Perception &c Psychophysics
`1992, 52 {1), 18-36
`
`On the perception of shape from shading
`
`DOROTHY A. KLEFFNER and V. S. RAMACHANDRAN
`University of California, San Diego, La Jolla, Calif:orp.~a
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`Perception & Psychophysics
`/992, 52 (/). 18-36
`
`On the perception of shape from shading
`
`DOROTHY A. KLEFFNER and V. S. RAMACHANDRAN
`University of California, San Diego, La Jolla, Cali[or;TJ.ia
`
`The extraction of three-dimensional shape from shading is one of the most perceptually com(cid:173)
`pelling, yet poorly understood, aspects of visual perception. In this paper; we report several new
`experiments on the manner in which the perception of shape from shading interacts with other
`visual processes such as perceptual grouping, preattentive search ("pop-out"), and motion per(cid:173)
`ception. Our specific findings are as follows: (1) The extraction of shape from shading informa(cid:173)
`tion incorporates at least two "assumptions" or constraints-first, that there is a single light source
`illuminating the whole scene, and second, that the light is shining from "above" in relation to
`retinal coordinates. (2) Tokens defined by shading can serve as a basis for perceptual grouping
`and segregation. (3) Reaction time for detecting a single convex shape does not increase with the
`number of items in the display. This "pop-out" effect must be based on shading rather than on
`differences in luminance polarity, since neither left-right differences nor step changes in luminance
`resulted in pop-out. (4) When the subjects were experienced, there were no search asymmetries
`for convex as opposed to concave tokens, but when the subjects were naive, cavities were much
`easier to detect than convex shapes. (5) The extraction of shape from shading can also provide
`an input to motion perception. And finally, (6) the assumption of "overhead illumination" that
`leads to perceptual grouping depends primarily on retinal rather than on "phenomenal" or gravita(cid:173)
`tional coordinates. Taken collectively, these findings imply that the extraction of shape from shad(cid:173)
`ing is an "early" visual process that occurs prior to perceptual grouping, motion perception, and
`vestibular (as well as "cognitive") correction for head tilt. Hence, there may be neural elements
`very early in visual processing that are specialized for the extraction of shape from shading.
`
`We use three-dimensional (3-D) depth perception to
`find our way around the world and to manipulate ob(cid:173)
`jects that we encounter. Although the retinal image is
`two-dimensional, somehow the brain is able to use the
`information from this image to yield an experience of so(cid:173)
`lidity and depth.
`Of the numerous mechanisms used by the visual system
`to recover the third dimension, the ability to use shading
`is probably phylogenetically one of the most primitive.
`One reason for believing this is that in the natural world,
`animals have often evolved the principle of countershading
`to conceal their shapes from predators; they have pale bel(cid:173)
`lies that serve to neutralize the effects of the sun shining
`from above (Thayer, 1909). The prevalence of counter(cid:173)
`shading in a variety of animals (including fishes) suggests
`that shading must be a very important source of informa(cid:173)
`tion about 3-D shapes.
`Although artists have long recognized the importance
`of shading, there have been few studies of how the hu(cid:173)
`man visual system actually extracts and uses this infor(cid:173)
`mation. Since the time when Leonardo da Vinci first
`
`We thank the Air Force Office of Scientific Research (Grant 89-{)414)
`and the Office of Naval Research (Grant N00014091J-1735) for funding
`this research, and H. Pashler, D. Plununer, D. Rogers-Ramachandran,
`A. Yonas, and T. Sejnowski for stimulating discussions. Requests for
`reprints and other correspondence should be sent to V. S. Ramachan(cid:173)
`dran, Psychology Department 0109, University of California, San Diego,
`La Jolla, CA 92093-0109.
`
`thought about this problem, there have been only a small
`handful of systematic psychological studies on it (Ber(cid:173)
`baum, Bever, & Chung, 1983; Brewster, 1847; Howard,
`1983; Ramachandran, 1988a, 1988b; Rittenhouse, 1786;
`Todd & Mingolla, 1983).
`We began our investigations by creating a set of sim(cid:173)
`ple computer-generated displays (Figure 1). The impres(cid:173)
`sion of depth perceived in these displays is based exclu(cid:173)
`sively on subtle variations in shading that we made sure
`were devoid of any complex objects and patterns. Our
`purpose, of course, was to isolate the brain mechanisms
`that process shading information from other mechanisms
`that may also contribute to depth perception in real-life
`visual processing. So the displays are intended to serve
`the same role in the study of shape from shading that
`Julesz's stereograms (Julesz, 1971) do in the study of
`stereopsis.
