Noise location and the slope of the psychometric function for simple motion stimuli.

We measured subject performance as a function of luminance for both detection and discrimination of increment stimuli; some were static, and some were arranged to give two-step apparent motion. Our aim was to examine a prediction for the shape of the psychometric function for motion: an accelerating function due to the presence of a multiplicative nonlinearity contained in many low-level motion models. For the tasks with static stimuli we found psychometric function slopes (of log d' versus log luminance plots) between 1.9 and 2.4 in two subjects, as previously reported. For the tasks with apparent motion stimuli in the same range of detectability, however, the slopes are between 1.2 and 1.7. The lower slopes indicate that many low-level motion models are either incorrect or incomplete as currently specified, and changes in nonlinearities and noise placement are discussed.

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.

Relative efficiency for the detection of apparent motion.

We measured the relative efficiency for motion and position discriminations of brief, localized spot stimuli with a technique that makes no assumptions about sites of noise or information loss in the visual system. In one task, the observer had to discriminate whether an increment was located at one (left) or another (right) closely spaced spots. In the other task, the observer had to discriminate two successive brief increments of the left spot from a left spot increment followed by a right spot increment. Ideal observer theory predicts identical performance on the two tasks. Observers' thresholds, however, were significantly lower in the motion task at all intervals between flashes (ISIs) less than 60 msec in one observer and all ISIs less than 150 msec in two other observers (P < 0.01, t-test). We conclude that this apparent motion stimulus is seen more efficiently than a non-moving stimulus, and that the higher efficiency may be due to use of a motion sensitive channel in addition to independent position sensitive channels.