Anim Cogn (2003) 6 : 253–258 DOI 10.1007/s10071-003-0188-5
O R I G I N A L A RT I C L E
Tomoko Imura · Masaki Tomonaga
Perception of depth from shading in infant chimpanzees (Pan troglodytes )
Received: 1 March 2003 / Revised: 29 July 2003 / Accepted: 29 July 2003 / Published online: 11 November 2003 © Springer-Verlag 2003
Abstract We investigated the ability to perceive depth from shading, one of the pictorial depth cues, in three chimpanzee infants aged 4–10 months old, using a preferential reaching task commonly used to study pictorial depth perception in human infants. The chimpanzee infants reached significantly more to three-dimensional toys than to pictures thereof and more to the three-dimensional convex than to the concave. Furthermore, two of the three infants reached significantly more to the photographic convex than to the photographic concave. These infants also looked longer at the photographic convex than the concave. Our results suggest that chimpanzees perceive, at least as early as the latter half of the first year of life, pictorial depth defined by shading information. Photographic convexes contain richer information about pictorial depth (e.g., attached shadow, cast shadow, highlighted area, and global difference in brightness) than simple computer-graphic graded patterns. These cues together might facilitate the infants’ perception of depth from shading. Keywords Depth from shading · Reaching · Looking time · Chimpanzees · Infants
Introduction Humans can extract three-dimensional information from two-dimensional stimuli such as photographs and pictures on the basis of linear perspective, occlusion, shading, and so on. These cues are called pictorial depth cues. The ability to perceive pictorial depth has been discussed with reT. Imura Graduate School of Humanities, Kwansei Gakuin University, Uegahara, Nishinomiya, Hyogo 662-8501, Japan M. Tomonaga (✉) Section of Language and Intelligence, Primate Research Institute, Kyoto University, Inuyama, Aichi 484-8506, Japan Tel.: +81-568-630549, Fax: +81-568-630549, e-mail:
[email protected]
spect to its biological basis and cultural influences (Gunderson et al. 1993; Hershberger 1970; Hess 1950, 1961). Some pictorial depth cues provide ambiguous clues for extracting three-dimensional information. For example, shading is defined as gradation in brightness. Since brightness is determined by three parameters, luminance of the light source, reflectance of the surface of the object, and the orientation of the surface, combinations of these parameters yield infinite types of gradations. To explain how depth is perceived from such ambiguous information, Ramachandran (Kleffner and Ramachandran 1992; Ramachandran 1988) proposed that our visual system has two hypotheses for the processing of shading information. First, there is only one light source, and second, this light source provides illumination from “above” the retinal coordinates. These hypotheses are ecologically valid for humans, who have evolved in an open environment with only a single sun shining from above. Therefore, we perceive a shaded circle with top bright and bottom dark as convex and vice versa. Kleffner and Ramachandran (1992) found that humans detected circular targets more easily when these were shaded vertically (top bright and bottom dark or vice versa) than when shaded horizontally (left bright and right dark or vice versa) in a visual search task. These results support the hypotheses described above. In addressing the questions concerning nature–nurture bases in the perception of pictorial depth, there exist various possible avenues for research. The first is a cross-cultural approach. The second is a developmental approach. The third is a comparative or comparative–developmental approach. Regarding the first approach, many studies have investigated cultural differences in the perception of visual illusions, such as the Müller–Lyer and Ponzo illusions (e.g., Jahonda 1966; Segall et al. 1966). The second approach has also received considerable attention, and there are increasing numbers of studies with human infants from the age of 3 to 7 months focusing on the perception of pictorial depth (familiar size: Yonas et al. 1982; relative size: Yonas et al. 1985; occlusion: Granrud and Yonas 1984; shading: Granrud et al. 1985; shading and line junctions: Bhatt and Waters 1998; line junctions: Bhatt and
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Bertin 2001; linear perspective: Arterberry et al. 1989; texture gradients: Yonas et al. 1986). It is plausible that organisms that have adapted to environments different from that inhabited by humans may have different hypotheses concerning pictorial depth information. Furthermore, animals have no “cultural” experiences in various types of pictorial depth perception, such that the third approach is indeed useful for the study of pictorial depth perception. There exists some previous work on the perception of pictorial depth using nonhuman animals from the comparative–cognitive, comparative–developmental, and neurophysiological perspectives (chicks: Hershberger 1970; Hess 1950, 1961; Regolin and Vallortigara 1995; pigeons: Reid and Spetch 1998; pig-tailed macaques: Gunderson et al. 1993; Japanese macaques: K. Fujita and S. Kanazawa unpublished data; Hanazawa and Komatsu 2001; Tsuji et al. 2000; horses: Timney and Keil 1996). Tomonaga (1998), for example, examined the perception of shape from shading in adult chimpanzees and humans using visual search and texture segregation tasks. Chimpanzees have evolved in three-dimensionally rich