A variant of the anomalous motion illusion based upon contrast and visual latency

We examined a variant of the anomalous motion illusion. In a series of experiments, we ascertained luminance contrast to be the critical factor. Low-contrast random dots showed longer latency than high-contrast ones, irrespective of whether they were dark or light (experiments 1 -3). We conjecture that this illusion may share the same mechanism with the Hess effect, which is characterised by visual delay of a low-contrast, dark stimulus in a moving situation. Since the Hess effect is known as the monocular version of the Pulfrich effect, we examined whether illusory motion in depth could be observed if a high-contrast pattern was projected to one eye and the same pattern of low-contrast was presented to the other eye, and they were binocularly fused and swayed horizontally. Observers then reported illusory motion in depth when the low-contrast pattern was dark, but they did not when it was bright (experiment 4). Possible explanations of this inconsistency are discussed.     

The aperture problems in the Pulfrich effect

The Pulfrich effect yields a perceived depth for horizontally moving objects but not for vertically moving ones. In this study the Pulfrich effect was measured by translating oblique lines seen through a circular window, which made motion direction ambiguous. Overlaying random dots that moved horizontally, vertically, or diagonally controlled the perceptual motion direction of the lines. In experiment 1, when the lines were seen to move horizontally, the effect was strongest in spite of the same physical motion of the lines. Experiment 2 was performed to test the above conditions again, excluding the Pulfrich effect of the dots on the depth of the lines. The overlaid dots were presented to one eye only. The result showed that the Pulfrich effect of the lines was persistently strong in spite of the perceptual changes in motion direction. Experiment 3 also showed that the Pulfrich depth was independent of the perceived horizontal speed in a plaid display. The Pulfrich effect was determined by measuring the horizontal disparity component, independently of the perceived motion direction. These results demonstrate that the aperture problems in motion and stereopsis in the Pulfrich effect are solved independently.     

“Parts” of perceived visual forms: New evidence

The Pulfrich effect (the illusory displacement of a moving stimulus when it is viewed with an optical filter over one eye) was measured repeatedly during a 20 min period following the initial filtering of the eye. For each of 10 filter densities, the Pulfrich effect always occurred immediately upon filtering the eye, but subsequently increased greatly during the adaptation period. The results indicate that the perceptual latency of the filtered eye is increased by two factors: the reduction in stimulus luminance due to the filter and progressive dark adaptation. Dark adaptation is apparently the more important of these two factors.     

Some consequences of stereoscopic depth constancy

Retinal disparity decreases in proportion to the square of the distance of the corresponding objective depth interval from the eyes. Up to a distance of 2 m, stereoscopic depth perception compensates well for this decrease in disparity with observation distance; for a given disparity, experienced depth increases approximately in proportion to the square of the observation distance. When disparities are artificially produced, by anaglyphs or vectograms, or by spectacles, they decrease only in proportion to the first power of the observation distance. The same is true when the Pulfrich effect gives rise to equivalents of disparity. Depth perception, however, compensates for the normal disparity loss. As a result, there should be a net gain in perceived depth approximately in proportion to the first power of observation distance. When perceived depth caused by horizontal magnification in one eye or by the Pulfrich effect was measured, it was found to increase approximately in proportion to observation distance.     

A saccadic suppression explanation of the Pulfrich phenomenon

A latency explanation of the Pulfrich phenomenon of binocular vision provides for the seen path with an oscillating pendulum to be symmetrical and at right angles to the line of sight. Since the experience of asymmetries in the seen path of a pendulum, when viewed with one eye filtered, is more than the exception, an explanation which has the potential to provide for both symmetry and asymmetry is to be preferred. A saccadic suppression explanation offers this possibility. A saccadic suppression explanation would provide that vision would be suppressed in the filtered eye first followed by suppression in the unfiltered eye. Both eyes would recover vision simultaneously. The predicted resultant disparate stimulation is consistent in direction with that necessary to the Pulfrich phenomenon. The details of the required stimulation have been checked using simple and compound episcotisters. The results with the episcotisters are consistent with the Pulfrich phenomenon.     

Two forms of retinal disparity

The retinal disparities in stereograms where the vertical alignment of pairs of homologous points in one eye differs from that in the other eye were found to be more effective than disparities that do not involve that kind of binocular difference. The presence of such “transverse disparities” was found to shorten the time elapsed until perceived depth was reported in four instances, in two simple stereogram pairs and in two different pairs of random dot pattern stereograms. In an experiment where binocular parallax was in conflict with an effect of past experience, the presence of transverse disparities caused binocular parallax to prevail. The presumption that the amount of perceived depth depends only on the amount of disparity (provided distances from the eyes are unchanged) and not on the configuration in which it manifests itself was found not to hold in stereograms containing transverse disparities.     

