The Surrogate Functions of Lines in Visual Perception: Evidence from ...

Perception, 1974, volume 3, pages 3 1 3 - 3 2 2

The surrogate functions of lines in visual perception: Evidence from antipodal rock and cave artwork sources

J M Kennedy, Judy Silver Scarborough College, University of T o r o n t o , T o r o n t o , Canada Received 20 March 1974; in revised f o r m 6 June 1974

Abstract. Lines in outline pictures may function as surrogates for features of the visible environment, without explicit tutoring in a convention (unlike representation in words). This interpretation of line representation is supported by the finding that drawings from ancient sources—in antipodal regions of the world—contain outline depiction of the same features as are found in modern outline sketches. 1 Introduction Gibson (1951) challenged a tradition in perceptual research that had used simple line forms as stimuli, by claiming that all too often simple line displays can be seen as pictures, a fact that may prevent these forms from counting as the basic building blocks of perception. Gibson did not ask quite how deeply rooted in the perceptual system is the pictorial power of lines, though it is clear that he thought pictorial form influences apparent depth and attention. In this paper we will examine line and pictorial perception, by responding to recent developments in Gibson's general ideas about perception, by noticing what lessons can be learnt from recent experiments on line depiction, and by calling attention to important regularities that may run as universals through man's earliest attempts in the caves of prehistory to explore the stimulation afforded by simple line displays. The thrust of this paper will be the claim that lines can be 'surrogates' (Hochberg, 1962) for what we may call, following Gibson, the basic sources of optical structure, and this surrogate function is one that lines can perform in the absence of training in any convention. 2 Pictorial reactions Consider lines drawn on a flat surface, i.e. an outline sketch. When one merely notices the number, length, color, and distribution of the lines on the plane surface, one is seeing form in itself, form as such, and one is taking the normal 'visual world' attitude. Sometimes the line display can be seen in quite a different way. Seen with a pictorial attitude, it can be regarded as a representational outline drawing, i.e. as picturing a quite differently shaped object than the collection of lines, flat on the paper (cf Arnheim, 1954, pp.97, 244). For example, converging lines viewed with the pictorial attitude might be seen as depicting the parallel edges of a slanted surface. Thus, if the experimenter asked for judgements of size and location, two observers might give radically different judgements, if one was taking the visual world attitude exclusively, and the other was engaging in pictorial perception. Are there any constraints on the range and variety of different pictorial reactions? Is the 'generalization' from a line to a solid object governed by any consistent rules? Firstly, of course, subjects say they do not literally see the depicted object, only its representation, a fact which distinguishes pictorial perception from visual-world perception. Secondly, as Gibson as well as Rubin (1915) noted in studies of the pictorial functions of simple line sketches, brief line forms often phenomenologically represent the edges of a surface, a surface arranged in the shape of an object or part of an object,

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wherein there are components ('edges') and a whole form ('object') made up of or circumscribed by these components. How are the components (lines) and wholes (patterns) in the outline drawing related to the components (edges) and wholes (objects) in pictorial perception? Gibson (1951) emphasized perception as opposed to 'convention' as a basis for outline representation. Gibson rejected the view that learning to view pictures is like learning to read a conventionalized writing system. The implication was that there was some intrinsic connection between human perception of objects and human perception of line forms, albeit line forms (1) omit veridical color and texture, (2) are self-evidently all in one plane unlike the objects they depict, and (3) are made of traces like elongated pencilmarks with two color-changes or contours while an edge of an object comprises a single change in slant or depth where there may be no color-change whatsoever. Gibson was taking a bold step, for at the time it was popular to assert that pictures were artificial devices, based on learned conventions, abstruse to untutored 'natives' or 'unacculturated tribesmen'(Kidd, 1904; Bieusheuvel, 1947; Herskovits, 1948). Since 1951, evidence from children (notably Hochberg and Brooks, 1962), retardates (O'Connor and Hermelin, 1961), and other cultures [e.g. South Africa: Hudson (1960, 1967), Page (1970); Ghana: Mundy-Castle (1966); Ethiopia: Deregowski et at (1972)] suggests that Gibson was correct. Little if any explicit training is necessary to obtain consistent identifications of line sketches by quite diverse groups of subjects. This evidence has been summarized by Kennedy (1974a). At this point the evidence seems strong enough to warrant a more substantial analysis of the relationship between lines and common objects than Gibson offered in 1951. To what components of objects are outlines related? While a whole word stands for an object though its parts are not related to the parts of the object (as 'wh' at the start of 'whale' does not stand for the snout of a whale), it may be that the lines at the beginning of the whale's picture indeed stand for features at the head of a whale. But what are those features? 3 Lines and components of the visual world Since 1951, Gibson has systematized his analysis of the visible features of the human environment—the sources of the optical structure of the eye—although his writings on pictures remain at a general theoretical level (Gibson, 1954, 1960, 1966, 1971). As variation of the environment that patterns the light at the eye, he mentions, at various times; variation in the layout of surfaces, variation in pigmentation in a surface, variation in illumination on a surface, and variation in the texturing of a surface (cf Clifford, 1946; Metzger, 1936; on texture see Gibson, 1950). Variation in surface layout is the most common source of optical structure depicted in outline drawings. Surfaces can be planar or curved, background or foreground, and thus outline sketches typically are surrogates for the following different arrangements: (a) Convex comers, like the outside corner of a room. (b) Concave corners, like the interior corner of a room. While both of these arrangements are standard in contemporary drawings (and it has recently been shown by Perkins, 1972, that an uncomplicated naturalistic perspective geometry governs their appearance in depictions), it will be tentatively suggested later that, despite their apparent simplicity, they are unexpectedly rare in early pictures (i.e. cave pictures), perhaps because they refer to parts of an ecology built by man—the carpentered world—rather than the ecology surrounding man the hunter (Lee and De Vore, 1968) and man the iceage cave-dweller (Cuvay, 1963).

