Journal of Comparative Psychology 2014, Vol. 128, No. 4, 402– 413

© 2014 American Psychological Association 0735-7036/14/$12.00 http://dx.doi.org/10.1037/a0037499

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Do Primates See the Solitaire Illusion Differently? A Comparative Assessment of Humans (Homo sapiens), Chimpanzees (Pan troglodytes), Rhesus Monkeys (Macaca mulatta), and Capuchin Monkeys (Cebus apella) Christian Agrillo

Audrey E. Parrish and Michael J. Beran

University of Padova

Georgia State University

An important question in comparative psychology is whether human and nonhuman animals share similar principles of perceptual organization. Despite much empirical research, no firm conclusion has been drawn. The Solitaire illusion is a numerosity illusion in humans that occurs when one misperceives the relative number of 2 types of items presented in intermingled sets. To date, no study has investigated whether nonhuman animals perceive the Solitaire illusion as humans do. Here, we compared the perception of the Solitaire illusion in human and nonhuman primates in 3 experiments. We first observed (Experiment 1) the spontaneous behavior of chimpanzees when presented with 2 arrays composed of a different number of preferred and nonpreferred food items. In probe trials, preferred items were presented in the Solitaire pattern in 2 different spatial arrangements (either clustered centrally or distributed on the perimeter). Chimpanzees did not show any misperception of quantity in the Solitaire pattern. Next, humans, chimpanzees, rhesus monkeys, and capuchin monkeys underwent the same testing of relative quantity judgments in a computerized task that also presented the Solitaire illusion (Experiments 2 and 3). Unlike humans, chimpanzees did not appear to perceive the illusion, in agreement with Experiment 1. The performance of rhesus monkeys and capuchin monkeys was also different from that of humans, but was slightly more indicative of a potential Solitaire illusion. On the whole, our results suggest a potential discontinuity in the visual mechanisms underlying the Solitaire illusion between human and nonhuman primates. Keywords: visual illusions, solitaire illusion, chimpanzees, rhesus monkeys, capuchin monkeys

tion (Koffka, 1955; Wertheimer, 1938). Visual illusions in particular have captured much attention in comparative psychology, because of their possibility to shed light into the mechanisms surrounding ordinary perception. Nonhuman primates perceive a wide range of visual illusions, such as the Delboeuf illusion (chimpanzees: Parrish & Beran, 2014a), the Ponzo illusion (rhesus monkeys and chimpanzees: Fujita, 1996, 1997), the corridor illusion (baboons: Barbet & Fagot, 2002), and the Zöllner illusion (rhesus monkeys: Agrillo, Parrish, & Beran, 2014). Other mammals (cats: Bravo, Blake, & Morrison, 1988; mice: Kanizsa, Renzi, Conte, Compostela, & Guerani, 1993), birds (pigeons: Fujita, Blough, & Blough, 1991; domestic chicks: Regolin & Vallortigara, 1995, and Rosa Salva, Rugani, Cavazzana, Regolin, & Vallortigara, 2013), and fish (e.g., goldfish: Agrillo, Miletto Petrazzini, & Dadda, 2013) also showed behavioral responding that suggests a humanlike perceptual experience of different illusory patterns. However, mixed results also are reported in the literature. For example, although one study showed that pigeons exhibited amodal completion (Nagasaka, Lazareva, & Wasserman, 2007), other studies did not report the same outcome (Fujita, 2001, 2004; Fujita & Ushitani, 2005). Similarly, baboons do not seem to perceive the Ebbinghaus-Titchener size illusion, in which humans misperceive target circle size as a function of inducing circle size (Parron & Fagot, 2007), although distantly related domestic chicks perceive the illusion in a humanlike fashion (Rosa Salva et al.,

The issue of how nonhuman animals see the world has interested philosophers and scientists from ancient times. Although it is undeniable that sensory information is captured and processed differently across species because of the evolutionary differences in the vertebrate eye, the question remains as to whether such sensory information in nonhuman species is ever processed and grouped according to the Gestalt laws described in human percep-