A generalized visual latency explanation of the Pulfrich phenomenon

The classical visual latency explanation of the Pulfrich phenomenon is generalized to allow for the possibility that the visual system uses velocity information from motion detectors to compensate for spatial lag due to time lag, in identifying the instantaneous position of a moving object. The results of the reported experiment verify one consequence of the generalized version: the elliptical apparent path is skewed away from the filtered eye. The results suggest also that the extrapolation factor increases with visual latency and that there is a different extrapolation factor for each eye when the eyes are illuminated unequally.     

A generalized visual latency explanation of the Pulfrich phenomenon.

The classical visual latency explanation of the Pulfrich phenomenon is generalized to allow for the possibility that the visual system uses velocity information from motion detectors to compensate for spatial lag due to time lag, in identifying the instantaneous position of a moving object. The results of the reported experiment verify one consequence of the generalized version: the elliptical apparent path is skewed away from the filtered eye. The results suggest also that the extrapolation factor increases with visual latency and that there is a different extrapolation factor for each eye when the eyes are illuminated unequally.     

刺激结构对视觉定位的影响

本实验采用多点定位作业,结合部分报告法,研究视觉刺激的整体结构对定位成绩的影响。所呈现的刺激为1、2、3或4个颜色点。其中2、3、4点刺激分别是直线、正三角形和正方形。结果发现,当呈现时间较长时,1点和2、3、4点的定位误差无显著差异。但是1点的定位反应时却显著少于2、3、4点。这提示出现视觉定位的结构效应。而当呈现时间较短时,却没有这种结构效应。实验还发现,这种结构效应并不因刺激结构的不同而有区别。     

A moving cast shadow diminishes the Pulfrich phenomenon

It is demonstrated that the presence of a moving cast shadow diminishes the Pulfrich phenomenon. This complements previous work by Kersten, Knill, Mamassian, and Bülthoff [1996 Nature (London) 379 31] indicating that visible cast shadows can override monocularly based cues to the perceived trajectory of a moving object. The present finding with the Pulfrich phenomenon indicates the effectiveness of shadows for overriding binocularly based cues to the perceived trajectory of a moving object.     

Figural goodness and the predictability of figural elements

All 126 patterns composed of five dots distributed over the cells of a 3 by 3 matrix were examined for the predictability of their elements. The predictability of a given dot in a given pattern was measured as the percentage of Ss who indicated that dot as one “implied or suggested” by the subpattern composed of the remaining four dots of the pattern. The dots comprising a figurally good five-dot pattern were generally more predictable, one from the others, than the dots comprising a poor pattern. This finding accords with the Gestaltist conception of a good figure as one whose elements are well organized, and it is the state of affairs required by Garner’s hypothesis that better figures are perceived to have fewer alternatives than poorer figures. A mechanism mediating the prediction of an element of a pattern from the other elements was suggested.     

An opaque surface influences the depth from the Pulfrich phenomenon

When a pendulum, swinging in a frontoparallel plane, is viewed with a neutral density filter before one eye, the pendulum bob appears to rotate in an elliptical path. This illusion, known as the Pulfrich phenomenon, was used to examine depth cue interaction in a natural environment. An opaque surface was placed behind the pendulum. This intervention put the 'Pulfrich depth' of the pendulum in conflict with the information provided by other depth cues, such as interposition, and observers reported that the 'far' part of the pendulum's Pulfrich trajectory appeared to be flat. This was not the case when the surface was transparent. If the opaque surface was placed farther behind the pendulum's physical path, the far trajectory became increasingly elliptical. These results suggest that the opaque surface influences the pendulum's depth qualitatively and quantitatively. That is, the depth order between the surface and the pendulum is determined by interposition, but the absolute depth of the pendulum's Pulfrich trajectory is constrained by the surface's depth.     

Perceiving surfaces in depth beyond the fusion limit of their elements

An isolated dot appears double outside a small disparity range called Panum's fusional area. In random-dot stereograms (RDSs), however, this doubling, or diplopia of dot elements is not evident at any disparity. Nevertheless, depth is perceived up to disparities that greatly exceed Panum's fusional limit. Either one is unaware of dot diplopia at disparities exceeding Panum's fusional limit or the fusion limit is extended. To examine these possibilities, we developed a novel RDS in which dichoptically color-coded dots have a distinctive color when fused, and return to their intrinsic colors when diplopic. We measured the fusion limit of dots in this RDS, and compared it to the patent stereopsis limit of the perceived surface in similar RDSs. We found that the fusional area of dots in the RDS was comparable to Panum's fusional area. Furthermore, there was clear dissociation between the fusion limit and the patent stereopsis limit in the RDS. We conclude that the elements composing a surface are not necessarily fused when a large disparity surface is perceived in depth.     

The Perception of Self-Motion Induced by Peripheral Visual Information in Sitting and Supine Postures

The effects of speed and direction of a moving peripheral stimulus on the perception of self-motion were studied in sitting and supine postures. Observers sat or lay on their backs with one monitor on each side of their head. On the monitor screens, random dot patterns moved vertically or horizontally under three speed conditions. The latency of the onset of the induced self-motion was shorter under the high-speed condition than under the lower speed conditions. In the sitting posture, the latency was shorter when the patterns moved vertically than when they moved horizontally. In the supine posture, the latency depended neither on the physical nor on the egocentric verticality of the pattern motion. This shows that the effect of direction of moving patterns was not the same in the sitting posture and the supine posture. The results were explained by an informational difference of visual-vestibular interaction in each of these two postures.     

