Discrimination of coherent motion when local motion varies in speed and direction.

Random-dot cinematograms (RDCs) consist of multiple local motion signals that can vary in direction and speed. These local motion signals can result in coherent motion: the percept of an overall direction and speed of motion in an RDC. Thresholds were obtained for discriminating differences in the strength of coherent motion. Observers were found to easily discriminate the strength of coherent motion on the basis of the elements' direction or speed under optimal conditions. However, a nonreciprocal relation was evident when this discrimination was performed under nonoptimal conditions. Discrimination of coherent motion that was based on the elements' direction was unaffected, but discrimination that was based on speed was impaired. Results indicate that humans are sensitive to small differences in coherent motion strength and suggest that the visual system processes direction and speed information nonreciprocally.

Memory for visual motion.

Observers briefly viewed random dots moving in a given direction and subsequently recalled that direction. When required to remember a single direction, observers performed accurately for memory intervals of up to 8 s; this high-fidelity memory for motion was maintained when observers executed a vigilance task during the memory interval. When observers tried to remember multiple directions of motion, performance deteriorated with increasing number of directions. Still, memory for multiple directions was unchanged over delays of up to 30 s. In a forced-choice experiment, observers viewed 2 successive animation sequences separated by a memory interval; for both sequences, dots moved in any direction within a limited bandwidth. Observers accurately judged which animation sequence was more coherent, even with memory intervals of 30 s. The findings are considered within the context of cognitive bias and memory for other aspects of perception.

Another means for measuring the motion aftereffect.

A new procedure for measuring the motion aftereffect (MAE) is described. The procedure involves adaptation to an animation sequence depicting dots moving in a given direction followed by presentation of a test sequence depicting dots moving in all possible directions. Under adaptation, the test sequence appears to have a directional bias opposite the direction experienced during adaptation. This MAE can be nullified by viewing an animation sequence in which a percentage of dots is constrained to move in a direction opposite the aftereffect. Using a method of constant stimuli, this percentage can be varied to find the value yielding incoherent motion. This dynamic MAE exhibits the same characteristics as the conventional MAE.

Misdirected visual motion in the peripheral visual field.

An object moving against a textured background is accurately perceived when viewed foveally, but when viewed peripherally the object's perceived direction of motion may deviate from veridical by as much as 90 deg. The illusory direction is oblique to the orientation of the background contours, which may themselves be moving or stationary. In several experiments, we examined the boundary conditions for occurrence of the illusion and tested hypotheses concerning its basis. This illusion of perceived direction dramatizes differences in motion processing between the fovea and the periphery.

Direction repulsion in motion transparency.

A series of experiments investigated perceived direction of motion and depth segregation in motion transparency displays consisting of two planes of dots moving in different directions. Direction and depth judgments were obtained from human observers viewing these "bi-directional" animation sequences with and without explicit stereoscopic depth information. We found that (1) misperception of motion direction ("direction repulsion") occurs when two spatially intermingled directions of motion are within 60 deg of each other; (2) direction repulsion is minimal at cardinal directions; (3) perception of two directions of motion always results in separate motion planes segregated in depth; and (4) stereoscopic depth information has no effect on the magnitude of direction repulsion, but it does disambiguate the depth relations between motion directions. These results are developed within the context of a two-stage model of motion transparency. On this model, motion directions are registered within units subject to inhibitory interactions that cause direction repulsion, with the outputs of these units pooled within units selective for direction and disparity.

Another perspective on the visual motion aftereffect.

Prolonged adaptation to motion in a given direction produces distinctly different visual motion aftereffects (MAEs) when viewing static vs. dynamic test displays. The dynamic MAE can be exactly simulated by real motion, whereas the static MAE cannot. In addition, the magnitude of the dynamic MAE depends on the bandwidth of motion directions experienced during adaptation, whereas the static MAE does not. Evidently a stationary pattern does not directly activate the neural mechanisms affected during motion adaptation, whereas a dynamic visual display does. These results imply that the traditional explanation of the MAE needs modification.

Anisotropies in visual motion perception: a fresh look.

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.