Perception

The minimum temporal thresholds for absolute motion detection were measured for sinusoidal grating patterns in foveal vision. Test patterns of relatively low temporal frequencies and low velocities were examined. The thresholds clearly decreased with test velocities rather than with test temporal frequencies. Modified velocity–time reciprocity was observed (ie the relationship between test velocity and temporal thresholds was described by a simple equation including two constants which indicate temporal and spatial limits). The temporal constant was about 35 ms and the spatial constant was about 1 min of arc. These constants are thought to provide the basic constraints on motion detection.

To examine the relationship between visual motion processing for perception and pursuit, we measured the pursuit eye-movement and perceptual responses to the same complex-motion stimuli. We show that humans can both perceive and pursue the motion of line-figure objects, even when partial occlusion makes the resulting image motion vastly different from the underlying object motion. Our results show that both perception and pursuit can perform largely accurate motion integration, ie the selective combination of local motion signals across the visual field to derive global object motion. Furthermore, because we manipulated perceived motion while keeping image motion identical, the observed parallel changes in perception and pursuit show that the motion signals driving steady-state pursuit and perception are linked. These findings disprove current pursuit models whose control strategy is to minimize retinal image motion, and suggest a new framework for the interplay between visual cortex and cerebellum in visuomotor control.

An object's location is best retrieved from the orientation in which it was learned. Otherwise, retrieval necessitates a mental effort to restore the original perspective. In this case there is a cost to speed and accuracy of location responses known as the alignment effect. We hypothesised that one can attenuate this alignment effect by systematically referring objects in an exocentric frame of reference during learning. Sixteen male students were asked to learn the location of five objects disposed in a totally new environment either by locating the objects in an egocentric or in an exocentric spatial frame of reference. After the learning phase, the participants were asked to imagine orienting themselves to an object in the scene and to point to another object. The analysis of pointing accuracy, orientation, and pointing times showed that the performances of participants engaged in the exocentric condition remained insensitive to the augmentation of the angle between their actual position on the path and the imagined orientation. On the other hand, the participants engaged in egocentric learning were disoriented when the difference between their actual orientation and the imagined orientation was great. We conclude that when an object's location is intentionally referred to in an exocentric reference frame, alignment effect can be significantly reduced.

Self-movement through an environment generates optic flow, a potential source of heading information. But it is not certain that optic flow is sufficient to support navigation, particularly navigation along complex, multi-legged paths. To address this question, we studied human participants who navigated synthetic environments with and without salient optic flow. Participants used a keyboard to control realistic simulation of self-movement through computer-rendered, synthetic environments. Because these environments comprised series of identically textured virtual corridors and intersections, participants had to build up some mental representation of the environment in order to perform. The impact of optic flow on learning was examined in two experiments. In experiment 1, participants learned to navigate multiple T-junction mazes with and without accompanying optic flow. Optic flow promoted faster learning, mainly by preventing disorientation and backtracking in the maze. In experiment 2, participants found their way around a virtual city-block environment, experiencing two different kinds of optic flow as they went. By varying the rate at which the display was updated, we created optic flow that was either fluid or choppy. Here, fluid optic flow (as compared with choppy optic flow) enabled participants to locate a remembered target position more accurately. When other cues are unavailable, optic flow can be a significant aid in wayfinding. Among other things, optic flow can facilitate path integration, which involves updating a mental representation of place by combining the trajectories of previously travelled paths.

The duration of a short empty time interval (typically shorter than 300 ms) is often underestimated when it is immediately preceded by a shorter time interval. This illusory underestimation—time-shrinking—had been studied only with auditory temporal patterns. In the present study, we examined whether similar underestimation would take place with visual temporal patterns. It turned out that underestimation of the same kind takes place also in the visual modality. However, a considerable difference between the auditory and the visual modalities appeared. In the auditory modality, it had been shown that the amount of underestimation decreased for preceding time intervals longer than 200 ms. In the present study, the underestimation increased when the preceding time interval varied from 160 to 400 ms. Furthermore, the differences between the two neighbouring intervals which could cause this underestimation had always been in a fixed range in the auditory modality. In the visual modality, the range was broader when the intervals were longer. These results were interpreted in terms of an assimilation process in light of the processing-time hypothesis proposed by Nakajima (1987 Perception 16 485–520) in order to explain an aspect of empty-duration perception.

Orthogonally oriented sinusoidal luminance gratings were dichoptically presented to the observers' left and right eyes. During the subsequent binocular rivalry, a small target was briefly presented (4AFC) to probe the strength of interocular suppression at various temporal latencies. Both stationary and moving rivalrous patterns were investigated. The purpose of experiment 1 was to compare the temporal characteristics of stationary and motion rivalry (0 and 1.2 deg s⁻¹), while that of experiment 2 was to examine rivalry suppression for higher speeds (2 and 4 deg s⁻¹). In all cases, it was found that the strength of suppression remained essentially constant throughout a single phase of binocular rivalry. The results of the investigation also revealed that moving rivalrous patterns lead to greater magnitudes of interocular suppression than static patterns. Despite these differences in the strength of suppression, the results of both experiments show that the temporal characteristics of motion and static rivalry are essentially identical.

In exploring stereokinesis, we devised flat cycloidal display figures which, when rotated on a disc in the frontal plane, are perceived as illusory three-dimensional forms with movement in depth; the dominant percepts were of twisted loops with an internal writhing motion. These dominant forms could be convincingly represented by stereo pairs derived from the flat display; related forms, not seen in the illusion, could also be constructed, seeming to show a selectivity for preferred stereokinetic forms by the perceptual system. Models were made of the stereo forms; when rotated, they showed similar illusions and selectivity. We suggest that the illusions arise because some components of the real motion do not appear in the sensory field. The perceptual system accommodates for this by constructing percepts which are not necessarily veridical but do reconcile form and motion into a coherent unity. The results are discussed in relation to concepts of invariance and rigidity, and with regard to the creative response to sensory data by the perceptual system.

Two overlapping transparent surfaces forming a two-dimensional pattern stand out in front of each other alternately. Let y denote the luminance of the region where these surfaces overlap and b the luminance of the background. In achromatic patterns, the probability that the lighter transparent surface appears to be in front of the other surface is known to increase with y and with b. The present results show that grouping by achromatic colour similarity cannot explain the effect of b. An alternative conjecture is that the luminance factors that control perceived surface segregation can explain the effects of y and b. Such an explanation predicts a new effect: the probability that one transparent surface appears to be in front increases with the absolute difference in luminance between the surface and the background. The present results confirm this prediction.

Cleomedes (Kleomedes) is a little-known Greek author (c. 1st century AD) who produced what is probably the earliest extant statement of size – distance invariance. He supported the Stoic philosophy and was concerned to discredit the Epicurean position that we perceive objects as having their true size. He explained the celestial illusion (the apparent enlargement of the sun near the horizon) in two ways: partly as a refractive effect of the atmosphere similar to angular enlargement when looking into water; and partly as a linear enlargement due to increased apparent distance in a misty atmosphere. He is the earliest extant author to offer apparent distance as a clear explanation of the celestial illusion. He attributed these views to Posidonius (c. 135–50 BC). His explanations remained at the geometrical level, and he did not speculate on sensory mechanisms.