tropical rain forests, which have different features from the savanna environment to which humans have adapted. Results of this study do not differ from those reported by previous authors in the case of the human subjects, but opposite patterns were obtained for the chimpanzees. Chimpanzees showed easier detection in the horizontal than in the vertical shading condition. Tomonaga suggested two possible explanations for this discrepancy. The first is that chimpanzees had different hypotheses from those of humans for the processing of shading information as a result of adaptation to relatively different environments. The second is that they relied more on local cues than on pictorial depth cues from shading, such as brightness contrast (cf. Miura and Kawabata 1999, 2000). There are relatively few studies from the standpoint of comparative development (e.g., Gunderson et al. 1993; Hershberger 1970; Hess 1950, 1961). Some of these studies tried to address more directly the questions related to hypotheses concerning shading (Hershberger 1970; Hess 1950, 1961). In these studies chicks were raised in artificial environments where a controlled light source provided illumination. The results were, however, inconsistent across researchers. Hess (1950, 1961) found that chicks were influenced by the position of the light source (top or bottom), but Hershberger (1970) found no effects of lighting conditions. In another comparative–developmental study, Gunderson et al. (1993) tested infant pig-tailed macaques on the perception of pictorial depth from linear perspective and relative size differences. They found that pig-tailed macaque infants at the age of 7–8 weeks reliably reached with their arms toward the “near” object when they viewed the stimuli monocularly. There are no reports of studies with nonhuman primate infants focusing on the perception of depth from shading. In the present study, we examined the perception of depth from shading in three infant chimpanzees at the age of 4–10 months under the preferential reaching task. We recorded reaching and looking time as behavioral measures
based on Gunderson et al.’s (1993) study, using photographs of convexes and concaves as stimuli. As in the case of human infants (Granrud et al. 1985), the aim was to examine the transfer of reaching responses from three-dimensional convex objects to two-dimensional convex patterns in the chimpanzee infants.
Methods Subjects Three chimpanzee infants (Ayumu: male, born on 24 April 2000, Cleo: female, born on 19 June 2000, and Pal: female, born on 9 August 2000) participated in the present experiment. Ayumu and Pal were conceived through artificial insemination and Cleo through natural copulation. Ayumu and Pal had the same biological father. All three infants were raised by their biological mothers and lived in an outdoor compound (approximately 700 m2) with 11 adult chimpanzees including their mothers and fathers. Ayumu was tested at 20–39 weeks of age (4–9 months old), Cleo at 23–41 weeks of age (5–10 months old), and Pal at 18–39 weeks of age (4–9 months old). Before the present experiment (2–5 months old), they were preliminarily tested for the perception of shape from shading using a habituation–dishabituation procedure with computer-generated shading patterns. During the present experiment, none of the infants were ever deprived of food or water. Care and use of the chimpanzees adhered to the “Guide for the care and use of laboratory primates” published by the Primate Research Institute, Kyoto University. Stimuli and apparatus We used three sets of stimuli in the present experiment (Fig. 1). The first set included three-dimensional toys and their photographs (see Fig. 1A). We prepared 12 different toy–photograph pairs (e.g., scale models of humans, animals, cars, half-cut fruit toys; Fig. 1A). The average size of the toys was 5.7×4.9 cm. Photographs were printed so that the size of the image equaled that of the real toy. Photographs were taken with a digital still camera and retouched using computer software (Photoshop and Paintshop Pro). These toys and photographs were attached to the left and right sides of gray panels (actual size: 30.0×12.0 cm). The second set included three-dimensional (real) convexes and concaves. Gray half-cut table tennis balls (3.6 cm in diameter) were attached to the gray panels as convex and concave. The third set comprised two-dimensional (photographic) convexes and concaves (Fig. 1B); these were photographic versions of the three-dimensional real convexes and concaves. Photographs of convexes and concaves differed in brightness, attached shadows, and cast shadows. The toy set was used for baseline training and for baseline trials in test sessions. The remaining two sets were used in the test sessions (see Procedure). These panels were attached to a micro CCD camera (SONY, Type CCD-MC100), which recorded the infants’ gaze directions. Procedure Experimental sessions were conducted in a test booth for chimpanzees (1.8×2.1×1.9 m). Each mother–infant pair came to this booth from the outdoor compound. One human experimenter entered the booth together with the pair (Fig. 1A). Each infant either was held by the mother or sat on the floor or on the experimenter’s lap during testing. Infants could move their arms freely during the experimental sessions. Each trial began by the presentation of the stimulus panel 30 cm away from the infant’s face and as close to the midline of the infant’s body as possible and was terminated when the infant reached out with the arm and touched one of the stimuli on the panel within