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(c) Occluding edges with background surfaces. Lines depict arrangements of a foreground flat surface overlapping a background surface in figure-ground cases where the background has the appearance of a surface color (cf Rubin, 1915, pp 4 1 - 5 6 , 1 1 4 - 1 1 5 ; Hochberg, 1962, p.33). (d) Occluding edges without background surfaces. Lines depict layout where a foreground surface is 'suspended', as it were, in a structureless surround, as though the foreground surface is silhouetted against the sky or some other medium with no definitively-located surface, in figure-ground cases where the background has the appearance of a volume color (cf Rubin, 1915, pp 5 6 - 6 4 ; Arnheim, 1954, pp 220-221, 223-224). (e) Occluding bounds with a background surface, like the occluding parts of a sphere resting on a ground. The term occluding bound is not used by Gibson, but was introduced to the Gibsonian lexicon in a thesis supervised by Gibson (Kennedy, 1971). It is intended to refer to the case where a single continuous rounded surface gradually slants back from the observer, so that a rounded object has a visible front and a hidden back, divided by a tangent from the point of observation. A sphere has no edges, strictly speaking, but can have a front and back divided by an occluding bound, behind which can be a background surface. In a closely related case, a line may also depict: ( 0 An occluding bound with no background surface, like the horizon against a cloudless sky. It is quite common for an outline drawing to contain depictions of each and all of the above six features. In addition, there is a special case, where a line in an outline drawing depicts a wire, a ridge, a crack, or the like. While corners and edges are made of only one change of slant or depth, things like wires and cracks are physically made up of several edges, close together and parallel and elongated. Perhaps all of these cases can be considered under one category—a seventh category of line depiction: (g) Parallel features of surface layout, close together and elongated. In referring to this category later, the phrase 'parallel features' will be used, for the sake of brevity. For example, the question will be asked: "Do early cave paintings contain widespread use of single lines depicting parallel features?" One reason for supposing line depiction of surface edges of the above kinds is that line drawings of 'impossible' combinations of surfaces (Penrose and Penrose, 1958), like that shown in figure 1, elicit perceptual confusion. The effects in figure 1 are almost impossible for the normal observer to analyze and explain. A useful contrast is a picture of a contemporary person—say Mao Tse Tung—in an anomalous historical setting—say as Washington crossing the Delaware—which the

Figure 1. An impossible object that produces perceptual conflict.