`We have recently used these computer-generated dis(cid:173)
`plays to discover a simple set of "rules" or constraints
`that the visual system uses in the interpretation of 3-D
`shape from shading (Ramachandran 1988a, 1988b). For
`example, Figure I depicts a set of objects that conveys
`a strong impression of depth. The sign of perceived depth,
`however, is ambiguous, since the visual system has no
`way of knowing where the light source is. Consequently,
`the display can be perceived as consisting of either con(cid:173)
`vex objects illuminated from the right or concave objects
`lit from the left ("eggs" or "egg-crate"). The reader can
`generate a depth inversion as though mentally "shifting"
`the light source.
`
`Copyright 1992 Psychonomic Society, Inc.
`
`18
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`PERCEPTION OF SHAPE FROM SHADING
`
`19
`
`Figure I. These computer-generated displays convey an impression of depth based exclusively
`on subtle variations in luminance. The sign of perceived depth is ambiguous. Each object can be
`perceived as either convex and lit from the right or concave and lit from the left, but all of the
`objects tend to be viewed with the same sign of perceived depth.
`
`Interestingly , when a depth inversion occurs , it tends
`to occur simultaneously for all objects in the display . Is
`this propensity for seeing all objects in the display as be(cid:173)
`ing simultaneously convex (or concave) based on a ten(cid:173)
`dency to assign identical depth values to all of them, or
`is it based on the tacit assumption that there is only one
`light source in the image? To find out, we used a mixture
`of objects that were mirror images of each other (Fig(cid:173)
`ure 2) . In this display, when the top row of objects was
`seen as convex, the bottom row was always perceived as
`concave, and vice versa. It was in fact impossible to see
`all the objects as being simultaneously convex or concave.
`This observation suggests that when interpreting shape
`
`from shading, the visual system incorporates the tacit as(cid:173)
`sumption that there is only one light source illuminating
`the entire visual image (or a large portion of it; Ramachan(cid:173)
`dran, 1988b). Hence the derivation of shape from shad(cid:173)
`ing cannot be a strictly local operation; it must involve
`" global " assumptions about light sources .
`Note that, as in Figure 2, a row can be seen as either
`convex or concave if the other row is excluded. When
`both rows are viewed simultaneously, however, seeing
`one row as convex forces the other row to be perceived
`as concave. Some powerful inhibitory mechanisms must
`be involved in the generation of these effects. The single(cid:173)
`light-source assumption is, of course, implicit in many
`
`Figure 2. The single-light-source assumption, demonstrated through the use of a mixture
`of shaded objects that are mirror images of each other: Objects in one row can be seen as
`either convex or concave if the other row is excluded; but when both rows are viewed simul(cid:173)
`taneously, seeing one row as convex forces the other row to be perceived as concave.
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`20
`
`KLEFFNER AND RAMACHANDRAN
`
`Figure 3. This computer-generated photograph demonstrates that the visual system has a built(cid:173)
`in "assumption" that the light source is shining from above. Note that the depth in these displays
`is conveyed exclusively through shading, with no other depth cues present. The shaded objects in
`the top panel are usually seen as convex, whereas those in the bottom panel are usually seen as
`concave. Note, however, that the illusion (i.e., the difference between convex and concave) is not
`as pronounced as it is in Figure SA, in which the objects are intermixed.
`
`artific"ial intelligence models, but Figure 2, as far as we
`know, is the first clear-cut demonstration that such a rule
`actually exists in human vision for the extraction of shape
`from shading. Bergstrom (1987) has pointed out that such
`a rule may also be involved in the computation of surface
`lightness.
`In addition to the single-light-source constraint de(cid:173)
`scribed above, there appears also to be a built-in assump(cid:173)
`tion that the light is shining from above, a principle first
`suggested by Sir David Brewster (1847). This would ex(cid:173)
`plain why, in Figure 3, objects in the top panel are usually
`seen as convex, whereas those in the bottom panel are
`often perceived as "holes" or "cavities." The sign of
`depth can be readily reversed by simply turning the fig(cid:173)
`ure upside down. The effect is weak, however, since
`either panel can be seen as convex if the other is excluded
`from view to eliminate the single-light-source constraint.
