The oblique effect depends on perceived, rather
than physical, orientation and direction
Xin Meng and Ning Qian, Vision Research,
2005, 45:3402-3413.
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Abstract
Observers can better discriminate orientation or direction 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 axis of the stimuli. Using the simultaneous tilt illusion,
we generated perceptually different orientations for the same inner
(target) grating by contrasting it with differently oriented outer
gratings. Subjects compared the target orientation with a set of
reference orientations. If orientation discrimina- bility was
determined by the physical orientations, the psychometric curves for
the same target grating would be identical. Instead, all subjects
produced steeper curves when perceiving target gratings near
vertically as opposed to more obliquely. This result of orientation
discrimination was confirmed by using adaptation-generated tilt
aftereffect to manipulate the perceived ori- entation of a given
physical orientation. Moreover, we obtained the same result in
direction discrimination by using motion repul- sion to alter the
perceived direction of a given physical direction. We conclude that
when the perceived orientation or direction differs from the physical
orientation or direction, the oblique effect depends on perceived,
rather than physical, orientation or direction. Finally, as a
by-product of the study, we found that, around the vertical direction,
motion repulsion is much stronger when the inducing direction is more
clockwise to the test direction than when it is more counterclockwise.
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