Short-term retention of visually presented stimuli: some evidence of visual encoding.

Subjects saw two visual stimuli on each trial, separated by an interval ranging from 0 to 8 sec. Each stimulus was composed of a subset of the dots of a 7 × 5 dot matrix. In particular, it was a composite of two patterns, one of which formed a capital English letter and the other of which was an arrangement formed by 8 randomly selected dots. The task was to decide as quickly as possible whether the two stimuli contained the same letter. Under one condition (correlated noise) the same noise pattern was used with both letters; thus the decision as to whether the letters were the same could be based on a test of congruence of the two stimuli, noise and all. Under another condition (uncorrelated noise) the noise patterns differed, assuring that the composite patterns differed, and thus precluding congruence testing as an adequate way to determine whether the two letters were the same. Performance was better (RTs and error rates were smaller) with the correlated noise than with the uncorrelated noise. The result was taken as evidence that visual information was retained in memory, and used to advantage, when the situation clearly warranted the direct comparison of visual patterns.     

Dynamic Touch by Hand and Head During Walking: Protective Behavior for the Head?

We investigated perception of the extent of an object held in the hand or attached to the head during walking. Participants wielded an occluded, weighted rod by hand and by head and reported the perceived location of the rod end (relative to a virtual vertical line) while walking and while standing. There was an unexpected effect of body part on perceived location of the rod end. In particular, participants stood or walked so that the rod end was farther away from the vertical line when wielding with the head than wielding with the hand. It is possible that differences between the tasks and the relative costs of a collision influenced the locomotory behavior in each condition.     

两种参照作用下的视觉定位

本实验采用多点视觉定位作业,研究视觉定位中的两种参照对定位成绩的影响。实验采用一些附加线条与三个刺激形成有特征的刺激图和无特征的刺激图,并与没有附加线条的情况进行了比较。有特征的刺激图是指这些附加的线条与刺激点构成一定的特征,如一个字母或图形等;无特征的刺激图指刺激点不能与附加线条构成有规律的特征。结果发现,当有附加线条的图形呈现时,被试的定位误差大于控制组,表现出一级参照对视觉定位的干扰作用,而且在呈现时间为100ms、200ms、300ms时,都是如此。在有特征组内,不同的特征对定位精确性的影响依赖于时间因素。     

Some aspects of the Pulfrich effect

Krekling, S. Some aspects of the Pulfrich effect. Scand. J. Psychol., 1973, 14, 87–90.-The course of the Pulfrich effect was studied as a function of binocular differences of intensity. A pendulum and a pointer were matched with respect to apparent distance, with one eye progressively dimmed by neutral density filters. The stereoscopic effect increased as a function of filter density, reached a maximum value and then decreased. At the point of reversal the apparent rotatory movement of the pendulum disappeared, coinciding with an abrupt increase in the variability of the subject's responses, probably indicating the transition to a qualitatively different visual process. The binocular-rivalry explanation of the reversal was not supported by the results.     

The rod-and-frame illusion in erect and supine observers

Previous studies have found that the effect of a tilted frame on egocentric rod adjustments is greater when an overhead display in a horizontal plane is viewed from a supine body position than when a vertical display is viewed from an erect body position. The present studies were designed to see whether this phenomenon could be attributed to an intravisual orientation contrast effect or to the effects of visually induced eye torsion. No significant erect-supine differences were found on measures of either effect. Errors in the direction of frame tilt were significantly greater in the supine position when observers were asked to align a visible rod or an unseen hand-held disk with the head, but no effect of body position was found in matching the orientation of the disk with the rod. The data suggest that erect-supine differences in frame effects are not attributable simply to intravisual factors. The results are discussed in terms used by Harris (1974) to describe “straight-ahead shifts” in judging spatial directions with respect to the median plane.     

Enumeration of dots: An eye movement analysis

The present study reports the measurement of response latencies and the recording of eye movements in a task in which adults had to enumerate dots in figures that differed in number of dots (n^d = 19–23) and grouping of dots. The functional relationship between latencies per dot and mean group size was in agreement with earlier findings (van Oeffelen & Vos, 1982). Temporal information from eye movement data indicated that the relative contribution of fixation durations to overall latency was far larger than the contribution of saccades, which superseded the contribution from eyeblinks. Spatial information in the form of eye movement trajectories indicated that, in general, there occurred one or two fixations at the starting position. From this position onward, eye movements were directed toward areas of dots rather than to each dot in particular. Scanning behavior was sometimes reiterative, in the sense that groups of dots were visited more than once. The results are discussed with respect to the nature of strategies employed during a dot-enumeration task.