This article was published Online First August 18, 2014. Christian Agrillo, Department of General Psychology and Center of Cognitive Neuroscience, University of Padova; Audrey E. Parrish, Psychology Department and the Language Research Center, Georgia State University; Michael J. Beran, Language Research Center, Georgia State University. This research was supported by funding from the National Institutes of Health to Michael J. Beran (Grant HD-060563); by the Duane M. Rumbaugh Fellowship and a 2CI Primate Social Cognition, Evolution and Behavior Fellowship from Georgia State University to Audrey E. Parrish; and by FIRB Grant 2013 (RBFR13KHFS) from Ministero dell’Istruzione, Universita` e Ricerca (MIUR, Italy) to Christian Agrillo. We thank Ted Evans for assistance with apparatus development and data collection, and the Language Research Center Staff for their care of the primates. Correspondence concerning this article should be addressed to Christian Agrillo, Department of General Psychology, University of Padova, Via Venezia 8, 35131 Padova, Italy. E-mail: [email protected] 402

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DIFFERENTIAL PERCEPTION OF THE SOLITAIRE ILLUSION

2013). One possible explanation for these discrepant results is that different studies often use different stimuli and procedures, thus making cross-species comparisons difficult to interpret. For instance, most of the studies comparing human and nonhuman species make use of extensive training to test nonhuman animals, whereas human participants are rarely trained because verbal instructions can be easily provided at the beginning of the experiment. It has even been shown that intraspecific and interspecific differences in the presence of the same illusory patterns can be reported if different training procedures are used (Beran, 2006). However, it is becoming more and more common (e.g., Fujita, 1996, 1997, 2001; Nakamura, Fujita, Ushitani, & Miyata, 2006) to use the same stimuli and a procedure that is, as far as possible, similar when comparing the performance of different species to assess potential similarities and dissimilarities regarding the universality of perceptual systems. A well-known category of illusory patterns consists of “numerosity illusions.” In these illusions, misperceptions of quantity or number occur through underestimation or overestimation, depending on the spatial arrangements of items within the arrays. For instance, adult humans required to judge numerosities of sets of circles are slower and more imprecise when circles are arranged such that some are inside others (nested) compared with when the circles do not overlap each other (Chesney & Gelman, 2012). This phenomenon of underestimating nested sets also has been reported in monkeys (Beran & Parrish, 2013). A similar phenomenon occurs with randomly arranged arrays; also known as the regular random numerosity illusion (RRNI), humans are affected by the presence of homogeneous sets such that we tend to overestimate the number of items in a regular pattern compared with when the same number of items are randomly arranged (Ginsburg, 1976, 1980). Nonhuman primates also are susceptible to the RRNI (Beran, 2006). Another numerosity illusion is called the Solitaire illusion. It occurs when one misperceives the relative numbers of two different colors of otherwise identical items in intermingled sets. This occurs because of the perception of internal clusters as being more numerous than external items that are not clustered in their appearance. In Figure 1, both arrays include the same number of dark and light items (N ⫽ 16), but centrally located items appear to be

Figure 1. Experimental setting used with chimpanzees. Chimpanzees were presented with two arrays that consisted of intermixed blue M&M’s and yellow cereal pieces. In this picture, the two arrays contained the same number of food items of each type (16 M&M’s and 16 cereal pieces in each array), but the spatial arrangement of the two arrays creates the Solitaire illusion, in which it appears to many humans that there are more M&M’s in the array at left than the array at right.

