Anisotropies in visual motion perception: a fresh look

Abstract

We measured motion-detection and motion-discrimination performance for different directions of motion, using stochastic motion sequences. Random-dot cinematograms containing 200 dots in a circular aperture were used as stimuli in a two-interval forced-choice procedure. In the motion-detection experiment, observers judged which of two intervals contained weak coherent motion, the other internal containing random motion only. In the direction-discrimination experiment, observers viewed a standard direction of motion followed by comparison motion in a slightly different direction. Observers indicated whether the comparison was clockwise or counterclockwise, relative to the standard. Twelve directions of motion were tested in the detection task and five standard directions (three cardinal directions and two oblique directions) in the discrimination task. Detection thresholds were invariant with direction of motion, but direction-discrimination thresholds were significantly higher for motion in oblique directions, even at low-coherence levels. Results from control conditions ruled out monitor artifacts and indicate that the oblique effect is relative to retinal coordinates. These results have broad implications for computational and physiological models of motion perception.

Unequal representation of cardinal vs. oblique orientations in the middle temporal visual area

“…Additionally, the demonstration of a prominent oblique effect in visual motion perception suggests that higher visual areas, such as area MT, contribute to this perceptual anisotropy (7). Our present finding of a greater representation of cardinal orientations in MT supports previous proposals (7,8) that the oblique effect may originate from neural machinery located central to V1 (also see ref. 13 for related data in cats), even though V1 does show a greater representation of cardinal over oblique orientations in some species (5,30,31).…”

“…Although some have proposed that this enhanced representation in V1 is responsible for the ''oblique effect'' (the observation in human psychophysical studies that detection of oriented stimuli is more sensitive for cardinal than oblique orientations), data from other studies showing prominent oblique effects with pattern components that are Ͼ10°apart suggest that the small receptive fields of V1 neurons are not responsible, and that some oblique effects originate in ''higher'' visual areas (8). Additionally, the demonstration of a prominent oblique effect in visual motion perception suggests that higher visual areas, such as area MT, contribute to this perceptual anisotropy (7). Our present finding of a greater representation of cardinal orientations in MT supports previous proposals (7,8) that the oblique effect may originate from neural machinery located central to V1 (also see ref.…”

“…Neurons of similar orientation or direction-of-motion selectivity are clustered into functional columns in MT (15-18). In addition, an oblique effect has been observed in humans for the perception of objects moving in cardinal vs. oblique directions (7,(19)(20)(21)(22). Because of a preponderance of direction-selective neurons, MT is thought to be important to the processing of visual motion (14).…”

“…One of the unexpected observations is that more cortical machinery is devoted to representing vertical and horizontal (cardinal) than oblique orientations, because more neurons are selective for cardinal orientations, and cardinal orientations produce a greater neuronal response (1)(2)(3)(4)(5). Although the functional consequences of greater representation of cardinal orientations are uncertain, a popular proposal is that such an anisotropy in human visual cortex underlies the ability of humans to better discriminate gratings and other visual stimuli with horizontal and vertical rather than oblique orientations (6)(7)(8). This psychophysical observation is known as the ''oblique effect,'' and the effect can be eccentricity-dependent (9,10).…”

Do perceptual biases emerge early or late in visual processing? Decision-biases in motion perception

“…Several studies have compared psychophysical and oculomotor performance in direction discrimination [17,18], but the results are equivocal. In particular, although effects such as reference repulsion and the oblique effect are consistently observed across different psychophysical experiments (see [19] as an example), studies investigating these effects with eye movements disagree in their conclusions [20,21].…”