Temporal selectivity of changing-size channels

Computation of surface slant from optic flow: orthogonal components of speed gradient can be combined

In previous work [Meese et al. (1995). Vision Research, 35, 2879-2888)] we showed that one-dimensional (1D) speed gradients are sufficient to produce a compelling impression of surface slant. Summing a 1D vertical shearing gradient or, less intuitively, a 1D horizontal shearing gradient with a random field of horizontally translating dots produces perceived slant about a horizontal axis. Similarly, a 1D vertical or horizontal compression gradient produces perceived slant about a vertical axis. Appropriately combining orthogonal 1D shears or compressions produces a purely deforming flow pattern. Here we asked whether both the vertical and horizontal components in such a stimulus contribute to perceived slant. Using a matching technique we found that for surfaces inclined about a vertical axis, this was indeed the case and that horizontal and vertical compression gradients contributed roughly equally to perceived slant. Similarly, for surfaces inclined about a horizontal axis, both vertical and horizontal shearing gradients contributed to the perceived slant, though here the horizontal gradient was given less weight than the vertical. We conclude that under appropriate conditions, the human visual system combines orthogonal speed gradients prior to the computation of slant from retinal flow.

Discrimination of motion in depth trajectory following acute alcohol ingestion

Evoked potentials were recorded at four scalp sites O1, O2, T5, T6; before and after the consumption of one of two alcohol dose levels, to the occurrence of disparate stimuli in dynamic random dot stereograms. Twenty young adult subjects, all of whom had vision which was normal or corrected to normal, were randomly assigned to the two dosage groups. Subjects viewed stimuli which embodied crossed disparity and followed one of three trajectories in depth. The task was to distinguish between and identify, by specific button presses, these trajectories. One half of the stimuli were presented with monocular cues to motion in depth (dark condition); in the remainder only stereoscopic depth information was available (uniform condition). Responses to both conditions showed alcohol-related reduction of evoked responses across all sites. It was concluded that the visual systems associated with the processing of motion in depth were susceptible to alcohol effects, as revealed by reductions in the amplitude of evoked responses.

Apparent motion in depth resulting from changing size and changing vergence

Two experiments are reported in which the effect of combining stimuli of changing size and changing vergence on the perception of motion in depth was examined. Changing size and changing vergence corresponded to in-phase or anti-phase sinusoidal motions of an outline circle, with different amplitudes. In-phase stimuli had approximately additive effects on the estimated peak-to-peak amplitude of apparent motion in depth. Anti-phase stimuli did not cancel each other; apparent motion was in-phase with one or the other stimulus. When apparent motion was in-phase with one stimulus, there was only a limited influence of the other stimulus. The results are discussed with regard to a model proposed by Regan and Beverley for the combination of changing size and changing disparity.

Adaptation to apparent motion

A spot alternating between two positions can produce apparent motion (AM). Following prolonged inspection, the AM degenerates into flicker. This adaptation effect was found to depend on spacing and timing; the probability of seeing motion during a 30-sec inspection period declined linearly with log spatial separation (over a range from 0.1 to 1 deg), and with log alternation rate (over a range from 2 to 4.5 Hz). Cross-adaptation, in which subjects were adapted to one alternation rate and tested at another, showed that low alternation rates gave stronger motion signals than high rates did. Adaptation to real motion (RM) strongly suppressed AM, which suggests that AM must be stimulating the same neural pathways as RM. Flickering spots (i.e. in-phase flicker) produced less adaptation than did a spot alternating between two positions (i.e. counterphase flicker), so the adapting mechanism must be responding to relative temporal phase. Embedding the adapting spots in configurations of other spots, which altered the pattern of perceived adapting motion without altering the local retinal stimulation, minimized the adaption, so the adapting mechanism must be responding to the path of seen motion. Adaptation can be used to measure the strength of AM and shows that AM is strongest for small separations, low alternation rates and high luminance contrast.