The oblique effect depends on the perceived, rather than physical, orientation and direction

Abstract

Our discriminability of orientation and direction is significantly better near the cardinal axes than near an oblique axis. We investigated whether this well-known oblique effect is determined by the physical or the perceived orientation and direction of the stimuli. Using the simultaneous tilt illusion, we generated perceptually different orientations for the same central target grating by contrasting it with different surround orientations. Both square-wave and sinusoidal gratings were used for the target and surround. The stimulus parameters were chosen so that the same physical target orientation in the center appeared either vertically or obliquely. For each of these perceptual conditions, two slightly different target orientations were presented in a trial and a psychometric function was measured with a 2AFC procedure. Trials for the two perceptual conditions were randomly interleaved. If orientation discriminability were determined by the physical orientations of the target, the two curves would be identical as the set of target physical orientations was the same. Instead, all three subjects we tested showed a steeper curve when the target gratings were perceived near vertically than when they were perceived obliquely. We also performed a similar experiment for direction discrimination using motion repulsion to manipulate the perceived direction of the same physical direction. Overlapping random dot patterns were used to generate repulsion. Due to large variation of repulsion among subjects, the stimulus parameters were tailored for each subject via a staircase procedure before the main experiment. Here again we found that for the same set of physical directions, the psychometric curve was steeper when the perceived directions were near vertical than when they were oblique. We conclude that the oblique effect depends on the perceived, rather than physical, orientation and direction, and may thus be a higher level phenomenon than previously thought.