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normal observer can indicate is inappropriate and explain why. Figure 1 contains perceptual conflict, while Mao as Washington is a conceptual conflict. Further support for the inherently perceptual interpretation of line depiction of features of surface layout can be sought with ambiguous drawings like a circle divided by a single diameter (figure 2). The diameter can be seen as a wire or an occluding edge or a crack between adjoining surfaces or an occluding bound like a horizon seen through a porthole. Standard figure-ground perceptual phenomena accompany perceptual switches in appearance of the diameter from one surrogate function to any of the other surrogate functions (Attneave, 1971). Edges of surfaces are not the sole source of optic structure. A division in the light reaching the eye may have originated at a plane surface because of change of pigmentation, or shadows, or texture. Kennedy (1970, 1971, 1974a, 1974b) finds that subjects (untrained in the particular convention) can identify line depictions of these features of the visible world, although subjects generally report that such line depiction lacks the immediacy and 'illusory' power of line depiction of corners and change of depth. As will be noted later, line depiction of pigment, shadow, and texture patterns is uncommon in a wide variety of cultures. Another kind of support for the inherently perceptual interpretation of line depiction might be sought with brain-injured subjects. In a revealing note, Rubin (1915, pp 163-164) described a subject investigated by Storck who could identify solid objects and colored pictures of objects (e.g. a watering-can, a house), but could not identify line drawings of the same objects. One interpretation of this case is that the surrogate functions of lines had been lost as a result of the injury, while other forms of perception including color perception were unimpaired. Further cases throwing light on this possibility would be useful; at present the case mentioned by Rubin can only be treated as a tantalizing lead. In sum, it seems that line drawings depict whole objects in ways understandable by untrained eyes, apparently by acting as surrogates for the physical ecological features that normally structure light to the eye.

Figure 2. The diameter of a circle can have different figure-ground appearances. 4 Lines in early outline drawings from antipodal regions The striking impression created by perceptual figures, the phenomenological effects— notably figure-ground effects— of some outline drawings and their recognizability by young, untutored and retarded subjects—these threads of evidence all combine towards a conclusion that line representation is a deeply-rooted ability, inherent to normal vision. If indeed the surrogate functions of lines are perceptual rather than conventional then one might seek further evidence in the artifacts of different cultures, in different ages and different locations around the globe, on the hypothesis that wherever outline drawings are found they will have uniform bases. While the

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words for 'goat' or 'spear' or 'man' may vary from culture to culture, perhaps line depictions of objects may be as readily understood today as when they were drawn, centuries ago, because identical elements are depicted in the same way now as then. While language relates objects and elements of sound on an arbitrary basis, perhaps outline depiction relates lines to features of the environment on a uniform basis wherever they are found. The idea is that there should be a common core to outline depictions from different isolated cultures in the antipodal regions of the world. Despite local variations in ornament or style or type of object depicted, or choice of materials used, or choice of tools used to create the pictures, outline pictures may always depict the same features, irrespective of the culture producing them. With this hypothesis in mind, representations of single objects in outline drawings from early or 'primitive' art—on cliff-faces, on naturally-occurring stone surfaces like the inside of caves, the class of artifact called 'parietal' or rock-art—were collected and analyzed. A classification scheme based on the features listed above was devised, tested for inter-judge reliability, and exemplars of each of the basic categories in the scheme were sought in the artwork from different regions. Monographs treating rock art from various parts of the world were obtained from the Toronto University research library. The search yielded 30 volumes (a list is available on request). The reproductions in each volume were then examined for appropriate outline art work. A figure was rejected if (a) it was an inscription rather than a picture, (b) it was a formal or geometrical design rather than a picture, (c) it was scribbles (a 'macaroni' network of lines as these are called) rather than a picture, or (d) it was a 'serpentine' network of lines, resembling snakes but without clear identifying internal features like eyes, tongues, or mouths. Ten features were coded. Each feature is based on the list of environmental variations that structure light to the eye, as described above. The ten were: concave corner, convex corner, occluding edge, occluding edge overlapping a background surface, occluding bound, occluding bound overlapping a background surface, 'parallel features' as described above, pigment change in a surface (contours), margins of shadows on a surface, and texture change on a surface. Each of the figures selected from the rock-art monographs was examined and coded for the presence or absence of these features. The second author selected and coded all the parietal art. The reliability of the coding was checked in three ways. Firstly, a set of contemporary outline drawings were selected by the first author and coded by both authors. The agreement was 88% and all disagreements were quickly settled after joint discussion. Secondly, a set of parietal drawings were selected by the second author and coded by both authors. The agreement was 92% and all except two cases were settled by discussion. The last two cases, both authors agreed, were dubious, but in a forced-choice situation the two cases would be coded differently by the two authors. The third check was applied to rare cases. Where very few instances of a feature were discovered, both authors considered each instance and discussed them. Of these rare cases only those cases where the authors concur are used in the analysis below. The parietal art that was coded comes from widespread parts of the globe. The sizes of the samples from each region varied considerably, from 20 from New Zealand to 222 from Northern Africa, i.e. from Egypt to Morocco (see table 1). The regions sampled in this study do not, of course, exhaust the regions in which parietal art is found. The present analysis simply attempts to cover widely separated regions of the globe and does not attempt to be exhaustive. Both percentages and some absolute numbers are given here, but caution must be used in interpreting these figures, for these numbers reflect not only the work of the original artists, but possible biases introduced by differential weathering of early and later art, selection biases of the authors whose books have been sampled in this study,

