Perception

The perception of relations between stimuli is considered to be essential for perceptual grouping. The present study investigated the ability of five-month-old infants to perceive the relative motions between simultaneously moving dots. Two of the dots moved horizontally back and forth in parallel trajectories, and a third dot moved vertically up and down between the other two. As a consequence of the relative motions, adults perceived the vertically moving dot to be moving an average of 32° clockwise or counterclockwise from the vertical depending on the phase of motion between the vertical and horizontal dots. An habituation paradigm was used to determine whether infants also perceive these relative motions. A control group of infants was presented a display with the same phase of motion between the vertical and horizontally moving dots for twelve trials. The experimentals were presented with the same display as the controls during the first ten trials, but on the last two trials the phase of motion was reversed. Fixation of the display continued to decline during the last two trials for the control, but not for the experimental group. A second study in which the phase of motions could be varied in two simultaneously presented three-dot displays supports the results of the first study. By five months of age, infants seem to be sensitive to relative-motion cues, and presumably have the ability to group moving stimuli perceptually.

To investigate the role of flat surface information for the plane of projection in pictorial perception, three studies were designed in which varying amounts of such information were made available to adult subjects. The first study tested preferences for true or modified linear perspective under conditions of presence or absence of surface texture cues for the plane of projection. In the second and third studies, the absence of texture cues for the plane was coupled with the addition of motion parallax and binocular information respectively. It was found that adults showed a consistent preference for parallel perspective in pictures when the flat-surface information was provided either by visible texture or by motion parallax; but no consistent preference for either true or modified perspective in the absence of all three sources of flatness information or when the flat surface information was given only by binocular cues in the absence of visible surface texture or head motion.

Nine squirrel monkeys were required to select from various sets of stimuli—differing in size or brightness—either in terms of relational criteria or on an absolute stimulus basis. The level of information processing required by each task was assessed by means of stimulus transformation techniques, variations in set size, and by the elimination of the visible context. It was found that some relational judgements make fewer processing demands on the subject than do absolute stimulus judgements; the ‘middle’ relation, however, appears much more difficult to use than selection of a stimulus on an absolute basis and may be beyond the competence of the squirrel monkey. The results are seen as support for the thesis as advanced by McGonigle and Jones that the criteria of judgement, when varied, change the depth of stimulus processing by monkey as well as man.

A series of experiments is reported on rivalrous-texture stereograms composed of narrow-band-filtered random noise. Experiment 1 found that the apparent depth—disparity function for such stereograms was different from that observed with similar but nonrivalrous stimuli. In particular, rivalrous divergent disparities produced the same depth as rivalrous zero disparity and this latter disparity itself produced a significant degree of protruding (i.e. ‘convergent’) depth in a certain type of rivalrous-texture stereogram. Free inspection was permitted and disparities were in the range 16 min convergent to 16 min divergent. Experiment 2 found no convincing evidence for reliable qualitative depth discriminations from tachistoscopic presentations of rivalrous-texture stereograms, using a forced-choice task requiring a discrimination between 16 min convergent and 16 min divergent conditions. This task was solved easily for equivalent nonrivalrous stimuli. Experiment 3 measured a hitherto unreported binocular depth effect, termed ‘paradepth’, which is produced by presenting a target in one field only. This effect appears to be a genuine binocular depth effect and not just the result of an ordinary monocular masking depth cue. The size of the depth effect was found to be a function of the width of the target. The overall conclusion derived from the series of experiments is that rivalrous-texture stereograms are complex stimuli capable of yielding curious and unexpected depth effects which are not readily explained in detail within any existing theoretical framework.

Moiré patterns in depth are reported when spatially periodic stimuli are viewed stereoscopically. Near-vertical square-wave gratings with horizontal disparity effected by orientation difference produce a ‘venetian blind’ effect. A single grating is observed divided into horizontal layers, the number of layers being a direct function of orientation difference. Lines within each layer are slanted with respect to the frontoparallel plane, becoming more so with increasing orientation difference. Subjects with stereo deficits report a decreased effect. Quantitative reports implicate stereoscopic mechanisms responsible for this unique, internal preservation of moiré patterns.

A fundamental process underlying motion perception is the matching of corresponding elements in different views. In this correspondence process spatial separation between elements plays a major role. The relevant separation is shown by the current study to be the two-dimensional, uninterpreted distance, a finding that has an implication to the level at which the correspondence process is carried out. The current findings are compared with earlier results concerning ‘optimality’ of apparent motion to conclude that optimality cannot serve as a measure for the correspondence strength.

Visual object recognition was investigated in a group of eighty-one patients with right- or left-hemisphere lesions. Two tasks were used, one maximizing perceptual categorization by physical identity, the other maximizing semantic categorization by functional identity. The right-hemisphere group showed impairment on the perceptual categorization task and the left-hemisphere group were impaired on the semantic categorization task. The findings are discussed in terms of categorical stages of object recognition. A tentative model of their cerebral organization is suggested.

Visual acuity (A) for a two-dimensional multiplicative sinusoidal contrast grid (sinusoidal chessboard) was measured as a function of spatial frequency (ω) of modulation in one of the dimensions. The reduction in acuity as frequency was increased was well described by the equation: A = −2πkω + c, where k and c are constants. At suprathreshold contrasts, the frequency of modulation appeared to be doubled relative to the underlying modulation frequency. This doubling is not related to the frequency of the harmonic components of the chessboard, but suggests the existence of a perceptual mechanism sensitive to areas of sinusoidal modulation which are seen at twice the frequency of the variation in contrast.

Figurai aftereffects were measured by using square patches of high-contrast grating on a dark background as inspection and test figures. The orientation of the outer square border and the enclosed grating were varied independently in order to evaluate their relative influence on the strength of the induced change of overall apparent size of the test figure. The largest effect is obtained when inspection and test figures are identical in the orientation of both outer border and enclosed grating. The strength of the aftereffect is reduced as a difference in orientation is introduced between inspection and test figure for either the outer contour or the contained grating, although the former is a more potent factor than the latter.