Perception

Ptolemy is often wrongly credited with an explanation of the moon illusion based on the size-distance invariance principle. This paper elucidates the two Ptolemaic accounts: one in the Almagest, based on atmospheric refraction, and the other in the Optics, based on the difficulty of looking upwards. It is the latter passage which has been thought to refer to size-distance invariance, but it is more probable that it refers to the idea that the visual rays are diminished by the force of gravity (i.e. that the retinal image is reduced in size). Alhazen was probably the first author to explain the illusion by the size-distance invariance principle, and Roger Bacon the first to explain the enlarged apparent distance of the horizon by the presence of intervening objects. Della Porta was the first to credit Ptolemy with these explanations, and this mistake was repeated by many subsequent authors.

Perceived figures tend to have surface colour and to be comparatively dense in texture and saturated. These figure characteristics are found in many of the illusory contour figures examined heretofore but new figures presented here do not conform to these characteristics. Consequently, rather than explain subjective brightness effects as offshoots of figural processes, it may be better to begin with concepts of local brightness effects induced by the display, which are made perceptually effective when grouped by the eye.

‘Good form’ theories of perception leave some latitude concerning how casually or cautiously order is imposed on the stimulus. Exploring this issue, the present experiment introduced a series of figures admitting up to three alternative ‘good’ readings. Eight college students estimated two angles in each of fifty-six pictured spatial forms. Theory predicted that geometrical regularities of rectangularity and symmetry would dominate their estimates. But a regularity would rarely appear when inconsistent with projective geometry and the given figure. The accuracy of subjects' estimates was also assessed. The results confirmed the hypotheses at high significance levels, arguing that such figures are interpreted through an order-imposing process restrained by projective geometry, and that subjects could make roughly accurate estimates based on the imposed order. Parallels with computer scene analysis are discussed. It is concluded that perceptual presumption of certain ‘good forms’ runs little risk of misinterpreting the stimulus.

In this paper, a revised Pandemonium-like model of visual-feature processing is formulated and a preliminary test of its feasibility is reported. The model differentiates visual-feature processing into a series of hierarchical stages organized by increasing complexity, with the output of each stage going both to the next higher stage, and directly to a more central processor. In the experiment, subjects sorted decks of cards into piles according to the presence or absence of a target stimulus which differed from nontargets in a variety of different features; detection of a feature was sufficient for detection of a target. The data generally supported the revised Pandemonium model, in that targets which differed from nontargets in features thought to be low in the hierarchy were processed faster than targets whose difference was in a high level feature. An extension of the revised model did somewhat less well in predicting the results of sorting for targets in which detection of any one of several features was sufficient for target detection.

This paper examines the structure underlying textural patterns. To this end, aspects of a model for texture are developed on the basis of both perceptual processes of the observer and structural properties of the patterns. Local constructs called primitives serve as generators for globally perfect, but unobservable, ideal textures. Study of the graph representation of these ideal textures reveals the basic regularity underlying visual textures. These perfect patterns are then transformed into observable textures by transformations which must not violate psychophysically based restrictions. Interpretively ambiguous textural patterns are used to demonstrate the influence that local primitives have on the formation of global pattern organizations.

A theory is presented of how a driver might visually control his braking. A mathematical analysis of the changing optic array at the driver's eye indicates that the simplest type of visual information, which would be sufficient for controlling braking and would also be likely to be easily picked up by the driver, is information about time-to-collision, rather than information about distance, speed, or acceleration/deceleration. It is shown how the driver could, in principle, use visual information about time-to-collision in registering when he is on a collision course, in judging when to start braking, and in controlling his ongoing braking. Implications of the theory for safe speeds and safe following distances are discussed, taking into account visual angular velocity detection thresholds, and some suggestions are made as to how safety on the roads might be improved.

This experiment examined the effects of adding five different kinds of prominent monocular features to a large-disparity random-dot stereogram. It was found that features which enclosed the disparate area produced the shortest initial perception times for fusion. The longer initial perception times for stimuli containing features without this enclosing property are explained in terms of less-helpful guidance of saccadic eye movements prior to the establishment of fusion. Subsequent reductions in perception times for these latter stimuli could be due to perceptual learning within the eye movement control system.

An irrelevant noise was turned on at various points during a visual reaction-time task. Latencies were reliably smaller than on quiet trials, provided the noise was turned on when a subject was looking at a relevant visual stimulus. This effect occurred regardless of whether the distribution of interstimulus intervals was constant or varied during a session, as well as independently of a subject's base rate of response and level of practice in the task. But it disappeared if reaction times in completely noisy sessions were compared to performances in quiet sessions. This effect was found to vary systematically with the orientation of and the hemiretina stimulated by a test form, but not with the particular ear stimulated by a noise. A model is suggested for these and related results in which the arousal elicited by a loud sound in the central nervous system either activates associative memories of form, inhibits early components of striate cortical unit response to the intensive properties of a visual stimulus, or both.

A set of black dots grouped on a white background to create an impression of radiating lines which fade towards the centre makes possible a percept of a central glow.