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and the selection criteria described above. Where figures are enormously differentlike a 50% compared to a 5%—some interpretation may be justified. Also one clear example of some particular feature may constitute an important 'existence proof demonstrating that line depiction of this feature was meaningful to some early men. Accordingly, some rare cases will be given respectful prominence alongside larger average figures in this summary of findings. A grand total of 657 examples of parietal art were given detailed analysis. Of these, 99% contained examples of occluding bounds; in 30% the occluding bounds overlapped background surfaces. 66% contained 'parallel features' like cracks or wires, 3% had occluding edges and 1% (5 cases) had depiction of pigment change. Not one single example from the 657 included line depiction of corners, boundaries of shadows, or the locus of a texture change. (Ten doubtful or 'mixed' cases will be considered later.) Interestingly, almost all the background surfaces that were depicted were internal to the object being depicted, like one limb being overlapped by another in the same animal. Only in 10 cases was an occluding bound seen as overlapping a surface detached from (i.e. external to) the object. Only 2 occluding edges were shown as overlapping a surface external to the depicted foreground object. In some respects there is considerable uniformity across the regions in the kinds of features depicted. By far the most common feature is an occluding bound, and all of the regions except New Zealand manifested this feature in more than 98% of the figures coded. The reason for a lower proportion (80%) in the New Zealand sample was a preponderance of 'stick-figures' where each line depicted an entire limb of an object. At the opposite extreme, occluding edges were, by and large, uniformly rare in all regions. In percentage terms, the Australian and Northern Scandinavian rock art revealed the most occluding edges—7% and 12% respectively—but one should note that the absolute numbers (3 and 6 respectively) are quite small. The Spanish sample revealed no occluding edges. The 'parallel features' category was present frequently in all regions, ranging from 45% in the sample from Australia to 79% in the examples from Northern Africa. Table 1. Line representation in rock-art outline drawings. Country

Number of drawings

Australia France New Zealand North Africa North America Norway Southern Africa Spain

44 190 20 222 47 50 51 33

Totals Per cent

657 100

Occluding edgesa

Occluding boundsb

Parallel features

3 3 1 6 1 6 2

44 190 16 222 46 49 50 33

20 110 13 175 32 37 31 16

22 3

650 99

434 66

Pigment contours

Mixed casesc

a

A background surface was indicated in 202 depictions of occluding bounds. Of these 10 had the background surface detached from the foreground object and the occluding surface. b A background surface was indicated in 5 depictions of an occluding edge. Of these 2 had the background surface detached from the foreground object and the occluding surface. c

In the mixed cases the line depicted an environmental feature where depth and color and texture changed concurrently.

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More varied results are shown by lines depicting the kind of overlap called here 'occluding bounds with background surfaces'. In one area these are not found (New Zealand). In the North American sample only one example was found—in this case the background surface was not part of the same object as the foreground overlapping surface, i.e. the background was external to the object. The Norwegian sample also had one case—in that case too the background surface was external to the foreground object, i.e. the background was detached from the foreground object. There were 2 cases in the Australian sample where a line depicted overlap and in both of these cases the background surface was internal to the object. However, fully 46% of the French sample and 42% of the Spanish sample included depictions of occluding bounds with background surfaces (almost all these cases were of overlap internal to the depicted object, with only one case in each region of a foreground object overlapping a detached or external background object). Some features that are uncommon in contemporary outline depictions are equally uncommon in parietal art. Lines are rarely used in modern outline drawings to indicate pure color or texture change in the surface of objects. There are only a few cases (5 in all, from Australia and France) of parietal depictions that may incorporate depiction of patterns of contours (pure pigment-change). The discussion of these will be reserved until a little later. Corners (none) and occluding edges (22 overall) were rare in the parietal art though such features are common in contemporary art. Discussion of these will also be reserved for later, and will be incorporated into a brief description of the kinds of objects depicted. 5 Objects depicted in parietal art The examples of depictions of contours in rock art were in the context of depictions of animals. Rather than reproduce the shades on an animal's hide, the artist had used lines to indicate the margins of patches of pigmentation, for example to indicate the banding on the tail of a kangaroo or to indicate change of color on the flanks of a horse. The rarity of lines depicting contours may be due to several factors. Firstly, it is not usually necessary to indicate patterns of plumage or hide-markings in order to make an object recognizable. Often it is only when the genus or the individual is to be depicted that pigmentation and markings are important. Secondly, to use lines to represent pigment contours is less satisfactory than using shading, for the line alone cannot indicate which side is darker than the other or what the hues on either side are. (The parietal artist might reasonably plan to first draw lines that depict layout change and to daub colors afterwards to show the correct pigmentation.) Thirdly, it may be that early man was more concerned with depicting objects like mammoths, his fellow man, and implements, rather than giraffes or zebras, the kinds of animals that have distinctive patterns in their hides. There were only a few examples of line depictions of contours—of these, one that was drawn on the flank of a horse was independently called a depiction of a pigment contour by Gidion, a Swiss aesthetician (1968), a fact which adds support to the present coding. Though the examples are few, it seems fair to conclude that early man did accept a line as a surrogate for a contour or change of color on an otherwise uniform surface. No pure examples of depiction of texture change were found, but there were some dubious or mixed cases that may be relevant to texture change. There were 10, all from France and Northern Africa, where lines depicted the boundaries of a hoof or a beak, a boundary where the color and texture of the animal's body change simultaneously. There is no obvious reason why these cases are found in only two regions. One might conjecture that these aspects of an animal had become important

