Perception

Detection of vertical bilateral symmetry has previously been studied in patterns composed of black or white dots on a grey background under four conditions: (a) same contrast (black or white) for all dots (called BB or WW, for ‘all black or all white’); (b) half of the dots black and half white with positive correspondence between symmetrical dot pairs (called MA for ‘matched’); (c) half of the dots black and half white with negative correspondence between symmetrical dot pairs (called OPP for ‘opposite’); and (d) black (white) dots on one side of the axis and white (black) dots on the other (called BW for ‘one side black the other white’). It was found that performance was ordered BB (or WW) = MA > OPP =BW, where > indicates better performance. That experiment was repeated here in experiment 1 with symmetry axes not only at vertical but also at horizontal and the two diagonals. It was found overall that BB = MA > OPP, BW. However, OPP > BW when random trials were included in the analysis but when they were excluded BW > OPP. This was due to a very high false-alarm rate in condition BW which could be accounted for if grouping by colour occurs prior to symmetry detection. In experiment 2 it was shown that vertical-symmetry salience over other orientations remained about the same as OPP patterns progressively changed into BB patterns by varying the percentage same polarity between 0% and 100% in 12%–13% steps. Thus, dot-pair polarity affects performance without affecting relative axis salience, as was also found recently when dot pattern outlines were masked. All of the data indicate that although opposite dot polarity does reduce performance slightly, the symmetry-detection mechanism is remarkably resilient to such perturbation. The high false-alarm rate in the BW condition of experiment 1 may be accounted for by extremely salient global grouping of dots by luminance which effectively creates an integral stimulus which is perceptually difficult to break down into its component dot pairs, prohibiting the required point-by-point matching necessary to reject symmetry detection. The small detrimental effect of nonmatched polarity might be due to the polarity differences masking the grouping of dots into ‘clumps’ on either side of the axis, a process for which there is a great deal of independent evidence.

The effect of selective amputations of the angle components in the wings-in (underestimated) and wings-out (overestimated) forms of the Müller-Lyer illusion was examined in two experiments. The stimulus figures consisted of one, two, or four angles. In experiment 1 the method of paired comparisons was used to scale the figures on the psychological continuum of length, and in experiment 2 the method of reproduction was used to obtain quantitative measures of illusion magnitude. There was good agreement between the scaling and the length-reproduction measures of the illusion. The illusory effects in all figures were significant, and the extent of the underestimation and overestimation of the wings-in and wings-out figures, respectively, increased as the number of angles increased. In general, selective amputation of the angle components produced similar patterns of illusory effects in the wings-in and wings-out figures. These findings are discussed with reference to the issue of whether the two forms of the conventional (ie four-angle) Müller-Lyer illusion are similar or distinct illusion types.

A study of size interactions of objects in three-dimensional space is reported. The canonical form of the Ebbinghaus illusion—test circles surrounded by large or small inducers—was used. Both monocularly visible (M) and purely cyclopean (C) objects were displayed stereoscopically to isolate the monocular and cyclopean components of the illusion. The results of two experiments indicate that: (i) depth plays a significant role when the test circles are cyclopean, but not when they are monocularly visible; and (ii) the size of C objects is affected equally by C and M inducers, but the size of M objects is affected much more strongly by M than by C inducers. In conclusion, possible explanations are offered for the main trends in the data, the most interesting of which is that cyclopean tests seem to be interacting only with the cyclopean component of monocularly visible inducers.

The generic-view principle (GVP) states that given a 2-D image the visual system interprets it as a generic view of a 3-D scene when possible. The GVP was applied to 3-D-motion perception to show how the visual system decomposes retinal image motion into three components of 3-D motion: stretch/shrinkage, rotation, and translation. First, the optical process of retinal image motion was analyzed, and predictions were made based on the GVP in the inverse-optical process. Then experiments were conducted in which the subject judged perception of stretch/shrinkage, rotation in depth, and translation in depth for a moving bar stimulus. Retinal-image parameters—2-D stretch/shrinkage, 2-D rotation, and 2-D translation—were manipulated categorically and exhaustively. The results were highly consistent with the predictions. The GVP seems to offer a broad and general framework for understanding the ambiguity-solving process in motion perception. Its relationship to other constraints such as that of rigidity is discussed.

