Perception

Three experiments investigated the perception of substance and shape as invariant properties of objects by three-month-old infants. In experiment 1, infants were habituated to two differently shaped objects undergoing a rigid motion. After habituation of the infants, the objects were presented undergoing a different rigid motion, or undergoing a deforming motion, or undergoing the same rigid motion. Habituation was maintained to the new rigid motion, indicating that the two rigid motions were perceived as sharing an invariant property. Dishabituation, on the other hand, occurred when a deforming motion followed a rigid one. In experiment 2, infants were habituated to one shape undergoing two different rigid motions. After habituation, the shape was changed but the same two motions continued. Dishabituation occurred, compared to a group with no shape change, indicating that shape is distinguished as an invariant property over two rigid motions. In experiment 3, habituation to a shape undergoing two rigid motions was followed by a new shape presented motionless, or the same shape presented motionless. Cessation of motion did not prevent recognition of shape as invariant. Two properties of an object, substance and shape, thus appear to be detectable as invariant in an event sequence, an instance of ‘phenomenal doubling’ at an early age.

This study investigated whether reaction time to detect an item within an image increases as a monotonic function of the relative or the absolute distance from the starting item to the probed item within the image. It was found that when relative probed distances were manipulated and absolute distances kept constant and, conversely, when absolute distances were manipulated and relative distances kept constant, reaction times were invariant across relative distance but increased as a function of increasing absolute distance. This result may be interpreted either in terms of operations within an imaginal representation or in terms of operations on propositional statements underlying the image.

An experiment is reported which demonstrates that the features used to detect a character in arrays of similar forms are a function of the stimulus context in which the target is embedded. With the use of blocked and randomised design it was shown that prior knowledge of the background aided search when the feature which was relevant for one background was irrelevant for another, but no significant effect of prior knowledge was obtained when the same feature was useful for the backgrounds.

Studies of steady visual stimulation have suggested that the subjective meaning of a stimulus may determine the form and degree of its perceptual fragmentation. This was tested for steady fixation by presenting a letter of the alphabet (‘L’) in a sequence of other letters which were either normally oriented or inverted. No evidence was found for greater stability in the former case. An investigation of the categorisation of subjective disappearances showed that, although the stability of pattern components was not determined by reporting these singly or in conjunction, the disappearances of the whole pattern were significantly increased by reporting only these as opposed to reporting disappearances of each component. Response to simulated subjective disappearances also showed that the effect was due to the nature rather than the number of response categories. In the final experiment all subjects fixated a rotated letter, after half had received pretraining in the recognition of rotated letters. No effects of the pretraining were apparent, for either level of response categorisation, and it was concluded that the processes underlying subjective disappearances in steady fixation are not modified by the meaningfulness of a stimulus. The involvement of central processes was, however, indicated by the effects of response categorisation and the influence of simulated disappearances on pattern stability in a trial subsequent to their presentation.

Many subhuman species and human infants, children, and adults can use two-dimensional information of relative rate of angular-size change to anticipate collisions between the self and approaching objects or surfaces. But extant studies have not determined what information is used when subjects view simulated approach events providing two-dimensional information and three-dimensional information (distance and distance change), as well as lower-order visual information contained in real approach events. Three experiments suggest that, given these several possibilities, adults' judgments of collision time are best predicted by two-dimensional spatiotemporal values which are invariant over object sizes, distances traversed, approach velocities, and several lower-order variables such as absolute angular size. However, collision time is substantially underestimated, with absolute amount of underestimation increasing as a function of actual time-to-collision. Large constant errors and loss of judgment linearity beyond about 10 s to contact time suggest that current models of human performance based on use of time-to-collision information require modified assumptions of operator efficiency.

Two instances of a new illusion of motion were discovered. In the first, when observers tracked a spot of light moving vertically in a field of stationary vertical lines, the lines appeared to move in the same direction as the spot. If observers did not track the spot, the lines appeared stationary. The second instance was designed to see if the same illusion would occur when the spot appeared to move as a result of induced motion. In this display a vertically moving grid of horizontal lines surrounded a stationary vertical line. A stationary spot of light could be projected on the line. If the subject fixated the spot, both spot and line appeared to move against the grid. If the spot was absent, the line appeared to move along with the grid. The implications of this illusion for theories of induced motion are discussed.

The perception of certain figures with illusory contours entails a reversal of figure and ground. It is hypothesized that this process occurs in two stages. First, some factor must suggest or cue the reversal. Experiments are described that isolate three such factors, namely, alignment of physically present contours, recognized incompletion of parts of the stimulus array, and set. Once cued, however, other experiments indicate that in a further stage of processing the solution is examined with respect to its compatibility with the stimulus display or with other perceptual properties to which the display gives rise. Only if such compatibility is present will the perception of a figure with illusory contours be maintained.

Visual field differences in stereoscopic form recognition using Julesz-type random dot stereograms were investigated. Dot size was varied in order to test the possibility that variations in the carrier dimension have contributed to past estimates of visual field differences. Twelve male and twelve female subjects, all right-handed, appeared for three test sessions—one with each different dot size. In each session the stimuli were flashed twenty-four times in each visual field, for 120 ms. Results showed no overall visual field effect, but a highly significant interaction between visual field and dot size. For small dots, left visual field superiority was observed, as previously reported by Durnford and Kimura. With large dots, however, the right visual field was superior. This reversal of visual field differences as a function of dot size implies that there is no consistent cerebral hemispheric specialization for stereopsis or stereoscopic form recognition per se. Instead, it appears that there is relative hemispheric specialization for responding to the carrier of stereoscopic information.

Surfaces possessing steep variations in depth present severe difficulties for orientationally tuned filter models of stereopsis. These difficulties are discussed in connection with a random-dot stereogram depicting a surface with steep horizontal corrugations. As expected on theoretical grounds, we find that a vertical ±45° orientationally filtered version of this stereogram cannot be fused. Moreover, it is demonstrated that a horizontal ±45° filtered version can be fused only with difficulty and its stereo percept is poor compared to that of the unfiltered original. It is concluded that orientated filters seem ill-designed to mediate the extraction of disparity cues, at least in the cases under consideration.

The centrality of individual differences in the visual component of perceptual adaptation was examined in a massed-practice—terminal-exposure, prism-viewing paradigm. With positive (adaptive) adjustments in the judgment of the visual straight-ahead, target-pointing aftereffects were found to be equivalent to the sum of the visual and proprioceptive (head—arm) aftereffects. For subjects showing negative visual adjustments to prism exposure, the target-pointing aftereffect was not significantly different from the change in proprioception alone. Implications of these findings for hypotheses concerning the process of perceptual adaptation are discussed.

Adaptation to a 9 cycles deg⁻¹ sine wave makes the apparent brightness profile of a 3 cycles deg⁻¹ square wave look less ‘square’ even when it does not look like that of a sine wave. Adaptation to a 3 cycles deg⁻¹ sine wave has no noticeable effect. Hypotheses based on ‘excitation patterns’ across spatial-frequency-selective channels can account for these results; it is not clear whether variations in local light adaptation can also account for them.

It is known that the sum of a random-dot array with vertical bilateral symmetry and one with horizontal bilateral symmetry appears as a random array. Here we show that if the vertically and horizontally symmetrical arrays are spatially filtered, so that their respective spectra are 2 octaves apart, then their superposition does not appear random, but both symmetries can be simultaneously perceived. The low-band array has a stronger perceptual weight than the high-band array. These demonstrations give further evidence that frequency channels are before symmetry perception.