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in the cultures of these regions. Tentatively, the conclusion is that some early cultures, but not all, used lines to indicate a combination of texture and color change. Parietal art seems to have used line to depict 'parallel features' a good deal. Lines were used to represent tufts of hair on animals, poles, appendages, limbs of animals and human beings, divisions between adjoining parts of an animal (e.g. the crack in the hoof of an antelope), and so forth. Lines used in this fashion serve as an extremely convenient pictorial device. It was not necessary to show much of the hair on the back or the mane of an animal—a few judicious strands would suggest the whole fur coat, with a kind of etcetera principle (Gombrich, 1960, p.331; Arnheim, 1971, p.18; Kennedy, in press). In some regions the entire object was often depicted by a few lines and shown as a 'stick-figure', for example in Australia and New Zealand. Lines used to depict 'parallel features' were found in all regions, which suggests this use of line is easily understood by the human visual system. There were remarkably few examples of an object being shown against a background. The subject of the picture was typically depicted without any reference to his habitat. This does not mean, however, that depicted overlap was not understood by parietal artists. In several regions (Australia, France, Spain, Northern Africa and Southern Africa) many examples were found where within the depicted object one part of the body overlapped another part. Evidently, overlap depth cues were widely understood in parietal art. A Gestaltist would say that lines were often used to serve a figure-ground function. In Gibsonian terms, the lines were used to show an occluding front surface against an occluded rear surface. Figure 3 is an instance from Southern Africa, where there is a foreground giraffe overlapping a more distant giraffe, and the lines of the fore giraffe act as surrogates for occluding bounds against background surfaces. This figure also shows internal parts of one giraffe overlapping each other. There were no convex or concave corners to be found in the sample. Also, there were only 22 instances of lines depicting occluding edges; the 22 were widely distributed, Spain being the only region without an instance. Corners and edges are made by flat surfaces and it seems reasonable to assume that the world of the parietal artist contained few flat surfaces or sharp edges for him to depict. Indeed, one of the few Would be the comparatively flat rock surface used as the canvas for the art-work. As corroboration of this interpretation it is interesting to note that most

Figure 3. Part of a rock-art display from Southern Africa (after Leo Frobenius).

