Abstract

When three-dot Vernier acuity is measured as a function of separation, thresholds are approximately proportional to the separation of the dots. One interpretation for this result is that the visual system responds to a fixed orientation cue. An alternative hypothesis is that Vernier thresholds depend on positional information, so that as the separation and, therefore, the eccentricity of the reference dots increase, positional uncertainty increases, resulting in thresholds being proportional to the eccentricity/separation of the reference dots. To distinguish between these two hypotheses, separation was pitted against eccentricity. Vernier thresholds were measured for stimuli presented on an isoeccentric arc, so that separation could be varied while holding the eccentricity of the test stimuli constant.¹ Measurements were made with isoeccentric arcs at eccentricities from 0.625 to 10°. At each eccentricity, there was a range of small separations (less than half of the eccentricity) over which thresholds were proportional to separation. In this range, stimulus duration had a marked effect, as may be expected if Vernier thresholds depend on orientation tuned filters. At larger separations, Vernier thresholds were independent of separation and of stimulus duration but varied systematically with the eccentricity of the isoeccentric arc. The results suggest that both position and orientation cues are important but that they operate to limit performance over different ranges of separation.

© 1988 Optical Society of America