Perception

Simple pictures under everyday viewing conditions evoke impressions of surfaces oriented in depth. These impressions have been studied by measuring the slants of perceived surfaces, with probes (rotating arrowheads) designed to respect the distinctive character of depicted scenes. Converging arguments indicated that the perceived orientation of the probes was near theoretical values. A series of experiments showed that subjects formed well-defined impressions of depicted surface orientation. The literature suggests that perceived objects might be ‘flattened’, but that was not the general rule. Instead, both mean slant and uncertainty fitted models in which slant estimates are derived in a relatively straightforward way from local relations in the picture. Simplifying pictures tended to make orientation estimates less certain, particularly away from the natural anchor points (vertical and horizontal). The shape of the object affected all aspects of the observed-object/percept relationship. Individual differences were large, and suggest that different individuals used different relationships as a basis for their estimates. Overall, data suggest that everyday picture perception is strongly selective and weakly integrative. In particular, depicted slant is estimated by finding a picture feature which will be strongly related to it if the object contains a particular regularity, not by additive integration of evidence from multiple directly and indirectly relevant sources.

The linking of spatial information is essential for coherent space perception. A study is reported of the contribution of temporal and spatial alignment for the linkage of spatial elements in terms of depth perception. Stereo half-images were generated on the left and right halves of a large-screen video monitor and viewed through a mirror stereoscope. The half-images portrayed a black vertically oriented bar with two brackets immediately flanking this bar and placed in crossed or uncrossed disparity relative to the bar. A pair of thin white ‘bridging lines' could appear on the black bar, always at zero disparity. Brackets and bridging lines could be flickered either in phase or out of phase. Observers judged whether the brackets appeared in front of or behind the black bar, with disparity varied. Compared to conditions when the bridging lines were absent, depth judgments were markedly biased toward “in front” when bridging lines and brackets flashed in temporal phase; this bias was much reduced when the bridging lines and brackets flashed out of phase. This biasing effect also depended on spatial offset of lines and brackets. However, perception was uninfluenced by the lateral separation between object and brackets.

Both the physical elevation that appears to correspond to eye level and the visually perceived pitch of a visual field are linear functions of the physical pitch of a normally illuminated, complexly structured visual field. One of the possible bases for the large effect of physical pitch on the elevation of visually perceived eye level (VPEL) is that the visual field generates a mental representation which specifies spatial coordinates and these determine the VPEL elevation (‘implicit-surface model’; ISM). The influence on the elevation of VPEL is nearly as large when the visual field contains either one or two long pitched-from-vertical or rolled-from-vertical lines in otherwise total darkness as when it consists of a well-illuminated and complexly structured pitched room (L Matin and W Li, 1994 Vision Research 34 311–330), and, in order to examine the ISM, we employed a rolled-from-vertical, two-line configuration within a frontoparallel plane viewed in otherwise total darkness. Measurements of visually perceived pitch were made by a manual matching procedure and VPEL measurements were made by the psychophysical setting of the elevation of a small visual target to appear at eye level while each of three subjects viewed the two-line configuration at each of three horizontal eccentricities with the configuration at each of seven roll orientations. In direct contradiction to the ISM, the perceived pitch of the two-line configuration did not deviate significantly from the erect orientation (‘vertical’) for any roll at any eccentricity, but the elevation of VPEL changed systematically with the roll of the configuration both at left and at right eccentricities, and did not change at all with the two-line configuration centered on the median plane. Consistent with our previous work and with our previous interpretation regarding the basis for VPEL (L Matin and W Li, 1994 Vision Research 34 2577 – 2598), the variation of VPEL for the two-line visual field equals the average of the VPEL variations produced by viewing each of the single lines separately.

An investigation was undertaken into whether haptic comparison of curvature and of shape is influenced by the length/width ratio of the hand. For this purpose three experiments were conducted to test the curvature matching of curved strips (experiment 1), the curvature matching of cylindrically curved hand-sized surfaces (experiment 2), and the shape discrimination of elliptically curved hand-sized surfaces (experiment 3). The orientation of the stimuli with respect to the fingers was varied. The results of the two matching experiments showed that a given curvature is judged to be more curved when touched along the fingers than when touched across the fingers. The phenomenal flatness along and across the fingers was found to be different and subject dependent. The results of the shape-discrimination experiment showed that the orientation of ellipsoidal surfaces influences the judgments of the shapes of these surfaces. This influence could be predicted on the basis of results of the second matching experiment. It is concluded that similar mechanisms underlie the (anisotropic) perception of curvature and shape. For the major part the trends in the results can be explained by the length/width ratio of the hand and the phenomenal flatnesses.

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.

Models of color transparency suggest that a region in which colors of surfaces converge in color space will appear transparent. The convergence is described by a transparency parameter a and a target of convergence. To test such models psychophysically, observers were presented a display with four colored areas. The colors of three of the areas were chosen in advance by the experimenter. The task of the observer was to choose the color of the fourth area to make a central region appear transparent. Settings for the fourth color were collected for a total of twenty-four color combinations chosen from three planes in color space. Observers' settings agreed well with the model, which predicts that choices for the fourth color lie along a line segment in color space that is parameterized by a. The results suggest further that color discriminability and color opponency also influence transparency judgment.

This research was designed to examine the consistency of individual differences in timing. Subjects were tested initially on a temporal-signal-detection task. In a series of trials, subjects judged whether a stimulus figure was displayed for either 12 s or greater than 12 s. Task performance was used to classify the subjects into groups with high or low temporal sensitivity (d'). Later, the subjects were tested on two classic time-judgment tasks. In a temporal-interference task, subjects reproduced intervals of 8 – 16 s during which they had rehearsed 0, 3, or 7 digits. Absolute error in time judgments increased linearly as a function of task demands. However, subjects with low temporal sensitivity made more error under all conditions compared with those with high sensitivity. In an isochronous-tapping task, subjects produced a series of 2-s and 5-s intervals. The low-temporal-sensitivity group produced more variable and inaccurate responses than the high-sensitivity group. The results demonstrate cross-situational consistency in timing performance across different tasks, time-judgment methods, and stimulus durations.

A stereoscopic illusion is produced by manipulating the binocular disparity of the irises in stereo pairs of human faces. These stimuli elicit unnatural percepts of the irises popping out of the face or sinking in inside the head.