Abstract

Under a range of stimulus conditions, the threshold for discriminating spatial offset in a Vernier target is finer than the retinal photoreceptor mosaic, the defining characteristic of a hyperacuity (Westheimer 1975, 1979). Over the past several years, evidence has accumulated that the visual system achieves fine Vernier thresholds using different mechanisms, depending upon the characteristics of the stimulus. The most acute Vernier thresholds, obtained for highly visible, abutting line targets in the fovea, are thought to be mediated by the contrast responses of oriented cortical filters that straddle the Vernier offset (e.g., Findlay 1973; Wilson 1986; Waugh et al. 1993). Thresholds are higher, but can remain within the hyperacuity range, when the Vernier targets are separated by more than a few min of arc (e.g., Sullivan et al. 1972; Klein & Levi 1987; Levi & Klein 1990; Waugh & Levi 1993) or have opposite contrast polarities (O'Shea & Mitchell 1990; Levi & Waugh 1996). The thresholds obtained under these stimulus conditions have been attributed to a local-sign mechanism that separately computes the locations of the individual Vernier elements and then compares these position signals (Klein & Levi 1987).

© 1997 Optical Society of America