The influence of visual symmetry on the encoding of objects.

Two experiments examined how symmetry affects the visual encoding of simple flat objects (lamellae). Experiment 1 showed that subjects encode a lamella's shape as both a facsimile and its enantiomorph. In Experiment 2 the errors made when responding to symmetrical and asymmetrical stimuli showed that there were three factors affecting the reproduction of a symmetrical stimulus. These are, in order of decreasing influence, (i) the orientation of the axis of symmetry, (ii) the presence of overall symmetry (irrespective of orientation), and (iii) identity of the orientation of the elements of the response in relation to those of the stimulus. They also show that symmetry is likely to be reproduced even where the reproduction is not of the same shape as the original stimulus. The relevance of these results to representation of objects is discussed.

The role of left-right symmetry in the encodement of spatial orientations.

Three experiments investigated a hypothesis, suggested by studies of the difficulties of discriminating between shapes forming symmetrical pairs, that spatial orientations of thin flat plates (lamellae) may be encoded in a plane, the encodement consisting of two enantiomorphs. The results indicated that participants encoded the spatial orientation of lamellar stimuli in terms of the difference in cogency between their two enantiomorphic elements (Expt 1). The difference in the cogency of the two enantiomorphs is related to the orientation of the plane containing the lamellar stimulus with respect to the participant's fronto-parallel plane (Expt 2). The two possible orientations of a lamella which yield the same difference of cogency, but which differ in spatial orientation (e.g. lamella 'b' set at 30 degrees or set at 150 degrees) are distinguished by the manner in which the two enantiomorphic elements are arranged with respect to their axis of symmetry (Expt 3). The results suggest that the orientation of a lamella may be encoded as a two-dimensional representation and hence that three dimensions may be encoded by two by means of enantiomorphs. Implications of this finding for the encodements of three-dimensional solids, wherein pronounced contours may fulfil the same role as do the edges of lamella, are discussed briefly.

The role of typical contours in the encodement of objects in three-dimensional arrays.

Two experiments are reported in which the relationship between the orientation of the typical contour of a lamella in relation to the observer, and the ability of the observer to learn the correct position and orientation of sets of lamellae in three-dimensional arrays, was examined. For lamellae presented with their typical contour in a plane other than the observer's frontoparallel plane, the results of both experiments indicated that participants encoded lamellae in such a way that their typical contour appeared to be closer to the observer's frontoparallel plane than the respective stimulus. The relationship of the present findings to previous results on children's drawings and to the concept of the picture plane are considered.

Perceived similarity of shapes is an asymmetrical relationship: a study of typical contours.

Two experiments are reported in which the relationship between perceived similarity amongst simple geometric solids and the set of projections arising from rotation of the typical contour of the solids about a vertical axis in its plane are examined. The results indicate (1) that judgments of similarity are affected by pronouncedness of typical contours, (2) similarity of simple geometrical solids is an asymmetrical attribute, in that where a set of possible projections of model X contains a set of possible projections of model Y, then model Y is judged as more similar to model X than X is to Y. The implications of this asymmetry for perceptual constancy and theories of object recognition are discussed.

W H R Rivers (1864-1922): the founder of research in cross-cultural perception.

This paper presents a brief review of Rivers's contribution to psychology, especially to his cross-cultural studies of the basic perceptual processes of vision, and examines the lasting influence of his findings. It reports briefly on the scientific milieu in which Rivers carried out his work and traces the growth of his interest in the psychology of perception from the early days of his medical work to his appointment as a foundation lecturer of two English universities, London and Cambridge. It is argued that Rivers's contribution has been undervalued by psychologists.

Sex differences on Bartel's task: an investigation into perception of real and depicted distances.

A new method derived from Bartel's (1958) studies was used to investigate sex differences in spatial perception. Bartel employed two related tasks; one of these tasks called for responses to a pictorial stimulus representing spatial arrangement in perspective and the other for responses to an analogous task presented in three dimensions. Modified forms of both these tasks were used. Consistent differences between men and women were found, the men showing greater distance constancy in relation both to real and to depicted distances. In addition a decline of such constancy with age was observed. The pictorial task was also used to test two groups of students of architecture: one relatively inexperienced and the other more experienced with the discipline, on the assumption that experience of spatial judgments might influence performance on this task. It was found that whilst familiarity did not affect responses there was a consistent difference between responses of men and women.

The depiction of distance: a Bartelian analysis.

Work by Bartel has shown that observers make systematic errors when attempting to set a receding row of staves at equal intervals, and make even-more-pronounced errors in the same direction when drawing a similar row; consequently, the factors that contribute to this systematic distortion in spatial representation were examined. The results indicate that the tendency to overestimate nearer distances and underestimate further distances in pictorial space may be enhanced both by increasing the size ratio of the nearest and furthest pictorial elements and by increasing the angle of convergence of represented parallel contours. However, it is clear that size ratio is the more powerful independent variable. The reasons for this discrepancy are discussed and an explanation of the pattern of pictorial distortion, in terms of a compromise between an objective representation of the relative estimated distance of the depicted distances and an inaccurately anticipated optical projection, is proffered.

