Orientation anisotropy in vernier acuity

An oblique effect is evident in vernier acuity for abutting lines. In Experiment 1 we show that the oblique effect in vernier acuity exists over a range of contrast levels, and is evident even when the horizontal and oblique lines are equally detectable or discriminable. Since this oblique effect cannot be explained by the lower visibility of oblique vernier lines or of the "dipole" cue, it is unlikely to be a consequence of lower neuronal sensitivity. In Experiment 2 we measured the orientation and spatial frequency tuning characteristics of vernier acuity for horizontal and oblique (45 deg) stimuli using a simultaneous masking paradigm. Our results showed no significant differences between either the orientation or spatial frequency tuning for horizontal and oblique stimuli; thus the oblique effect is unlikely to result from differences in the tuning of neurons sensitive to the oblique meridians. Finally, in Experiment 3, we tested the notion that the oblique effect for vernier judgments might reflect limitations imposed beyond the initial filtering operation by measuring vernier acuity for horizontal and oblique lines with either the observer of the frame tilted at an angle of 45 deg. The oblique effect for vernier followed retinal (rather than gravitational) coordinates, and was unaffected by the orientation of the frame, suggesting a relatively low level cause. By exclusion, we suggest that the dependence of vernier acuity on orientation may result from increased positional uncertainty at oblique orientations, perhaps as a consequence of lower cortical neuronal density and/or increased topographic noise.