`On the other hand, if a mixture of such objects is pre(cid:173)
`sented, it is almost impossible to reverse any of them be(cid:173)
`cause of the combined effect of two constraints-the
`single-light-source constraint and the "top" -light-source
`constraint (Figure 5A).
`Next, we wondered what would happen to the interpre(cid:173)
`tation of shape from shading if one were to give the visual
`
`system conflicting information about the light source's lo(cid:173)
`cation. To explore this, we created the display shown in
`Figure 4. The central disks are identical in A and B, with
`a vertical gradient. The surround in A has a conflicting
`horizontal gradient, which could not occur with a single
`light source illuminating the display. The figure was
`shown to 48 naive subjects, who were asked to examine
`the two panels (A and B) carefully and compare the two
`central disks. Their task was to judge which of the two
`central figures (A or B) appeared more convex. The re(cid:173)
`sults were clear-cut; the central disk in panel B almost
`always appeared to be more convex than did the central
`disk in panel A (72 out of 96 trials) . In fact, many sub(cid:173)
`jects spontaneously reported that the disk in panel A
`almost appeared flat. We may conclude, therefore, that
`the magnitude of depth perceived from shading is en(cid:173)
`hanced considerably if objects in the surround have the
`opposite polarity, a spatial contrast effect that is vaguely
`reminiscent of the center-surround effects that have been
`reported for other stimulus dimensions such as motion
`(Nakayama & Loomis, 1974) and color (Land, 1983;
`Livingstone & Hubel, 1987). Another way of saying this
`would be that the perception of shape from shading is en(cid:173)
`hanced considerably if the information in the scene is com-
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`PERCEPTION OF SHAPE FROM SHADING
`
`21
`
`Figure 4. This display demonstrates "center-surround" interactions in the perception of shape
`from shading. The central disk in panel A is usually seen as less convex than the identical one in
`panel B. These effects are usually much more pronounced on the CRT than they are in the printed
`versions shown here. This effect demonstrates that the magnitude or perceived depth is also influ(cid:173)
`enced by the single-light-source constraint (after Ramachandran, 1989b).
`
`patible with a single light source. When the information
`from the majority of objects (e.g., panel A) suggests that
`the light source is on the left (or right), the shading on
`the central object is perceived as a variation in reflectance
`rather than depth (Ramachandran, 1989b).
`What if the location of the light source was revealed
`by some obvious means? This question was first raised
`by Berbaum et al. (1983). They asked subjects to view
`a muffin pan illuminated from below while holding a hand
`nearby to cast a shadow-thereby revealing the light
`source. Berbaum et al. found that many subjects now re(cid:173)
`ported a reversal of relief. Oddly enough, we did not find
`this to be true for our computer-generated displays. A hol(cid:173)
`low mask lit from above looks like a "normal" (convex)
`face lit from below. But if the eggs and cavities in Fig(cid:173)
`ure 5A are placed right next to it, their depth does not
`reverse (Ramachandran, l988a), in spite of the fact that
`the face now "reveals" the light to be corning from be(cid:173)
`low. Yet we found that if the eggs and cavities are directly
`pasted on the face with their outlines blurred in order to
`"blend" them into the face, then their depth does indeed
`reverse (i.e., the eggs become cavities, and vice versa).
`We may conclude, therefore, that the knowledge about
`the new light source location, revealed by the face, does
`not generalize to apply to other items in the display un(cid:173)
`less these items are seen as belonging to the face-that
`is, as being parts of the same object. Or, to put it differ(cid:173)
`ently, the single-light-source rule is adhered to more
`rigidly for different parts of an object than it is for differ(cid:173)
`ent objects in a scene.
