Proximal Stimulus for Brewster’s Phenomenon is Equivalent to That of Wheatstone’s Stereogram

We examined Brewster’s perceptual phenomenon that is produced by the same binocularly equivalent proximal stimulus as Wheatstone’s stereogram. We then constructed and observed the perceptual effect created by another distal stimulus that also created an equivalent binocular proximal stimulus. All three stimulus situations produced the appearance of a tilted line in the median plane which is consistent with Hering’s rule that the average of the two local signs and binocular eye position determine visual direction. The characteristics of the proximal stimulus are also relevant to the question raised by Helmholtz regarding the empirical vertical horopter.

Temperature Created by a Tilted Moving Heat Source: Heating Line and Cylinder

Temperatures created by a moving tilted line and a moving tilted cylinder are considered. Analytical expressions for low Peclet (Pe⪡1) and high Peclet (Pe⪢1) numbers are obtained for the whole range of possible tilt angles. These expressions almost overlap: It is shown that these analytical expressions describe very well the results of numerical calculations at any Peclet numbers except for a very narrow range of Pe close to unity. A method of calculation of the cut shape (variation of the tilt angle inside the cut) is discussed.

A Computational Model of the Perceived Velocity of Moving Plaids

Local motion detection mechanisms generally lead to one component of the optic flow becoming indeterminate. One way to solve this ‘aperture problem’ is to compute the optic flow which minimises some smoothing constraint. With iterative schemes the computed velocity array is suboptimal relative to the constraint until the process has converged. Under the original assumption that the iteration rate is sufficiently low to allow the perception of suboptimal flows at short stimulus durations, iterative gradient models give an accurate description of biases in the perception of tilted line velocity. We examine whether this approach can be applied to moving sinusoidal plaids. Our simulations are in agreement with a number of psychophysical results on both speed and direction perception. In particular we show that the effect of stimulus duration on the perceived direction of type II plaids [Yo and Wilson, 1992 Vision Research32(1)] can be accounted for without recourse to second-order mechanisms. The effects of contrast and component directions on the evolution rate of this bias are well reproduced. The model also successfully describes the effect of spatial frequency, and data obtained with gratings. These results suggest that iterative gradient schemes can model the dynamics of interactions between local velocity detectors, as revealed by psychophysical experiments with lines and plaids.

Terminator Position Affects the Perceived Speed of Line Segments Tilted with Respect to Direction of Motion

We investigated factors producing bias in the perceived speed of tilted lines in horizontal translation. The effects of grouping, collinearity, eccentricity, terminator proximity, and stimulus uncertainty on perceived speed were studied. The matched speed of vertical line compared to an inclined line was estimated with the use of a double random interleaved staircase for speed discrimination with a two-alternative forced choice. The speed of the tilted stimulus was held constant while the speed of the vertical stimulus was modified by the subject's response. Stimuli were single lines and groups of lines. The groups of short lines were arranged either in a collinear or in a randomly scattered fashion. The length of the line stimuli ranged from 0.33 deg to 7.0 deg of visual angle. Speed estimates were obtained for angles of tilt ranging from 0° to 90°. For line segments, collinearity was found to be the critical factor in determining perceived speed. Collinear segments showed a similar bias in perceived speed to single lines of the same overall length. However, randomly scattered segments were not subject to a bias in perceived speed. Random perturbation of the length or vertical position of a single line abolished the bias in perceived speed of a tilted line compared to a vertical line. Current models of the integration of motion measurements should be changed to account for the effects of topological arrangement and terminator position on the perceived speed of inclined lines.