Perception

Two experiments are reported in which the response of pigeons to perspective transformations of a pattern target was measured. In the first experiment an alphabet letter was taken as the pattern target (the ‘positive’) and the response to its perspective transforms was compared to the response to nontarget letters (‘negatives’) over the course of discrimination training. In the second experiment irregular quadrilaterals were used as positives and the responses to slight perspective deformations of the prototypes were compared to the responses to random transformations of the same magnitude under steady-state conditions. The amount of differential responding depended on the type of transformation. There was no differential response to targets rotated in the picture plane or around the horizontal axis in either experiment. There was differential response to small reductions and enlargements of the target. There was also differential response to translated targets, and this was seen irrespective of the amount of displacement. Results from targets rotated around the vertical axis were erratic, some target-plus-angle combinations elicited differential responding, but most did not. The erratic responses are attributed to symmetries in pattern elements that were abstracted as critical features. Pigeons therefore exhibit no true rotation invariance, limited size invariance, and complete shift invariance. It is argued that size invariance, but not position invariance, may depend on prior exposure to the alternatives.

Low-level preattentive vision processing is of special interest since it seems the logical starting point of all vision processing. Exploration of the human visual processing system at this level is, however, extremely difficult, but can be facilitated by the use of stroboscopic presentation of sequences of random-dot stereograms, which contain only local spatial and temporal information and therefore limit the processing of these images to the low level. Four experiments are described in which such sequences were used to explore the relationships between various cues (optical flow, stereo disparity, and accretion and deletion of image points) at the low level. To study these relationships in more depth, especially the resolution of conflicting information among the cues, some of the image sequences presented information not usually encountered in ‘natural’ scenes. The results indicate that the processing of these cues is undertaken as a set of cooperative processes.

Mirror spectacles which enhance binocular disparity by optically doubling the normal separation between the eyes were used to create conditions of combined perceptual and oculomotor conflict. Apparent depth and distance, as well as tonic accommodation, tonic vergence, and accommodative-vergence gain (response AC/A ratio), were assessed immediately before and after a 30 min exposure period of naturalistic viewing with the spectacles. Wearing the spectacles produced an increase in tonic vergence, and perceptual aftereffects consisting of increased apparent distance and depth. The results indicate that oculomotor conflict associated with enhanced interocular separation may be resolved through adaptation of tonic vergence, rather than through alteration of accommodative-vergence gain. The results also demonstrate that perceptual conflict between disparity and multiple veridical depth cues does not necessarily produce adaptive modification of the relationship between binocular disparity and apparent depth.

To determine how the visual system represents information about change in target direction, we studied the detection of such change under conditions of varying stimulus certainty. Target direction was either held constant over trials or was allowed to vary randomly. When target direction was constant the observer could be certain about that stimulus characteristic; randomizing the target direction rendered the observer uncertain. We measured response times (RTs) to changes in target direction following initial trajectories of varying time and distance. In different conditions, the observer was uncertain about either the direction of the initial trajectory, or the direction of change or both. With brief initial trajectories in random directions, uncertainty about initial direction elevated RTs by 50 ms or more. When the initial trajectories were at least 500 ms, this directional uncertainty ceased to affect RTs; then, only uncertainty about the direction of change affected RTs. We discuss the implications of these results for (i) schemes by which the visual system might code directional change; (ii) the visual integration time for directional information; and (iii) adaptational processes in motion perception.

The motion of two dots corresponding to the two endpoints of an oscillating rigid pendulum produces the perception of an elastic rod, not a rigid one. The possibility that structural information about a rigid object whose motion is simulated plays an important role in the perceptual restoration of the rigidity is examined. Two experiments are reported in which the motion of the endpoints of rigid objects (two endpoints for a rod, four for a sheet, and eight for a board) was displayed. Rigid structure was restored whenever structural information could be used to activate a knowledge data-base on the properties of the objects.

Vernier acuity has usually been tested with stimuli of the same contrast polarity (SC). This traditional vernier acuity was compared to that obtained with stimuli of opposite-contrast (OC) in which one target was brighter than the background and the other was darker. For both bar and dot targets vernier acuity with OC stimuli was about half as good as with SC stimuli. There were large individual differences in the size of the disadvantage with OC stimuli, although thresholds remained within the hyperacuity range. There were also individually-differing biases to see a dark vernier stimulus on one or the other side of a bright stimulus. Differences between OC and SC vernier acuities persisted over a wide range of interstimulus spacings, widths, and contrasts. At extremes of these spatial manipulations acuities became similar, but only because SC acuities were degraded to the level of OC acuities. Subjects showed little improvement in OC vernier acuity, even after 50 000 trials. It is concluded that finest judgements of spatial position arise in a level of the visual system at which light and dark stimuli are treated independently.

Time-to-contact is an important quantity for controlling activities which involve the timing of interactions with objects and surfaces in motion relative to an observer. Two alternative means for obtaining perceptual information that might be used to obtain the time-to-contact required to correctly time an interaction have been contrasted: a method based on the perception of distance and velocity, and a method due to Lee involving a perceptual variable called tau. A monocular version of the first method is presented and shown to place a highly unrealistic and arbitrary limitation on the capabilities of the visual system. The second method is reviewed and its limitations discussed. Several means by which these limitations can be overcome are presented. Recently reported results from experiments which involved catching self-luminous balls in the dark are interpreted in terms of timing information available to the subject, and the notions of intermodal and multimodal timing information are introduced. Finally, the possibility that timing information is available to an observer which does not involve the variable tau is considered. It is concluded that many questions regarding the perception of time-to-contact remain unresolved and that much empirical research remains to be done.

Two experiments were conducted. In experiment 1, part 1, binaural and monaural localization of sounds originating in the left hemifield was investigated. 104 loudspeakers were arranged in a 13×8 matrix with 15° separating adjacent loudspeakers in each column and in each row. In the horizontal plane (HP), the loudspeakers extended from 0° to 180° in the vertical plane (VP), they extended from −45° to 60° with respect to the interaural axis. Findings of special interest were: (i) binaural listeners identified the VP coordinate of the sound source more accurately than did monaural listeners, and (ii) monaural listeners identified the VP coordinate of the sound source more accurately than its HP coordinate. In part 2, it was found that foreknowledge of the HP coordinate of the sound source aided monaural listeners in identifying its VP coordinate, but the converse did not hold. In experiment 2, part 1, localization performances were evaluated when the sound originated from consecutive 45° segments of the HP, with the VP segments extending from −22.5° to 22.5°. Part 2 consisted of measuring, on the same subjects, head-related transfer functions by means of a miniature microphone placed at the entrance of their external ear canal. From these data, the ‘covert’ peaks (defined and illustrated in text) of the sound spectrum were extracted. This spectral cue was advanced to explain why monaural listeners in this study as well as in other studies performed better when locating VP-positioned sounds than when locating HP-positioned sounds. It is not claimed that there is inherent advantage for localizing sound in the VP; rather, monaural localization proficiency, whether in the VP or HP, depends on the availability of covert peaks which, in turn, rests on the spatial arrangement of the sound sources.