Previous research has shown that the perception of motion within a local region is influenced by other motions within neighboring areas (eg induced motion). Here, a study is reported of the perceived speed of dots moving within a circular target region, which was surrounded by other motions within a larger surrounding area. The perceived speed of the central dots was found to be fastest when the surround was stationary; it became slower as the speed of motion in the surround was increased. This decrease in the perceived target speed with increases in surround velocity occurred regardless of whether the direction in which the surround moved was the same as or opposite to the motion of the target region. This result cannot be explained by using simple models of perceived speed that depend only upon such factors as the magnitude of relative motion between center and surround. The spatial area over which these motion interactions occur was also investigated.

The physical elevation that appears to correspond to eye level (VPEL), as measured with a small visual target, changes systematically with the orientation in depth (‘visual pitch’) of a visual field consisting of one or two pitched-from-vertical lines in darkness. The influence is large and, with a one-line stimulus, is only 15% smaller than the influence exerted by a complexly structured, well-illuminated, pitched visual field. A line from a frontoparallel plane can be presented to the same retinal locus as a pitched-from-vertical line; the three experiments in the present report were aimed at determining the influence on VPEL from such lines. In the first two experiments the subject viewed a visual field consisting of a one-line or two-line pitched-from-vertical stimulus from a pitched-only plane or an oblique one-line or two-line stimulus from an erect plane. Each of the pitched-from-vertical stimuli was presented at seven different orientations separated by 10° over a ±30° range. Each of the oblique-line stimuli was presented at an orientation that resulted in stimulation to the same retinal locus as one of the conditions with pitched-from-vertical lines, and thus a range of ‘equivalent pitches’ was examined that corresponded to the range of pitches for the pitched-from-vertical lines. The variation in orientation of the oblique-line stimulus and the pitched-from-vertical stimulus each produced systematic changes in VPEL; the two were indistinguishable. A third experiment specifically designed to examine the possibility that either stimulus sequencing or lack of naivity of the subjects might have been involved turned up no such effects. It is concluded that the aspect of a line stimulus that controls the influence on VPEL is the orientation of the image of the line on a projection sphere centered on the nodal point of the eye or, as in the present experiments with viewing in primary position, the retinal locus stimulated; the orientation-in-depth of the stimulating line provides no additional influence on VPEL for the stationary, erect, monocularly viewing observer. The results are interpreted within the framework of the great-circle model.

Visually perceived eye level (VPEL) was measured while subjects viewed two vertical lines which were either upright or pitched about the horizontal axis. In separate conditions, the display consisted of a relatively large pair of lines viewed at a distance of 1 m, or a display scaled to one third the dimensions and viewed at a distance of either 1 m or 33.3 cm. The small display viewed at 33.3 cm produced a retinal image the same size as that of the large display at 1 m. Pitch of all three displays top-toward and top-away from the observer caused upward and downward VPEL shifts, respectively. These effects were highly similar for the large display and the small display viewed at 33.3 cm (ie equal retinal size), but were significantly smaller for the small display viewed at 1 m. In a second experiment, perceived size of the three displays was measured and found to be highly accurate. The results of the two experiments indicate that the effect of optical pitch on VPEL depends on the retinal image size of stimuli rather than on perceived size.

Davis and Driver presented evidence suggesting that Kanizsa-type subjective contours could be detected in a visual search task in a time that is independent of the number of nonsubjective contour distractors. A linking connection was made between these psychophysical data and the physiological data of Peterhans and von der Heydt which showed that cells in primate area V2 respond to subjective contours in the same way that they respond to luminance-defined contours. Here in three experiments it is shown that there was sufficient information in the displays used by Davis and Driver to support parallel search independently of whether subjective contours were present or not. When confounding properties of the stimuli were eliminated search became slow whether or not subjective contours were present in the display. One of the slowest search conditions involved stimuli that were virtually identical to those used in the physiological studies of Peterhans and von der Heydt to which Davis and Driver wish to link their data. It is concluded that while subjective contours may be represented in the responses of very early visual mechanisms (eg in V2) access to these representations is impaired by high-contrast contours used to induce the subjective contours and nonsubjective figure distractors. This persistent control problem continues to confound attempts to show that Kanizsa-type subjective contours can be detected in parallel.