Perception

A bistable pattern is shown with white and black bars with horizontal and vertical orientations which produce an impression of thin slabs stacked up in depth either toward or away from the observer. It is postulated that the ambiguity is induced by the observer's assumption of the direction of the light source.

The wallpaper illusion, first described over a century ago, can occur when a person with normal binocular vision views a pattern that is periodic in the horizontal meridian of the visual field. Escalator treads present such a pattern. Evidence is presented favoring the view that disorientation experienced by escalator riders is caused by this illusion. Possibly some of the estimated 60 000 escalator falls occurring in the United States each year are linked to it.

Conditions under which binocular unmasking (BU), as an analogue of binaural unmasking, occurs have been explored. Observers were to detect through a stereoscope a Gabor signal in patches of two-dimensional broadband gaussian noise surrounded by a frame of uniform noise. The right-eye gaussian field was displaced relative to the left eye so that it appeared either in front of or behind the frame. Performance when signal disparity was equal to that of the noise—a condition functionally equivalent to monocular processing—was compared to that obtained when signal disparity was zero—a case in which BU should occur. Enhanced signal detectability of up to 12 dB and of nearly constant magnitude was observed in the latter condition when uncrossed disparities of up to 67.60 min visual angle and display durations of 1 s were employed. Signal detectability declined appreciably with increasing disparity (both crossed and uncrossed) when display duration was reduced to 90 ms, thus preventing the occurrence of compensatory vergence eye movements. It is suggested that BU effects may result from a process of linear summation of monocular inputs.

Models of motion perception usually assume that the visual system references spatial displacements to retinal coordinates, and not to three-dimensional coordinates recovered by a parallel process. The present studies investigated whether moving elements viewed in the context of a static random-dot stereogram could lead to the appearance of motion in depth. Observers judged the velocity of a monocular element translating horizontally in the stereo context as ‘same as’ or ‘different to’ that of a standard. Based on velocity constancy, if there was apparent motion in depth, the relative velocity judgments would yield a predictable pattern of errors.

The first experiment compared two stereo contexts: a sloped surface versus a fronto-parallel plane at zero disparity. The results indicated an overall increase in the perceived velocity of the element moving in the sloped surface context. A similar pattern of results was found when surfaces differing in incline were compared. Experiment 2 explored the case of fronto-parallel planes at crossed and uncrossed disparities. Here depth differences did not systematically affect observers' judgments. It was concluded that in some cases motion analysis can be affected by three-dimensional disparity information and not by angular displacement alone.

A stationary window was cut out of a stationary random-dot pattern. When a field of dots was moved continuously behind the window (a) the window appeared to move in the same direction even though it was stationary, (b) the position of the ‘kinetic edges’ defining the window was also displaced along the direction of dot motion, and (c) the edges of the window tended to fade on steady fixation even though the dots were still clearly visible. The illusory displacement was enhanced considerably if the kinetic edge was equiluminous and if the ‘window’ region was seen as ‘figure’ rather than ‘ground’. Since the extraction of kinetic edges probably involves the use of direction-selective cells, the illusion may provide insights into how the visual system uses the output of these cells to localize the kinetic edges.

The role of low-spatial-frequency information in the processing of global stimuli made up of local elements was examined. After selective removal of low spatial frequencies two major changes occurred in the pattern of results. First, response times to global stimuli were significantly slower and the usual speed advantage of global over local processing was lost. Second, when processing local features the usual decrease in response speed when the local and global letters are not the same (consistency effect) was not obtained. These effects could not be explained by changes in error rate, by contrast variation resulting from the process of filtering, or by loss of visual sensitivity due to greater eccentricity of global images.

Structural visual information processing was investigated in peripheral vision. Perceptual completion, as observed at the blind spot, occurred across a vertical blank stripe in a test pattern, presented in the peripheral visual field. That is, a shape, organized by structural processing, was perceived under critical visual conditions when it was presented in peripheral vision. The results indicate that structural processing, as indicated by perceptual completion, occurs in the peripheral visual information handling process. The results suggest that structural processing of shape information is built into the peripheral visual system and reduces the information load on the higher visual processes under conditions in which visual information is not adequate.

