SPATIAL SCALE SHIFTS IN PERIPHERAL VERNIER ACUITY

Abutting line vernier acuity thresholds are markedly degraded in perip heral vision, while line detection thresholds are elevated to a much l esser extent. To study the spatial and orientation tuning properties o f the mechanisms underlying peripheral line vernier acuity, abutting v ernier thresholds were measured in the presence of one-dimensional ban d-limited spatial noise masks varying in orientation and spatial frequ ency. To examine the effects of these masks on target visibility, line detection thresholds were also measured. We find that in both the fov ea and the periphery, noise masking produces marked elevations of vern ier thresholds, which are tuned to both spatial frequency and orientat ion. (i) Spatial frequency tuning: in the fovea, the spatial frequency tuning is bandpass, with a bandwidth of approximate to 2.5 octaves, a nd a peak spatial frequency of about 10 c/deg. In the periphery the sp atial tuning is similar in bandwidth, however the peak shifts systemat ically to lower spatial frequencies with increasing eccentricity, impl ying that thresholds are mediated by spatial mechanisms tuned to progr essively larger spatial scales with eccentricity. (ii) Orientation tun ing: at all eccentricities there is a bimodal orientation tuning funct ion for vernier acuity, consistent with the hypothesis that the respon ses of at least two filters, whose orientations straddle the target li nes, 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 det ection and line vernier tasks, the scale of the most sensitive spatial mechanisms shifts systematically with eccentricity. The change in lin e 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 acui ty. The extra increase in peripheral vernier thresholds may be a conse quence of a high degree of positional uncertainty which adds noise at a stage following the combination of filter responses.