tuning function for vernier acuity. We propose that, for o ffset line targets, the differential responses of at least two filters with orientations which straddle the target lines are combined to ext ract relative position information. The spatial frequency tuning chara cteristics of the optimal mechanisms for mediating vernier information are similar to those optimal for detecting the target lines themselve s, except that they are tuned to a slightly higher spatial frequency a nd have a slightly narrower bandwidth. The spatial mechanisms most sen sitive to the vernier offset and to target detection exhibit similar r esponses to increases in mask modulation. This finding suggests that t hese tasks are limited by the same source of noise, and explains why u nder a variety of experimental manipulations, equally visible vernier targets result in similar vernier thresholds.