Viewing angle and the perceived orientation of pictorial elements: geometric or representational effects?

The role of geometric shape properties in determining the perceived orientation (heading) change of picture elements was investigated in four experiments. In experiment 1 the systematic change in perceived heading of each of a depicted team of horses which projected irregular geometric shapes was measured. In experiment 2, involving silhouettes, the perceptual axes of these same horse shapes, together with those of a pair of enantiomorphic deltoid shapes, were derived. These derived shape axes were used in experiment 3, along with single oblique contours, to determine the degree of heading shift as a function of the complexity and orientation of the shape. The degree of heading shift was remarkably similar for both silhouette shapes and the single contours when the axes were oblique but, whereas vertical orientation was predicted to abolish heading shift, this was only true for the deltoid shapes. In experiment 4 a possible explanation of the nonpredicted heading shift for the vertically oriented horse silhouettes was investigated. Subjects' individual estimates of the axes of the horse shapes, rather than a group mean value, were used to set the horse shapes to the vertical. When viewing was from two positions no significant heading change with view was found. Taken as a whole the data suggest that the geometric properties of patterns, rather than their representative nature, determine the very obvious heading shift when pictures are viewed from different angles.

Perceptual impulsions and unfamiliar solids: evidence from a drawing task.

Previous work (Dziurawiec & Deregowski, 1992) has shown that children's distorted drawings of animal models may be explained by the child's tendency to depict typical contours, the outlines of the surfaces which undergo pronounced change. The present paper investigates whether the typical contours notion can be extended to purely geometric solids. Results from a drawing task by children aged nine and eleven years, using unfamiliar models of varying complexity, indicate that the tendency to draw in perspective increases with the increase in figure complexity for both age groups, but younger children show a greater reliance on typical contours than older children. Recasting the data from previous drawing experiments (Bartel, 1928/1958; Cox, 1986) further confirms the utility of the typical contours approach. Finally, the advantages of such an approach over that of canonicity (cf. Palmer, Rosch & Chase, 1981) for the representation of solids are elaborated.

The perception of spatial structure with oblique viewing: an explanation for Byzantine perspective?

Earlier work has confirmed that (i) observers can judge divergent receding lines, placed directly in front of them, to be parallel, and (ii) converging lines which are displaced laterally, so that they are viewed obliquely, can also be judged to be parallel. The former observation is in accord with traditional views of perspective while the latter, which is in accord with the depictions of objects found in Byzantine painting, is not in accord with perspective but is predicted by the relative magnitude of the visual angles subtended by the near and far ends of the pair of lines. To investigate whether these effects occurred when the stimulus was clearly three-dimensional, experiments were conducted with a novel apparatus, consisting of a framework of computer-controlled motor-driven luminous rods. This could be remotely adjusted so that all visible sides appeared to be parallel, ie to resemble a cube. Results showed that observers set the sides of this trapezohedron framework as diverging when it was viewed immediately in front of them, a result which is concordant with linear perspective, ie they see the normal projection of a cube as having converging edges. When the framework was displaced from the median plane so that it was viewed obliquely, the sides were set as converging and the magnitude of this effect was significantly related to angle of view, ie observers see the normal projection of a cube as having diverging sides.(ABSTRACT TRUNCATED AT 250 WORDS)

Three-space inference from two-space stimulation.

Oblique contours sloping at 30 degrees with respect to the horizontal were presented alone, in combination to form chevrons, or with a vertical line to form arrowhead or Y patterns; they were projected onto a screen in the frontal parallel plane and viewed from positions that gave viewing angles of 90 degrees (normal to the screen's surface), 53 degrees, or 34 degrees. The perceived orientation of the contours, as assessed by a movable arm that the subjects set to be parallel to the obliques, changed monotonically as a function of viewing angle. The change was as great for single obliques as for combinations of obliques within the chevron, arrowhead, and Y patterns. The results of Experiment 1 were extended in Experiment 2, in which obliques at 30 degrees and 50 degrees with respect to the horizontal were presented singly or in combination as chevron patterns. It is argued that the results of both experiments indicate that single two-space oblique lines are immediately interpreted as lying in three-space and that the changes in perceived orientation are a consequence of this perceptual inference.

Convergent and divergent perspective.

Use of divergent (or inverse) perspective in pictures is often regarded as arbitrary or even as erroneous in spite of the fact that entire schools of art exist in which this kind of perspective is regularly used. An experiment is reported which shows that a significant trend towards divergent perspective is experienced by subjects viewing laterally displaced three-dimensional arrays. Centrally viewed arrays show the expected perceptual convergence. It is therefore argued that divergent perspective, under appropriate conditions, is as perceptually legitimate an experience as convergent perspective.

