Perception

The authors have previously hypothesised that colour vision has evolved not only to encode colour per se but also, perhaps principally, to enhance luminance-based visual processing, so that for colour information to be fully effective, luminance as well as chromatic variations should be present in visual targets. Results of previous experiments, testing detection of spatial gratings and detection and perceived brightness of Mach bands support the hypothesis. Further experiments are reported in which the hypothesis was tested by using a higher-level task of pattern recognition. Subjects had to discriminate between luminance (isochromatic), isoluminant (chromatic), or combined colour/luminance ellipses and circles. It was found that the ability to discriminate between a circle and an ellipse was greatly enhanced when both colour and luminance variations were present as compared with the pure luminance or colour presentations. Summation-square analysis shows linear colour-luminance summation which can be modeled by a single-analyser model.

The magnitude of the barber-pole illusion is measured as a function of the contrast, orientation, and phase difference between the moving ‘barber-pole’ grating and a moving ‘surround’ grating that forms the aperture surface. It is found that as the difference between the barber-pole grating and the surround grating increases, the influence of surround motion on barber-pole motion decreases (resulting in an increase in the magnitude of the barber-pole illusion). This pattern of results is interpreted as evidence for competition between processes that detect the motion of line terminators along the length of the rectangular aperture and processes that detect grating motion in directions perpendicular to grating orientation.

How do we interpret outline drawings of surfaces? Although pictorial depictions are projectively ambiguous, observers demonstrate definite preferences of interpretation. Additionally, they commit typical errors. A study is reported of one specific arrangement of surfaces as it is represented in outline drawings, namely the arrangement that results when two arbitrary surfaces are joined at a common edge to form an angle in 3-D (‘phenomenic folding’). With some of these arrangements, observers report that the angle formed by the two surfaces is zero (complete folding). With others, they report that the angles are greater than zero (incomplete folding). Both interpretations are actually valid. Several investigators have proposed that observer preferences such as these are due to a tendency to prefer a 3-D interpretation that will make the depicted 3-D shape regular.

Three experiments were performed to test this regularisation hypothesis. In the first, observers were shown pairs of four-sided polygons joined at one equal side. Their task was to imagine how the smaller polygon could be folded completely towards the larger, and, subsequently, to report on its position after the folding (‘mental folding’). Reported positions were consistent with 3-D interpretations that caused figural regularisations. In the second and third experiments, observers were shown drawings of diamonds and parallelograms folded along a number of differently positioned and oriented segments (‘folding edge’). Their task was to estimate verbally the extent of the dihedral angle formed by the two surfaces. Results indicated that the perception of incomplete folding is determined by 3-D interpretation of the orientation of the drawing with respect to the picture plane. In a fourth experiment, observers were asked whether projective equivalences might be disambiguated by animating two kinds of displays that yield the ‘incomplete folding’ effect but that should be distinguishable on the basis of the trajectories of the vertexes of the folding parts. Results demonstrated that even in these conditions observers are unable to interpret the foldings correctly. These results might be taken to indicate that projective, static information leading to a simpler and more regular interpretation of the display can prevail over explicit motion information that should force the system to achieve a nonregular solution.

Phenomenally strong visual illusions are described in which the motion of an object‘s cast shadow determines the perceived 3-D trajectory of the object. Simply adjusting the motion of a shadow is sufficient to induce dramatically different apparent trajectories of the object casting the shadow. Psychophysical results obtained with the use of 3-D graphics are reported which show that: (i) the information provided by the motion of an object's shadow overrides other strong sources of information and perceptual biases, such as the assumption of constant object size and a general viewpoint; (ii) the natural constraint of shadow darkness plays a role in the interpretation of a moving image patch as a shadow, but under some conditions even unnatural light shadows can induce apparent motion in depth of an object; (iii) when shadow motion is caused by a moving light source, the visual system incorrectly interprets the shadow motion as consistent with a moving object, rather than a moving light source. The results support the hypothesis that the human visual system incorporates a stationary light-source constraint in the perceptual processing of spatial layout of scenes.

The effect of attention of the perceived length of briefly presented peripheral lines was investigated. Attention was manipulated by engaging observers in a second concurrent task (letter identification). Observers used the method of adjustment to indicate the length of the stimulus lines. In two experiments it was found that the primary effect of attention was to reduce the variability of line length adjustments. Previously investigators had reported that attention reduces perceived line length. A third experiment suggested that these previously reported results might have been the outcome of a spatial interaction with the cue used to manipulate attention, but not the result of visual attention.

Caricatures, which increase the distinctiveness of faces, are generally recognised at least as well as undistorted images of those faces. However, caricatures seem to facilitate recognition more for some faces than others. An investigation was made into whether the effectiveness of caricaturing depends on a face‘s initial distinctiveness. In experiments 1–3, subjects learned names for unfamiliar faces (photographs) that varied in distinctiveness, and were tested on recognition of caricatures, anticaricatures, and undistorted images of those faces. The test images were line drawings in experiments 1 and 2 and photographic images in experiment 3. Experiments 1 and 2 were identical except that subjects had more exposure to the study photographs in experiment 1. In all three experiments, distinctive faces were recognised (named) more accurately than less-distinctive faces, and caricatures were recognised at least as accurately as undistorted images and better than anticaricatures. However, distinctiveness and caricature level did not interact. Nor did a face's initial distinctiveness correlate with the degree of recognition facilitation produced by caricaturing (experiments 1–3) or with the caricature level chosen as the best likeness (experiment 4). The effectiveness of caricatures varied across faces and experimental conditions, but these differences did not relate to differences in initial distinctiveness. These results prompted a more careful analysis of the expected relationship between initial distinctiveness and the power of caricatures, which indicated that the relationship may be curvilinear rather than linear. In addition, it was found that line-drawing caricatures functioned as superportraits (recognised better than undistorted images—experiment 1) but photographic caricatures did not (experiment 3), suggesting that the forensic potential of caricatures may be limited.

Bartlett and Searcy's recent account for the Thatcher illusion suggests that inversion impairs holistic facial information. This illusion is used to compare the effects of inverting and negating faces. Subjects made a speeded response to whether just the mouth and the eyes of a face have been inverted. Performance was found to be slower when faces were inverted rather than upright. Presenting faces in photographic negative also hindered performance implying that this transformation also disrupts holistic facial information.

Stewart et al (1993, Perception 22 1227–1244) attempted to demonstrate that the ratio of angular speed to angular acceleration, θ̇/θ̈, of the approaching object is preferred as the source of time-to-collision information to the ratio of visual angle to angular speed, θ/θ̇. It is demonstrated that this is not justified.

In an earlier paper, “Misperception of time-to-collision by drivers in pedestrian accidents”, it was proposed that time-to-collision is derived from the angular speed and angular acceleration of an approaching object, with the use of the algorithm 2θ̇/θ̈, rather than from θ/θ̇. This response to the critical paper by Tresilian is intended to support that conclusion, which was based on four main arguments:

(1)

the focus of expansion could not provide a reliable datum for judging θ;