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Fig. 1 A A chimpanzee infant (Ayumu) looks at the panel and reaches toward the three-dimensional toy (half-cut peach) in a baseline trial (photo courtesy of the Mainichi Shimbun). B An example of a test stimulus used in the two-dimensional convex/concave test trials. Left convex, right concave
30 s (Fig. 1A). Infants looked at the panel binocularly. When an infant did not touch any stimuli within 30 s, the trial also terminated and was coded as “no occurrence of response.” We used two types of reaching responses as behavioral measures. One was manual reaching, defined as extending the arm and touching the stimulus on the panel with the finger(s), and the other was oral reaching, defined as approaching and touching the stimulus with the lips and tongue. Incidental touching caused by sudden changes in posture or without looking at the stimulus was not counted as a response. Responses were judged by the experimenters inside and outside the booth independently. When both experimenters reported a response in a given trial, that trial was coded as “occurrence of response.” The present experiment consisted of two types of sessions. The first type was a baseline session in which only the toy set was used. A baseline session consisted of 12 trials for Ayumu and 6 trials for Cleo and Pal. The second type was a test session in which all three sets of stimuli appeared. A test session consisted of 8 toy trials (baseline trials), 2 three-dimensional convex/concave trials, and 2 two-dimensional convex/concave trials for Ayumu; and 4 toy trials, 1 three-dimensional convex/concave trial, and 1 two-dimensional convex/concave trial for Cleo and Pal. In each session, the positions of toy and photograph, or convex and concave, as well as the order of trials was randomized. On average, Ayumu received 2 baseline and 1 test sessions per week, while Cleo and Pal received 2 baseline and 2 test sessions per week. In total, Ayumu received 51 sessions (33 baseline and 18 test sessions), Cleo received 58 sessions (29 baseline and 29 test sessions, although video recording of 1 test session was missing), and Pal received 56 sessions (29 baseline and 27 test sessions).
We analyzed two response measures: the number of reaching responses, and looking-time duration until the reaching response. We conducted preliminary tests of correlations between these response measures and ages across subjects and found no systematic (i.e., increasing, decreasing, or U-shaped) trends. On the basis of these preliminary analyses, all the data collected during the experimental period were pooled, and we counted the number of trials in which the reaching response occurred in each type of session, and each type of trial within test sessions. These numbers were divided by the total number of trials for each condition to calculate the proportion of reaching responses. From the videotapes, we coded the infant’s eye directions every 0.1 s (three video frames) from the presentation of the stimulus panel to the reaching response. When the subject made a reaching response after the first 15 s or made none at all, we only coded the infant’s gaze within the first 15 s. Infants’ direction of eye gaze was categorized into left, center, right, and other (including looking away). On the basis of these coded data, we calculated looking times for the target side, the distractor side, and the central area for each trial. Since reaching latencies varied from trial to trial, we calculated the relative looking time for either side of the panel, with actual looking time for each side divided by the latency of reaching response for each trial. One author (TI) coded all the data, and another individual coded approximately 10% of the data. Intercoder reliability estimated with Cohen’s kappa was 0.67 and was considered as “substantial” according to Landis and Koch’s (1977) interpretation scale of kappa coefficients. Because of the small number of subjects (n=3), we conducted statistical tests on a within-subject basis. For reaching responses, we conducted binomial tests based on the frequency of reaching responses; for relative looking time, we conducted paired-comparison two-tailed t-tests based on each test trial for each subject. These tests were conducted separately for three- and two-dimensional convex/concave trials. To correct experiment-wise type I error, alpha levels for each test were corrected on the basis of Sidak’s inequality (cf. Holland and Copenhaver 1987). For each measure for each subject, we conducted two independent tests (three- and two-dimensional test trials), so that the corrected alpha levels for 0.05, 0.01, and 0.001 were set to 1–(1–0.05)1/2=0.0253, 1–(1–0.01)1/2=0.005, and 1–(1–0.001)1/2=0.0005, respectively.
Results Reaching response During the baseline sessions, all infants showed more reaching responses toward the real toys than toward the photographs (71.3% of the trials to the real toys and 9.9% to the photographs, averaged across subjects). During the test sessions, they again showed more reaching responses toward the toys than toward the photographs, much like in the baseline sessions (68.3% and 8.3%). Figure 2 shows the percentage of trials in which the subjects reached toward three- and two-dimensional convexes and concaves during test sessions. As this figure illustrates, each subject showed significantly more reaching responses toward the convex than the concave in the three-dimensional convex/concave trials [binomial tests, Ayumu, 27/27 (reaching to convex/total reaching responses), P