`Note that it is also possible to group all the convex
`shapes in Figure 5A together mentally to form a cluster
`that is clearly segregated from the background of con(cid:173)
`cave shapes. This result is surprising, for it is usually as(cid:173)
`sumed that only certain elementary stimulus features such
`
`as orientation, color, and "terminators" can be grouped
`together in this way (Beck, 1966; Julesz, 1971; Treisman,
`1985, 1986). Figure 5A shows that even 3-D shapes that
`are conveyed by shading can provide tokens for percep(cid:173)
`tual grouping and segregation (Ramachandran, l988a,
`1988b). To make sure that the effect was not due to some
`more elementary image feature (such as luminance polar(cid:173)
`ity), we produced a control stimulus (Figure 5B), in which
`the targets were similar to those in Figure 5A in terms
`of luminance polarity but did not convey any depth. In
`this display, it is difficult to segregate the tokens on the
`basis of differences in polarity, suggesting that the effects
`observed in Figure 5A must be based on 3-D shapes.
`Segregation is also much more pronounced for top-down
`differences in illumination than for left-right differences.
`For instance, if Figure 5A is rotated by 90°, the degree
`of segregation is also reduced correspondingly . This fur(cid:173)
`ther supports the view that the effect depends on the 3-D
`shapes of the tokens rather than on luminance polarity
`(Ramachandran, l988a, l988b).
`Our purpose in the rest of this communication is to de(cid:173)
`scribe some formal experiments that we carried out to con(cid:173)
`firm and extend our earlier observations (Kleffner &
`Ramachandran, 1989; Ramachandran, l988a, l988b).
`Our preliminary observations, described in Figure 5A,
`suggested that shape from shading can serve as an elemen(cid:173)
`tary feature for perceptual grouping; but would the same
`results also hold for effortless preattentive search or ''pop(cid:173)
`out''? Consider the case of a single egg displayed against
`a background of several cavities. The extent to which reac(cid:173)
`tion times vary with the number of items in a display is
`often used as a criterion to decide whether a particular
`visual feature is detected ''preattentively'' or not. If sub(cid:173)
`jects do not have to search for the target-that is, if they
`can spot it without inspecting every item on display-
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`22
`
`KLEFFNER AND RAMACHANDRAN
`
`®
`
`®
`
`Figure 5. (A) This figure contains a random mixture of shaded objects that have opposite luminance
`polarities. 1be ones that are light on top are usually perceived as spheres that can be mentally grouped
`together and segregated from tbe background of concave objects. Hence we may conclude that three(cid:173)
`dimensional shapes defined by shading can provide tokens for perceptual grouping and segrega(cid:173)
`tion. If the figure is rotated 90•, segregation becomes much more difficult. (B) Tokens in this con(cid:173)
`trol display have the same luminance polarity as the shaded images do, but they do not convey
`depth information. Segregation of the tokens is difficult to achieve.
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`then the feature in question is, by definition, "elemen(cid:173)
`tary. '' The reaction time for spotting such a target will
`not increase linearly with the number of distractors (Treis(cid:173)
`man , 1985, 1986). We decided to use this criterion to find
`out whether or not an egg would appear to pop out against
`a background of cavities. Subjects were simply asked to
`report the presence or absence of a single egg against a
`background consisting of a varying number of distractors
`(cavities).
`·
`
`EXPERIMENT 1
`Visual Search for 3-D Shape from Shading
`
`Method
`Subjects. Five subjects participated: the 2 authors, I other re(cid:173)
`searcher in the lab, and 2 undergraduate research assistants .
`Display. This display, as well as subsequent ones described in
`this paper, were all generated on a CRT driven by an Amiga micro(cid:173)
`computer. The targets and distractors are illustrated in Figure 6.
`Targets and distractor items subtended 1.0° of visual angle and were
`placed in random positions without overlap, within a display area
`6 . 1 o high x 6.6° wide . On each trial, I, 6, or 12 items were dis(cid:173)
`played, with half the trials containing one target and the remaining
`trials containing no targets. Targets and distractors were constructed
`from 16 luminance levels ranging from .057 to 136. 1 'cd/m2 and
`presented on a background of 14.6 cd/m2
`•
`Procedure. The subjects were seated . 75 m from the screen in
`) for
`a dark room. Each trial began with a dark screen (.057 cd/m2
`0 .8 sec, followed by the presentation of a fixation point on a gray
`background (0.76 cd/m2
`) for 1.8 sec. The experimental stimulus
`was then displayed. Two keys on the keyboard were used by the
`subjects to indicate whether the target was present or absent in the
`display, and the subjects' reaction times were recorded. A response
`from the subject ended the trial, and the screen was once again
`blacked out. Subjects were given feedback after each trial, consist(cid:173)
`ing of a '' + '' or ''- ' ' on a blank screen, which indicated whether
`or not the response was correct. This also served as the fixation
`point for the next trial.