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instances of occluding edges are in depictions of man-made articles such as spear tips, arrowheads, parts of canoes, clothing, and so forth. In contrast, contemporary outline drawings make frequent use of corners and edges. Their abundance in modern drawings and their rarity in parietal art seems best accounted for by the kind of carpentered or natural environments available to the artist; only the modern artist has many examples of corners and edges in his habitat—in buildings and furniture. This account can also explain why occluding bounds are common in both parietal and contemporary outline drawings. Both the natural and the carpentered environment contain many examples of rounded objects—limbs, balls, trees, and hills. Depiction of occluding bounds was probably essential to the parietal artist, if he was to make any part of his environment apparent in a picture. In overview, it seems that ancient and modern outline artwork present many commonalities. Whether the drawing is from early New Zealand or from primitive caves in Spain or cliff faces in northern Scandinavia or from the pages of today's newspaper, many similarities can be found. Lines are used to depict similar features of objects, like the edges of their surfaces, often with overlap, and perhaps also patterns marked out by the boundaries of areas of pigment on their surfaces. While modern man would probably have found the parietal artist's language baffling, he would have been able to understand much of his art work without question. In conclusion, the common fact, brought forcefully into perceptual psychology by J. J. Gibson, that lines can look like, or depict, objects seems to have deep roots in the visual system. The surrogate functions of lines have been shown to be present in untutored children and subjects with very low IQs, it may be a specific perceptual capacity one can lose in brain damage without loss of visual-world or color perception, and its universality in human visual systems is suggested by the similarity of early outline drawings from diverse, antipodal regions of the world. References Arnheim, R., 1954, Art and Visual Perception (University of California Press, Berkeley). Arnheim, R., 1971, Art and Entropy (University of California Press, Berkeley). Attneave, F., 1971, "Multistability in perception", Scientific American, 135, 62-71. Bieusheuvel, S., 1947, "Psychological tests and their application to non-European peoples", in Yearbook of Education, Ed. G. B. Jeffrey (University of London Press, London). Clifford, W. K., 1946, The Common Sense of the Exact Sciences (Knopf, New York). Cuvay, R., 1963, Prehistoric Cave Painting, translated by M. Shenfield (Methuen, London). Deregowski, J. B., Muldrow, E. S., Muldrow, W. F., 1972, ''Pictorial recognition in a remote Ethiopian population", Perception, 1, 417-425. Gibson, J. J., 1950, 77ze Perception of the Visual World (Houghton-Mifflin, Boston). Gibson, J. J., 1951, "What is a form?", Psychological Review, 58, 403-412. Gibson, J. J., 1954, "A theory of pictorial perception", Audio-visual Communication Review, 1, 3-23. Gibson, J. J., 1960, "Pictures, perspective and perception", Daedalus, 216-227'. Gibson, J. J., 1966, The Senses Considered as Perceptual Systems (Houghton-Mifflin, Boston). Gibson, J. J., 1971, "The information available in pictures", Leonardo, 4, 27-35. Gidion, S., 1968, "Outline in early art", Leonardo, 2, 181-192. Gombrich, E. H., 1960, Art and Illusion (Princeton University Press, Princeton). Herskovits, M. J., 1948, Man and His Works (Knopf, New York). Hochberg, J. E., 1962, "The psychophysics of pictorial perception", Audio-visual Communication Review, 10, 22-54. Hochberg, J. E., Brooks, V., 1962, "Pictorial recognition as an unlearned ability", American Journal of Psychology, 75, 624 - 628. Hudson, W., 1960, "Pictorial depth perception in sub-cultural groups in Africa", Journal of Social Psychology, 52, 183-208. Hudson, W., 1967, "The study of the problem of pictorial perception among unacculturated groups", International Journal of Psychology, 2, 89-107. Kennedy, J. M., 1970, "Outlines and shadows", paper presented at the meeting of the American Psychological Association, Miami.

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Kennedy, J. M., 1971, "Line representation and pictorial perception", Doctoral dissertation, Cornell University (unpublished). Kennedy, J. M., 1974a, Images and Information (Jossey-Bass, San Francisco). Kennedy, J. M., 1974b, "Icons and information", in 73rd National Society for the Study of Education Yearbook, Ed. D. Olson (University of Chicago Press, Chicago). Kennedy, J. M. (in press), Pictures, perception, etcetera", in Studies in Perception: Essays in honor of J. J. Gibson, Ed. H. Pick (Cornell University Press, Ithaca). Kidd, D., 1904. The Essential Kafir (Black, London). Lee, R. B., De Vore, I., 1968, Man the Hunter (Aldine, New York). Metzger, W., 1936, Gesetze des Sehens (Kramer, Frankfurt am Main). Mundy-Castle, A. C, 1966, "Pictorial depth perception in Ghanaian children", International Journal of Psychology, 1,289-333. O'Connor, N., Hermelin, B., 1961, "Like and cross-modality recognition in subnormal children", Quarterly Journal of Experimental Psychology, 11, 48-52. Page, H. W., 1970, "Pictorial depth perception: A note", South African Journal of Psychology, 1,45-48. Penrose, L. S., Penrose, R., 1958, "Impossible objects: A special type of illusion", British Journal of Psychology, 49,31-37. Perkins, D., 1972, "Visual discrimination between rectangular and non-rectangular parallelopipeds", Perception and Psychophysics, 12, 396-400. Rubin, T., 1915, Synsoplevede Figurer (Gyldendals, Copenhagen).

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