In an attempt to uncover the properties of the psychophysical spatial mechanisms which optimally respond to the vernier offset between two abutting lines, we investigated the effects of one-dimensional band-limited spatial noise masks superimposed with the target, on vernier thresholds. Unidirectional vernier thresholds were measured in the presence of masks varying in orientation, spatial frequency content and luminance modulation. Because of the dependence of vernier thresholds on target visibility, the effects of these masks on target detection thresholds were also measured. In accordance with the results of Findlay [(1973) Nature, 241, 135-137] but contrary to an hypothesis that the direction 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 offset line targets, the differential responses of at least two filters with orientations which straddle the target lines are combined to extract relative position information. The spatial frequency tuning characteristics of the optimal mechanisms for mediating vernier information are similar to those optimal for detecting the target lines themselves, except that they are tuned to a slightly higher spatial frequency and have a slightly narrower bandwidth. The spatial mechanisms most sensitive to the vernier offset and to target detection exhibit similar responses to increases in mask modulation. This finding suggests that these tasks are limited by the same source of noise, and explains why under a variety of experimental manipulations, equally visible vernier targets result in similar vernier thresholds.