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more numerous than those located in the perimeter of the same pattern (i.e., most people would say there are more darker objects than lighter objects at left, and vice versa for the image at right). It has been suggested that centrally located items would form a single unit, a Gestalt, because of their continuity, whereas items located in the perimeter would form separate small clusters and thus violate this unifying principle (Wertheimer, 1938). Items forming a better Gestalt appear more numerous, leading to perception of the Solitaire illusion by humans (Frith & Frith, 1972). It is currently unknown whether this phenomenon also occurs in nonhuman species. Studying the perception of the Solitaire illusion in species other than humans may permit us to better understand the evolutionary roots of our perceptual systems, including perceptual biases interrupting our numerical judgments. In the present study, we investigated whether human and nonhuman primates perceive the Solitaire illusion in a similar manner using three experiments. In Experiment 1, we investigated the Solitaire illusion in chimpanzees using a two-alternative forced choice task with food arrays composed of a high-preference food (blue M&M’s) and a low-preference food (yellow cereal pieces). In control trials, the numbers of items differed across the choice options, and chimpanzees were expected to select the array containing the larger number of higher-preference, blue items regardless of the quantity of lower-preference, yellow ones. In test trials, an equal number of blue and yellow items were arranged in the specific cross-pattern of the Solitaire illusion: in one array, blue items were centrally located, and yellow ones were located in the perimeter; in the other array, the opposite displacement of blue and yellow items was shown. If chimpanzees perceived the illusion, they were expected to select the array in which the blue items were centrally located more often than they selected the opposite arrangement. In Experiment 2, we extended our investigation of the Solitaire illusion using a different methodology and additional primate species to test whether this illusion was impacted by the procedural approach used in the first experiment, and also to test how this effect emerges within different but closely related species. We tested humans, chimpanzees, rhesus monkeys, and capuchin monkeys in a computerized task that required them to select the larger number of white dots between two arrays composed of white and black dots. Computerized testing allowed for comparable assessment of the Solitaire illusion among the different species using as similar a methodology as possible, in addition to high trial counts and the complete removal of potential experimenter cues. Stimuli used in test trials were arranged as in Experiment 1, including infrequent probe trials that presented the Solitaire arrangement. Because of individual differences in the perception of the illusion by the monkey species in Experiment 2, we retested rhesus monkeys and capuchin monkeys using a novel color set in Experiment 3 to investigate the robustness of the Solitaire illusion.

Experiment 1 In Experiment 1, we investigated whether chimpanzees perceived the Solitaire illusion in a spontaneous choice task with sets of food items that were spatially arranged to reproduce the illusory pattern under investigation.

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Method Subjects. Four adult chimpanzees—two males (Mercury, age 26; Sherman, age 39) and two females (Lana, age 42; Panzee, age 27)—were tested. All chimpanzees were group housed at the Language Research Center of Georgia State University. They were given their normal daily diet of fruits, vegetables, and primate chow each day. Chimpanzees were never food or water deprived for testing purposes, and they worked to earn preferred food items. Lana, Panzee, and Sherman were language trained based on a lexigram system in which arbitrary symbols represented food items, objects, people, places, and activities (see Rumbaugh & Washburn, 2003). All chimpanzees had participated in previous quantity-judgment studies (e.g., Beran, 2001, 2004, 2012; Beran & Beran, 2004). Apparatus. The chimpanzees were tested in their home enclosures. Trials were presented via a testing bench with a sliding tray that could be pushed toward the focal chimpanzee. We mounted two 20-cm ⫻ 18-cm display boards side by side on the sliding tray 13 cm apart (see Figure 1). Each display board had 32 shallow wells into which we placed food items (cereal pieces and M&M’s candies), distributed according to the appropriate trial type. The display boards were positioned at approximately a 40degree angle so as to provide a more vertical orientation for the chimpanzees to view. For food rewards, we used a combination of low-preference food items (yellow cereal corn puffs) and highpreference food items (blue M&M’s). The chimpanzees were highly motivated to maximize M&M’s intake, as they chose M&M’s over cereal pieces on every two-choice preference test that they received prior to the start of the experiment. Thus, their display-board choice preferences were assumed to indicate which board they perceived to contain the most M&M’s. Design and procedure. Chimpanzees were given a twooption choice task, from which they presumably attempted to select the array that they perceived to contain the larger number of blue M&M’s. Each array contained a combination of yellow and blue items. These items were either randomly arranged for control trials or arranged in a specific cross-pattern for the Solitaire trials. Critically, for each trial, the two arrays contained an equal amount of yellow and blue items combined, but were the inverse relation of one another with regard to specific quantities of each item type. For example, for a 12 versus 8 control trial, each array would contain 20 items, but one array would contain 12 blue items and 8 yellow items, whereas the second array would contain 8 blue items and 12 yellow items. For this example, chimpanzees should select the array that contained more blue items even though the arrays had the same overall quantity. The testing bench had a retractable miniblind to minimize the possibility of experimenter cuing. After each display board was baited out of view of the chimpanzee, the experimenter raised the blind and pushed the tray in toward the chimpanzee after 3 s of viewing time had elapsed. The chimpanzee selected one of the two display boards by pointing to the preferred board. To further reduce the possibility of cuing, the experimenter closed his eyes during this selection phase so that he could not see any response from the chimpanzees, and a second experimenter—who was not blind to the display boards, but was not in view of the chimpanzee (seated away from the testing area) and therefore could not cue the subject—announced the chimpanzee’s selection. These selections