Four experiments test the assumption that, in the visual perception of pictures, observers have reliable and direct access to the equivalence of shapes in projective geometry. The assumption is that perception of projective equivalence is the basis of shape constancy (‘the projective thesis’).

Observers matched or reproduced abstract planar shapes under conditions of rotation in the picture plane, and pictured rotation in depth. Departure from projective equivalence was assessed in each study by measuring the planar analogue of cross ratio. Projective equivalence was not found to be perceived uniformly where Euclidean equivalence was not judged uniformly, either in recognition tasks or in production tasks. When the projective thesis is put to a suitably general test, confidence in the thesis is undermined.

Two separate paradigms utilizing measurements of reaction time were employed to study facial feature saliency in schematic line drawn faces. In the first paradigm the speed of response to the omission of different facial components was measured, and in the second, the speed of response to feature substitution was measured. In both paradigms the facial features were presented in a random temporal sequence in order to minimise preferential scanning strategies. The two separate paradigms reflected the feature hierarchy most commonly found in the literature, ie the outline and eyes are more salient than the nose and mouth in terms of both speed of processing and error rate. In a third study the feature substitution paradigm was used to investigate the effects of feature saliency on the perception of emotional faces. The results suggest a change in the eyes/mouth hierarchy so that the mouth becomes the most salient feature in the surprised, happy, and sad target faces. This reverse in hierarchy, however, was not evident with the angry target face. These results are discussed in terms of changes in the focus of ‘attention’ and/or changes in ‘processing efficiency’.

Two experiments were performed to assess the accuracy and precision with which adults perceive absolute egocentric distances to visible targets and coordinate their actions with them when walking without vision. In experiment 1 subjects stood in a large open field and attempted to judge the midpoint of self-to-target distances of between 4 and 24 m. In experiment 2 both highly practiced and unpracticed subjects stood in the same open field, viewed the same targets, and attempted to walk to them without vision or other environmental feedback under three conditions designed to assess the effects on accuracy of time-based memory decay and of walking at an unusually rapid pace. In experiment 1 the visual judgments were quite accurate and showed no systematic constant error. The small variable errors were linearly related to target distance. In experiment 2 the briskly paced walks were accurate, showing no systematic constant error, and the small, variable errors were a linear function of target distance and averaged about 8% of the target distance. Unlike Thomson's (1983) findings, there was not an abrupt increase in variable error at around 9 m, and no significant time-based effects were observed. The results demonstrate the accuracy of people's visual perception of absolute egocentric distances out to 24 m under open field conditions. The accuracy of people's walking without vision to previously seen targets shows that efferent and proprioceptive information about locomotion is closely calibrated to visually perceived distance. Sensitivity to the correlation of optical flow with efferent/proprioceptive information while walking with vision may provide the basis for this calibration when walking without vision.

When elements in a pattern which would otherwise produce an illusory figure are modified by outlining or by increasing their regularity (in accordance with a proposed measure of the latter quality), the illusory effect is much reduced. Both results reflect the principle that ‘coincidences’ in a display play little or no role in the process of segmentation when other circumstances tend to force critical edges to ‘belong’ to individual elements of the display.

Perception, 1990, volume 19, pages 471–482

“An acceleration illusion caused by underestimation of stimulus velocity during pursuit eye movements: Aubert-Fleischl revisited” by Alexander H Wertheim and Peter Van Gelder

Four lines have been missed out from the second paragraph on page 480 starting with line 14 from the top. This paragraph should read:

Note that when eye velocity across the stimulus increases, both V_ret and V_Tef increase but their difference remains the same. However, the JND between them (and thus the threshold) increases in accordance with Weber's law. Consequently V_p is reduced. This explains the reduction of perceived velocity during the execution of an eye movement (ie the acceleration illusion) and also the dependence of this reduction on the threshold for motion.

The penultimate paragraph consisting of four lines starting with line 12 from the bottom:

Note that this cannot happen when the background moves in the same direction as the eyes, because here retinal image velocity is smaller than eye velocity. A reduction in reference signal size then reduces the difference between V_Kt and V_ref, ie it reduces V_p and therefore, if anything, enhances the acceleration illusion.

Should be removed as it is a repeat of the paragraph above it.

We apologise to the authors for this typesetting error.