`
`PERCEPTION OF SHAPE FROM SHADING
`
`23
`
`Each block of the experiment consisted of 48 trials presented in
`random order, 8 trials from each of 6 conditions (I, 6, or 12 total
`items, with the target item either present or absent). The subjects
`completed four experimental blocks for each target-distractor set.
`Prior to the collection of data for each condition, the subjects prac(cid:173)
`ticed the experiment with the test stimulus until they felt comfortable
`(this was done for at least dneblbck, but for less than four blocks) .
`
`Results
`The major findings of this study were that subjects' abil(cid:173)
`ity to detect targets shaded vertically was significantly dif(cid:173)
`ferent from their ability to detect either horizontal shad(cid:173)
`ing or a step change in luminance. These results are shown
`in Figure 7. In the first display, with shading from top
`to bottom, reaction times were not dependent on the num(cid:173)
`ber of items in the display (Kleffner & Ramacha~dran,
`1989). The slopes from this graph indicate an average
`reaction time of 4 msec per item when the target was
`present and 5 msec per item when the target was absent.
`But for the second display, shaded from left to right, reac(cid:173)
`tion times did increase with the number of items in the
`display, to 22 msec per item for the target present condi(cid:173)
`tion and 50 msec per item for the target absent condition.
`This difference in slopes suggests that a "serial search"
`strategy was being used. The third display, a step change
`in luminance, gave mixed results. For most subjects, the
`reaction times varied with the number of distractor items
`in the display, but there was substantial variability between
`subjects . The average reaction time was 8 msec per item
`for the target present condition and 18 msec per item for
`the target absent condition.
`A statistical comparison was made of the resulting
`slopes (reaction time vs. number of items in the display)
`from the graphs in Figure 7. A two-way analysis of vari(cid:173)
`ance (ANOV A) with repeated measures was performed,
`with the line slopes from the graphs as the dependent vari-
`
`Figure 6. Examples of the target -distractor sets used in Experiment 1. (A) An object shaded top
`to bottom had to be detected against a field of distractors shaded from bottom to top. (B) An object
`shaded from left to right had to be detected against distractors that were shaded right to left. (C) A
`step change in luminance in the vertical direction had to be detected against a background of dis(cid:173)
`tractors that had the opposite polarity.
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`24
`
`KLEFFNER AND RAMACHANDRAN
`
`®
`Visual Search Task
`Vertical Luminance Gradients
`
`Targe1 Presen1
`-
`- - Targe1 Absen1
`
`n = 5 subjects
`
`t
`
`Items In Display
`
`10
`
`12
`
`14
`
`c:
`0
`
`.. ..,
`u .,
`(/)
`.!:
`.,
`E
`j::
`c:
`.!!
`u ..
`.,
`a:
`
`1.4
`
`1.2
`
`1.0
`
`0.8
`
`0.6
`
`0.4
`
`0.2
`
`0.0
`
`1.4
`
`1.2
`
`1.0
`
`0.8
`
`0.6
`
`0.4
`
`0.2
`
`c:
`
`.. ..,
`0 u .,
`(/)
`.!:
`.,
`E
`j::
`c:
`
`.!! u ..
`.. a:
`
`®
`Visual Search Task
`Horizontal Luminance Gradients
`
`~ Target Present
`- . - - - Target Absent
`
`n = 5 subjects
`
`Items in Display
`
`10
`
`12
`
`14
`
`©
`Visual Sean;h Task
`Step Change in Luminance
`
`--a--
`Target Present
`----+--
`Target Absent
`n = 5 subjects
`
`c:
`0
`
`(/)
`.!:
`
`.. ..,
`u .,
`.,
`E
`j::
`c:
`
`.!! u .. .,
`
`a:
`
`1.4
`
`1.2
`
`1.0
`
`0 .8
`
`0.6
`
`0.4
`
`0.2
`
`0.0
`
`10
`
`12
`
`1 4
`
`Items In Display
`
`Figure 7. Results obtained from the visual search task, in which 5 experienced subjects participated. For vertical shading (A), the reac(cid:173)
`tion time is unaffected by the number of distractors in the display. For horizontal shading (B), however, subjects' reaction times increased
`monotonically with the number of items in the display. When the stimulus was a step change in luminance (C), reaction time generally
`increased with the number of items in the display, but there was considerable variability between subjects.