were unambiguous and were always clearly made by the chimpanzees, as all subjects were widely familiar with using pointing cues to indicate their choice in two-option discrimination tasks (e.g., Beran, McIntyre, Garland, & Evans, 2013; Parrish & Beran, 2014a, 2014b). Then, the experimenter gave the contents of the selected display board to the chimpanzee for immediate consumption. We recorded the specific array that was selected, along with the contents of that array and the side of the apparatus on which it was located. We included four trial types, including three control trials and one Solitaire illusion trial (test trial). For the control trials, we introduced three quantity comparisons including 4 versus 8 items of differing quality within each array, 8 versus 16 items of differing quality in each array, and 8 versus 12 items of differing quality within each array. In each trial, items randomly were placed in the wells of the tray. For Solitaire illusion trials, each array contained 16 blue and 16 yellow items, with one array presenting 16 M&M’s centrally located with 16 cereal pieces on the perimeter, and one array presenting 16 cereal pieces centrally located with 16 M&M’s on the perimeter (see Figure 1). Testing consisted of eight sessions with each chimpanzee, with five trials per session. These five trials included one of each of the control trials and two Solitaire illusion trials. Trial type was pseudorandomized within session so that each session presented two control trials, a Solitaire trial, another control trial, and, finally, the second Solitaire trial. For control trials, the display board side (left or right) that contained the larger number of M&M’s was randomized within session. For Solitaire illusion trials, we included one trial in which the M&M’s were centrally located in the array on the left side and one trial in which the M&M’s were centrally located on the right side, and these presentations were randomized across sessions.

Results and Discussion Control trials. Sherman, Panzee, and Mercury did not exhibit any difference in performance as a function of the side on which the larger array was presented (chi-square test, all ps ⬎ .05), but Lana showed significantly higher accuracy when the larger array was presented on her left, ␹2(1) ⫽ 4.80, p ⫽ .028. Figure 2 presents each chimpanzee’s performance on the three numerical contrasts. All chimpanzees successfully selected the larger array of M&M’s in control trials in which there was a difference between arrays. Binomial tests confirmed that performance was significantly better than chance (all ps ⬍ .05). For each subject, no significant difference in performance across the three numerical contrasts was found (chi-square test: Lana, ␹2[2] ⫽ 2.40, p ⫽ .301; Panzee, ␹2[2] ⫽ 2.09, p ⫽ .352; Sherman, ␹2[2] ⫽ 0, p ⫽ 1.0; Mercury, ␹2[2] ⫽ 2.29, p ⫽ .319), showing that they performed similarly in the three control conditions. A comparison of the performance on the first two sessions versus the last two sessions indicated no differences in performance over time (Fisher’s exact test for proportion of correct choices on first two and last two sessions: Lana, 0.83 and 1.0, p ⫽ 1.0; Panzee, 1.0 and 1.0, p ⫽ 1.0; Sherman, 1.0 and 1.0, p ⫽ 1.0; Mercury, 0.67 and 1.0, p ⫽ .455). Thus, when there was a true difference in the number of M&M’s in the arrays, the chimpanzees chose the array with the larger number. Solitaire illusion test trials. No left–right bias was reported for any ape (chi-square test, all ps ⬎ .05). No chimpanzee selected