Geometric restitution of perspective: Bartel's method.

Bartel's geometric restitution of perspective is explained and illustrated by application to two well-known works of art. Applications of the methods of Bartel and others are briefly discussed.

On a changing perspective illusion within Vermeer's The Music Lesson.

The effect of line of sight on the perception of spatial configuration has been investigated in a well-known painting (The Music Lesson by Vermeer) and in two control patterns. In experiment 1, subjects indicated the perceived inclination of two major contours which defined the sidewall-floor and backwall-floor joints in (i) a projected image of the painting, (ii) a three-line representation of the major spatial elements of the painting, and (iii) a three-dimensional wire model of these same contours, when standing in each of eighteen positions on a line running parallel to the surface of the screen. Results indicated a significant change in the perception of the sidewall-floor, but not of the backwall-floor contour, as viewing angle changed, in both the painting and the three-line representation. However, the angular setting in the latter case was significantly less than when the painting was used, ie subjects underestimated the depicted inclination. Settings for the wire model did not deviate with viewing angle and reflected geometrically correct adjustments. In experiment 2, the results of experiment 1 were confirmed using enantiomorphs. These findings are discussed in the light of other view-dependent illusions in paintings.

Seeing the impossible and building the likely.

Young & Deregowski (1981) state that the reason for some subjects' failure to perceive 'impossible' figures as confusing is probably that they do not integrate the stimuli, even though they see elements of the figures as depicting spatial arrangements. This suggests that such subjects should also tend to build distorted models of geometrical structures. This hypothesis was tested in the course of the present experiment. The results support the hypothesis.

A cross-cultural comparison of the use of a Gestalt perceptual strategy.

The ability to discriminate whether pictured box shapes (parallelopipeds) are projections of three-dimensional rectangular forms has been demonstrated by Perkins and Cooper in US populations and interpreted as a symptom of a general Gestalt strategy in perception. Deregowski suggested earlier that this perceptual strategy might not appear as strongly in less 'carpentered' cultures, among perceivers less familiar with Western modes of depiction. A study is reported in which the performance on the discrimination by US children in grades 1, 4, and 7; and children from Zimbabwe, Africa, in grades 1, 2, 4, and 7--children of less experience with pictures and urban environments--has been examined. All groups evinced the discrimination at high levels of statistical significance. However, the findings disclosed much less accurate performance in the Zimbabwe groups at all grade levels, and no improvement with age either in the US or in Zimbabwe. The absence of improvement argues against an explanation of the difference between the US and Zimbabwe groups in terms of either a carpentered-world hypothesis or a difficulty with picture perception, at least when those interpretations are taken in their simplest forms.

Within-race and between-race recognition of transformed and untransformed faces.

Numerous studies have shown that people from one racial group experience difficulties in remembering the faces of people from other racial groups. Two experiments are reported that explore the phenomenon more fully by examining subjects' ability to recognize faces transformed slightly in pose between study and test. Published work shows that white subjects can recognize such transformed white faces as well as they identify untransformed faces. In the first experiment, it was shown that white subjects were unable to recognize transformed black faces. The second experiment, following the pilot study, investigated recognition of both black and white, transformed and untransformed faces by Europeans and Africans. This study revealed that people from one racial group are particularly disadvantaged in recognizing other-race faces that have undergone transformation. The results are discussed in relation to learning approaches to face memory.

Learning to see the impossible.

Four experiments investigating children's ability to detect the impossibility of impossible figures are reported. In the first, children were required to identify the impossible figure from a pair of corresponding possible and impossible figures. Whilst seven-year-old children were able to detect the impossibility of certain impossible figures, their overall level of performance was lower than that of older children. Regardless of age, the impossibility of some types of figure was found to be relatively easy or difficult to detect. Experiment 2 confirmed this pattern of results using a task that required children to copy possible and impossible figures from memory. Experiment 3 showed that, when the impossibility of an impossible figure is not readily detected, this is not due to failure to understand the conventions used in that figure to represent depth and solidity. In experiment 4 predictions from different hypotheses concerning the principal factor responsible for the detection of impossibility were tested. Results support the view that the detection of impossibility requires the construction of a mental representation (internal model) of the inter-relations of the constituent parts of the depicted object. It is suggested that the construction of such internal models may be of general importance in picture perception.

Depicted angle of forms and perception of line drawings.

Subjects were presented with projections of either rectangular or elliptic forms which appeared either on their own or upon the following figures: (a) a face of a portrayed cube, (b) a parallelogram identical with (a), and (c) a rectangle of equal width and area to (a). Four different angles of projection were used for both the forms and for the background figures. In the composite figures these angles were varied independently. The subjects were required to judge the width of the forms. The forms were judged to be wider when presented on backgrounds (a) and (b) than when presented on background (c) or alone, especially at greater angles of projection. This suggests that the implicit-shape constancy effect is independent of relational effects resulting from the frame surrounding the form.