`
`able. The main effect for target type was significant at
`the .0 1 level [F(2, 16) = 8.129, p < .0038], indicating
`that subjects' performance was significantly different in
`the three experimental conditions. (The second factor in
`the ANOV A, whether the target was present or absent
`in each trial , was included in the analysis to account for
`variance. This factor, and the interaction between the fac(cid:173)
`tors, was not significant here or in the following three
`comparisons.) ANOVAs were also used to make a direct
`comparison between pairs of experimental conditions. In
`a comparison of top/bottom shading with left/right shad(cid:173)
`ing, the main effect for target type was significant at the
`
`.05 level [F(l ,8) = 10.886, p < .011]. Top/bottom shad(cid:173)
`ing against a step change in luminance also produced a
`significant main effect for target type at the .05 level
`[F(l ,8) = 9.058, p < .0168]. The difference between
`left/right shading and a step change in luminance, on the
`other hand, was not significant.
`
`Discussion
`These results suggest that the extraction of shape from
`shading can provide a basis for effortless or ''preatten(cid:173)
`tive" visual search, since reaction times do not increase
`with the number of distractors . The fact that such pop-
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`out is seen only for top-bottom differences in shading,
`and not for left-right differences, has two important im(cid:173)
`plications. First, it implies that the effect must be based
`on the extraction of 3-D shape from shading, not just from
`differences in luminance polarity. Second, the process
`must incorporate the assumption that the light is shining
`from above. Hence certain "scene-based" image charac(cid:173)
`teristics-such as the assumed location of light sources(cid:173)
`can influence visual search (Ramachandran,
`l988a,
`1988b), a point that has also been elegantly demonstrated
`in the recent experiments of Enns and Rensink (1990) .
`One anomalous finding is that the target defined by a step
`change in luminance also seemed to pop out more than
`one would expect from a casual inspection of Figure 58.
`The reason for this might be that even though no depth
`is visible in this display, the mere presence of a vertical
`luminance gradient (with white on top) is sufficient to
`stimulate whatever neural detectors are involved in signal(cid:173)
`ing convexity. The neurons may be excited suboptimally
`so that although the signal is strong enough to be detected
`in a search task, it is not strong enough to actually evoke
`a compelling sense of depth.
`
`EXPERIMENT 2
`Asymmetries in Visual Search
`
`Treisman and Gormican ( 1988) noted that it is easier
`to detect a "closed" circle against a background of Cs
`(open circles) than it is to detect a C against a background
`ofOs. They point out that such search asymmetries exist
`for a wide range of other types of visual features as well .
`Prompted by suggestions from A. Treisman and J. T.
`Enns, we decided to look for search asymmetries in the
`detection of 3-D shape from shading. In some prelimi(cid:173)
`nary experiments with experienced subjects, we found no
`evidence for an asymmetry, but we decided to repeat the
`experiments on naive subjects.
`
`PERCEPTION OF SHAPE FROM SHADING
`
`25
`
`Method
`Subjects. Six subjects from the undergraduate subject pool at the
`University of California, San Diego, participated in each of the con(cid:173)
`ditions of the experiment (18 subjects total).
`Display . The displays were identical to those in Experiment I ,
`with the exception that the target and distractor items were distin(cid:173)
`guished by top versus botton'l'shading, bottom versus top shading ,
`and left versus right shading. These are shown in Figure 8.
`Procedure. The procedu~e was identical to that used in Experi(cid:173)
`ment I , except that each set of 6 subjects participated in only one
`of the conditions (top vs . bottom shading, bottom vs. top shading ,
`and left vs. right shading). Comparisons were therefore made across
`subjects rather than within subjects.
`
`Results
`The results (see Figure 9) showed a striking asymmetry .
`Surprisingly, it was much easier to detect a cavity against
`a background of eggs than vice versa. 1 For detecting an
`egg, reaction times increased with the number of items
`in the display, suggesting serial search. The average reac(cid:173)
`tion time was 26 msec per item when the target was
`present and 50 msec per item when the target was absent.
