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 horizont al and oblique lines are equally detectable or discriminable. Since th is oblique effect cannot be explained by the lower visibility of obliq ue vernier lines or of the ''dipole'' cue, it is unlikely to be a cons equence of lower neuronal sensitivity, In Experiment 2 we measured the orientation and spatial frequency tuning characteristics of vernier a cuity for horizontal and oblique (45 deg) stimuli using a simultaneous masking paradigm, Our results showed no significant differences betwe en either the orientation or spatial frequency tuning for horizontal a nd oblique stimuli; thus the oblique effect is unlikely to result from differences in the tuning of neurons sensitive to the oblique meridia ns, Finally, in Experiment 3, we tested the notion that the oblique ef fect for vernier judgments might reflect limitations imposed beyond th e initial filtering operation by measuring vernier acuity for horizont al and oblique lines with either the observer or the frame tilted at a n angle of 45 deg. The oblique effect for vernier followed retinal (ra ther than gravitational) coordinates, and was unaffected by the orient ation of the frame, suggesting a relatively low level cause, By exclus ion, we suggest that the dependence of vernier acuity on orientation m ay result from increased positional uncertainty at oblique orientation s, perhaps as a consequence of lower cortical neuronal density and/or increased topographic noise.