Spatial scale shifts in peripheral vernier acuity

Abutting line vernier acuity thresholds are markedly degraded in peripheral vision, while line detection thresholds are elevated to a much lesser extent. To study the spatial and orientation tuning properties of the mechanisms underlying peripheral line vernier acuity, abutting vernier thresholds were measured in the presence of one-dimensional band-limited spatial noise masks varying in orientation and spatial frequency. To examine the effects of these masks on target visibility, line detection thresholds were also measured. We find that in both the fovea and the periphery, noise masking produces marked elevations of vernier thresholds, which are tuned to both spatial frequency and orientation. (i) Spatial frequency tuning: in the fovea, the spatial frequency tuning is bandpass, with a bandwidth of approximately 2.5 octaves, and a peak spatial frequency of about 10 c/deg. In the periphery the spatial tuning is similar in bandwidth, however the peak shifts systematically to lower spatial frequencies with increasing eccentricity, implying that thresholds are mediated by spatial mechanisms tuned to progressively larger spatial scales with eccentricity. (ii) Orientation tuning: at all eccentricities there is a bimodal orientation tuning function for vernier acuity, consistent with the hypothesis that the responses of at least two filters, whose orientations straddle the target lines, are combined to extract vernier offset information. In contrast, at all eccentricities, line detection is most strongly masked when the mask and line target have the same orientation. For both the line detection and line vernier tasks, the scale of the most sensitive spatial mechanisms shifts systematically with eccentricity. The change in line detection threshold with eccentricity is approximately proportional to the variation in spatial scale; however this shift in spatial scale is not sufficient to account for the degraded peripheral vernier acuity. The extra increase in peripheral vernier thresholds may be a consequence of a high degree of positional uncertainty which adds noise at a stage following the combination of filter responses.