`For detecting a cavity, however, reaction times did not
`increase with the number of items in the display. Aver(cid:173)
`age reaction times were 5 msec per item for both target
`present and target absent conditions. For the third dis(cid:173)
`play, which consisted of left to right shading, reaction
`times were again dependent on the number of items in
`the display-25 msec per item when the target was present
`and 60 msec per item when the target was absent.
`A comparison was made between the resulting graphs
`(plotting reaction time vs. number of items in the display).
`A two-way ANOV A without repeated measures was per(cid:173)
`formed, with the line slopes from the graphs as the de(cid:173)
`pendent variable. The main effect for target type was sig(cid:173)
`nificant at the .0001level [F(2,30) = 15.314,p < .0001],
`indicating that the subjects' performance was significantly
`different in the three experimental conditions. The sec(cid:173)
`ond factor, target presence/absence, was included in the
`
`Figure 8. Examples of the target-distractor sets used in Experiment 2: (A) An object shaded
`top to bottom had to be detected against a field of distractors shaded from bottom to top. (B) An
`object shaded from bottom to top had to be detected against distractors that were shaded top
`to bottom. (C) An object shaded from left to right had to be detected against distractors that were
`shaded right to left.
`
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`
`

`
`26
`
`KLEFFNER AND RAMACHANDRAN
`
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`Visual Search Task
`Detecting an 'egg'
`
`1.4
`
`Visual Search Task
`Detecting a 'cavity'
`
`--e--
`Target Present
`Target Absent
`-+---
`n "' 6 subjects
`
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`Target Present
`
`---- Target Absent
`
`n = 6 subjects
`
`10
`
`12
`
`1 4
`
`Items in Display
`
`Visual Search Task
`Horizontal Luminance Gradients
`1.4
`
`10
`
`12
`
`14
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`ilems in Display
`
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`~ Target Presen t
`- . - - Target Absent
`
`n "' 6 subjects
`
`0.0 +--~-.-~--.~-,-~.--~-,-~---.~--.
`0
`1 0
`12
`14
`
`Items in Display
`
`Figure 9. Visual search asymmetries in the extraction of shape from shading. Six naive subjects participated (see text). The reac(cid:173)
`tion time for detecting a "cavity" was unaffected by the number of items in the display. On the other hand, for detecting an "egg,"
`reaction time increased with the number of items in the display and the same was true for detecting left/right shading. These results
`demonstrate a striking asymmetry in the subjects' ability to detect cavities as opposed to eggs. This effect is seen only in naive sub(cid:173)
`jects. In subjects who have had considerable previous experience with such tasks (as have the authors), the asymmetries do not
`exist (Kleffner & Ramachandran, 1989).
`
`ANOV A to account for variance. The interpretation of
`this factor across experimental conditions is ambiguous,
`but it is included here for completeness. In the first
`ANOVA, this factor was significant at the .05 level
`[F(l ,30) = 12.649, p < .0013], while the interaction be(cid:173)
`tween the two factors was not significant. In order to com(cid:173)
`pare the experimental conditions directly, ANOV As were
`performed on the data from pairs of experimental condi(cid:173)
`tions. The ANOVA comparing top/bottom shading with
`bottom/top shading showed that these experimental con-
`
`ditions were significantly different at the .0001 level
`[F(1,20) = 48.325 , p < .0001]. The target present/
`absent factor was significant at the .01 level [F(1 ,20) =
`8.368, p < .009]; the interaction was not significant. The
`ANOV A comparing bottom/top shading and left/right
`shading was again significant at the .0001level [F(1 ,20)
`= 22 .295 , p < .0001] . (Both the target present/absent
`factor and the interaction were not significant.) In the
`ANOV A comparing top/bottom shading with left/right
`shading, the main effect for target type was not signifi-
`
`Legend3D, Inc. Ex. 2010-0010
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`
`

`
`cant. The target present/absent factor was significant at
`the .01 level [F(1 ,20) = 12.173, p < .0023], and the
`interaction was not significant.
`
`Discussion
`These results imply that naive subjects find cavities eas(cid:173)
`ier to detect than eggs. This seems surprising and counter(cid:173)
`intuitive, given the more widespread prevalence of "con(cid:173)
`vexity" in nature (Deutsch & Ramachandran, 1990;
`Hoffman, 1983), but since virtually nothing is known
`about the neural detectors that encode shape from shad(cid:173)
`ing, we should perhaps be prepared for such surprises.
`Treisman and G