Geometrical illusions can affect time-to-contact estimation and mimed prehension.

Results indicate that under some conditions the Sander parallelogram illusion can affect time-to-contact (TTC) estimation in a prediction-motion (PM) task and in an interceptive action (IA). The illusion also affected mimed manual prehension. The implication is that the timing of responses in the PM and IA tasks may involve an estimate of TTC that is based on the perceived dimensions of the environment. Further research is warranted in the development of models of perceived collision and of visually guided actions.

Size-arrival effects: the potential roles of conflicts between monocular and binocular time-to-contact information, and of computer aliasing.

With computer simulations of self-motion, participants approached a floating object and tried to "jump" over it without collision. Participants "jumped" significantly later over small objects than they did over larger objects. This occurred when the displays were viewed monocularly or binocularly, a finding that suggests that such size-arrival effects (DeLucia & Warren, 1994) were not due to a conflict between monocular and binocular cues to time-to-contact (TTC) information (Tresilian, 1994, 1995). Moreover, the results further suggest that size-arrival effects are not due to irregularities in TTC information that can occur from computer aliasing and that the latter does not always affect TTC estimation; visual information used in such judgments does not seem to be extracted on a frame-by-frame basis.

Cognitive motion extrapolation and cognitive clocking in prediction motion task.

An interruption paradigm was used to measure judgments that rely on cognitive extrapolation of approach and lateral motion. In some conditions the pattern of errors was consistent with that obtained with time-to-contact (TTC) judgments measured with a prediction motion (PM) task. Also, the slope of the relationship between estimated and actual TTC in judgments of approaching objects decreased when visual information about the environment between the observer and the display was minimized. Moreover, the accuracy of relative duration judgments of visual (but not auditory) stimuli decreased when a PM task was performed concurrently. Results are consistent with the notion that PM tasks involve cognitive motion extrapolation rather than solely a clocking process that counts down TTC.

Judgments of relative time-to-contact of more than two approaching objects: toward a method.

Observers reported which of as many as eight computer-generated approaching objects would "hit" them first. Accuracy was above chance probability except when two-object displays contained pictorial relative size information that contradicted relative time-to-contact (TTC) information. Mean d' and response time was greater, but mean efficiency (Barlow, 1978) was smaller with eight objects than with two. Performance was less effective when global expansion contradicted TTC information than when local expansion contradicted TTC. Results suggest that observers can judge relative TTC with as many as eight objects when certain sources of information are consistent with TTC and that observers rely on information other than, or in conjunction with, optical TTC. Also, the sources of visual information that affect performance may vary with set size, and identification (but not detection) judgments may be constrained by limited-capacity processing.

A pupillometric correlate of scotopic visual acuity.

While not easily fit into the classic descriptions of the pupillary light reflex, previous studies reported that changes in the spatial composition of the retinal image can evoke a pupillary response. The present study extends this observation by showing that the pupil constricts in response to scotopic as well as photopic spatial patterns. Moreover, the amplitude of the scotopic response decreases with increasing spatial frequencies suggesting a pupillary spatial acuity of about 3 c/deg. The scotopic pupil acuity is similar to the scotopic perceptual visual acuity measured in the same observers.

Pictorial and motion-based depth information during active control of self-motion: size-arrival effects on collision avoidance.

With computer simulations of self-motion, Ss controlled their altitude as they approached a floating object and, after getting as close as possible to the object, tried to "jump" over it without collision. Ss jumped significantly later for small objects, compared with larger objects that were approached from equal distances at equal speeds and were positioned at equal clearance heights. This occurred even when accretion-deletion information was present and when object width and length were varied independently. Results were consistent with studies in which Ss judged a large far approaching object to hit the viewpoint before a small near object that would have arrived sooner (P.R. DeLucia, 1991a, 1991b). Results suggest that pictorial information such as relative size contributes to active collision-avoidance tasks and must be considered in models of perceived distance and time-to-arrival.

Diamond-winged variants of the Müller-Lyer figure: a test of Virsu's (1971) centroid theory.

Diamond-winged variants of the Müller-Lyer figure were used to test predictions of Virsu's (1971) theory of the Müller-Lyer illusion based on efferent readiness for eye movements toward the figure's center of gravity. A Müller-Lyer figure with diamond-shaped wings resulted in a greater center-of-gravity distance than the corresponding, conventional Müller-Lyer figure, but fin length and the rest of the figure remained constant; in Virus's study, fin length and center-of-gravity distance covaried. Results were consistent with Virus's data when we used the stimulus conditions that he reported. Results from a wide range of stimuli challenge Virsu's theory, and thus are consistent with the conclusions of Brigell, Uhlarik, and Goldhorn (1977).

A quantitative analysis of illusion magnitude predicted by several averaging theories of the Müller-Lyer illusion.

This article describes a quantitative method to evaluate several averaging (i.e., confusion and assimilation) theories by comparing predictions of the absolute magnitude of the Müller-Lyer (ML) illusion with results of previous studies of the composite ML figure. The magnitude of illusion was best predicted by Davies and Spencer's (1977) theory and by integrative field theory (Pressey & Pressey, 1992). Furthermore, when the ML figure was at the point of subject equality, the average of shaft and intertip distances, and the configural dimensions proposed by Davies and Spencer, were most frequently closest to being equal in the apex-in ML and apex-out ML. Results indicate that a comparison of predicted and reported absolute magnitudes of the ML illusion can provide quantitative criteria to distinguish and evaluate averaging theories of the ML illusion.

Geometrical illusions in solid objects under ordinary viewing conditions.

The Müller-Lyer and Ponzo illusions were obtained under free binocular viewing of three-dimensional objects, and the function relating magnitude of illusion to fin angle, characteristic of converging-line versions of the Müller-Lyer pattern, was closely paralleled by volumetric (three-cone), line-free objects (but not with an erect, planar "walk-through" construction and moving observers). Illusions cannot be dismissed as artifacts of static, impoverished viewing, therefore, but must be explained within any general theory of perception. Perspective explanations have difficulties with such three-dimensional manifestations, and seem completely inapplicable to our further finding that approximately the same amount of illusion occurred in objects and patterns with no oblique lines or edges. Confusion or averaging theories, not themselves tested here, remain unthreatened by these data.

Pictorial and motion-based information for depth perception.

When computer-generated objects approached the viewpoint in midair, a large far object appeared to be nearer than a small near object and appeared to hit the viewpoint before the small object, which was specified by time-to-contact information to arrive sooner. These judgements were consistent with relative size and occurred even when motion-based information was potentially above threshold. The effects of relative size persisted with higher resolution animated films of approaching objects, were weakened by ground-intercept information, and were not as robust with laterally translating objects. Although it is often asserted that the kinds of information that have traditionally been called static depth cues are superseded by motion-based depth information, this article attempts to show that the reverse also can occur in distance perception, as has been shown in form perception.