Perception

Piaget has distinguished a number of distinct stages in the development of the concept of an enduring external object during infancy. I present a theory of a class of behaviours at one of these stages embodied in a working computer program. The behaviour of this program matches a class of perceptual behaviours of infants between about twelve and twenty weeks of age in a number of experimental situations studied by Bower. The theory argues that these behaviours are a result of the interaction between the perceptual and conceptual levels of the system, and the way in which conflicts between competing descriptions of an object are resolved. I locate the cause of several features of the behaviours in the procedures for managing the changing representation of the world, and the system's way of treating transitions between the states of an object (for example, moving to stationary). The basic conceptual primitives of the analysis are objects and events, not motion and place, as argued by Bower, or the infant's previous activity, as argued by Piaget. I argue that adequate explanations of experimental findings such as these require the construction of fairly detailed computational models.

Subjects adapted to rectangular targets whose opposite edges moved in opposite directions at any given instant (as when an object moves directly towards or away from the head). The distance that adaptation spread from the vertical edges was up to three times less when the horizontal edges were also moving than when the horizontal edges were stationary. Furthermore, for a square target (1 deg × 1 deg), adaptation spread least when the horizontal edges moved at the same speed as the vertical edges, whereas for a target whose height was half its width (0·5 deg × 1 deg) adaptation spread least when the horizontal edges moved at half the speed of the vertical edges.

We propose that the human visual system acts as though it contains detectors sensitive to the size and shape of an object and that these detectors enhance this sensitivity to shape and size by comparing the velocities of the horizontal and vertical edges. When a nonrotating solid object moves in three dimensions its shape severely restricts the possible relationships between the velocities of the vertical and horizontal edges of its retinal image. Our hypothetical detectors utilise these geometrically determined velocity relationships as a basis for their selective sensitivity to shape.

More speculatively, object perception and the perception of shape and size in everyday vision might involve visual sensitivity to the relationship between the velocities of an object's edges as well as sensitivity to the locations of these edges: the visual system may recognise which of the many edges in the visual field belong to a single object by comparing the velocities of orthogonal pairs of edges.

A target moving in discrete spatial steps with an appropriate interstep interval (ISI) can appear visually as if it is in continuous motion. The momentary spatial position of such a target is interpolated by the observer between its real physical positions. The extent of this interpolation was measured by a vernier alignment technique, and was found to decrease as the ISI was lengthened.

A discretely moving target may be described as a continuously moving target on which is superimposed a periodic modulation of spatial position. It is shown that the traditional ‘staircase’ stimulus for apparent motion can be generalized to include other kinds of periodic modulation. With the use of various analog-filtered and digitally filtered versions of staircase stimuli with different ISIs, it was shown that the phenomenal interpolation of a periodically modulated moving target was affected only when the frequencies of modulation were less than about 25 Hz. The spatial amplitude of the modulation also has some effect.

Observers adapted to motion by looking at rotating logarithmic spirals. They were tested with a stationary mirror image of the adapting spiral in which all contours were at 90° to those of the first spiral. Motion aftereffects were reported in the contrarotational direction—that is, observers who had seen clockwise rotating motion reported seeing counterclockwise aftereffects. These aftereffects lasted one-third as long as the aftereffects obtained when the adapting spiral was used as the test figure. These two aftereffects were shown to have different storage properties, thereby indexing the operation of at least two different mechanisms. We interpret the motion aftereffect that is obtained with the mirror-image stimulus as indicative of the existence of global rotation detectors.

Recent research is reviewed and its implications discussed. ‘On-the-Spot’ accident investigations have confirmed that errors of perception by the driver are a major contributory factor to accidents. However, the available evidence suggests that few of these are attributable to reduced or defective vision, since at best only a weak relationship has been found between a driver's level of vision (or visual performance) and his accident rate. A number of reasons for this general finding are considered, including driver compensation. For all drivers, the rapid fall in visual acuity with angular distance from the centre of vision presents particular problems, giving special significance to eye-movement patterns and the problems of visual search. Numerous physical and psychophysical restrictions on visibility could lead to the ‘looked, but failed to see’ type of accident, but their relative importance requires evaluation. There is now much evidence that the driver is quite often operating beyond his visual or perceptual capabilities in a number of key driving situations, including overtaking, joining or crossing a high-speed road, and a number of nighttime situations. It is concluded that ‘expectancy’, based on experience in both the long and the short term, has a profound influence on driver perception and assessment of risk. For all drivers, serious errors of judgement from time to time would seem inevitable. In general, these do not lead to accidents because of, among other things, the safety margins added by the driver and adjustments made by other road users. Thus, despite his limitations and fallibilities, the average driver is involved in surprisingly few serious incidents, particularly in view of the rapid rate of decisionmaking that is required. However, the present accident rate should not be accepted as inevitable and various countermeasures are discussed.

Experiments were carried out to test whether spatial relationships between local pattern features are important in the production of pattern-specific visual adaptation effects. Evidence was found for sensitivity to the shape of local pattern features, but not to their relative positions. These findings are contrasted with recent results for texture discrimination where the visual system can make use of local position information.

To answer the question ‘What is suppressed during binocular rivalry?’ a series of three experiments was performed. In the first experiment observers viewed binocular rivalry between orthogonally oriented patterns. When the dominant and suppressed patterns were interchanged between the eyes observers continued seeing with the dominant eye, indicating that an eye, not a pattern, is suppressed during rivalry. In a second experiment it was found that a suppressed eye was able to contribute to stereopsis. A third experiment demonstrated that the predominance of an eye could be influenced by prior adaptation of the other eye, indicating that binocular mechanisms participate in the rivalry process.

Black and white stripes were used to induce a tilt aftereffect near the blind spot. Stripe fragments on either side of the blind spot were seen as being completed across the blind spot, but the magnitude of the tilt aftereffect they induced suggested that the perceptually ‘filled-in’ portions of the stripes did not contribute to the aftereffect. So perceptually filled-in lines seem not to be fully potent percepts.

Black-on-white displays are presented which are capable of producing subjective figures that are not enhanced in brightness.