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DIFFERENTIAL PERCEPTION OF THE SOLITAIRE ILLUSION

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Figure 2. Results of Experiment 1. In control trials for which the number of food items of each type differed between the two arrays, the chimpanzees’ performance was significantly above chance level (shown by the horizontal line) and was equivalent across all the three numerical contrasts (4 vs. 8, 8 vs. 16, and 8 vs. 12). In the Solitaire illusion test, chimpanzees’ performance did not significantly differ from chance level, thus showing no influence of the different spatial arrangements of M&M’s and cereals. In the Solitaire illusion test, the y-axis indicates the proportion of trials in which chimpanzees selected the array with centrally located blue items.

either arrangement at levels that differed from chance (binomial tests, all ps ⬎ .05; see Figure 2). Thus, the chimpanzees did not appear to perceive the centrally clustered food items as more numerous than the same number of items arranged on the perimeter. Experiment 1 assessed whether chimpanzees fell prey to the Solitaire illusion when they chose between different quantities of food items. In control trials in which blue items differed in their number in the two choice options, chimpanzees accurately selected the array containing the larger number of M&M’s. This aligns with previous literature showing that chimpanzees can spontaneously assess the larger quantity of food items (e.g., Beran, 2001; Beran et al., 2013). However, in test trials with the Solitaire patterns, no chimpanzee selected either array at levels that differed from chance, suggesting that no visual illusion occurred in the present task.

Experiment 2 The results of Experiment 1 are not sufficient to claim that chimpanzees do not perceive the Solitaire illusion. For instance, motivational factors might have played a confounding role. As the illusion is likely to generate subtle numerical differences between the arrays, the possibility exists that chimpanzees did not spontaneously select any array in the presence of the Solitaire arrangements, as both alternatives might have provided seemingly equivalent amounts of preferred food. To assess whether the lack of bias reported in Experiment 1 was impacted by the procedural approach adopted, we set up a different experiment using a computerized task with a training procedure.

Furthermore, in order to form a broader assessment of the Solitaire illusion in nonhuman primates, we extended our study by testing two distantly related primate species in addition to chimpanzees, including rhesus monkeys (Old World monkeys) and capuchin monkeys (New World monkeys). As a control, we also tested a sample of adult humans. The ideal comparative approach would present all species with the exact same task, the same training parameters, and the same kinds of feedback with regard to correctness or incorrectness of responses. Therefore, we trained all species using a computerized task requiring them to select the larger number of white dots in two arrays composed by white and black dots. In test trials, two Solitaire patterns were presented. If subjects learned to discriminate very small differences between the arrays on trials in which there were true differences between the number of white and black dots, we should expect a bias in the presence of the Solitaire patterns to choose the pattern made of centrally located white dots if they perceived any kind of numerosity illusion.

Method Subjects. We tested four adult chimpanzees (Pan troglodytes; the same subjects tested in Experiment 1), six adult rhesus macaques (Macaca mulatta; all males, ages 10 to 26 years), seven adult capuchin monkeys (Cebus apella; four males and three females, ages 5 to 23 years), and 12 adult human volunteers (Homo sapiens; two males and 10 females, ages 22 to 25 years). Nonhuman primates were tested at the Language Research Center (Georgia State University). Human participants with normal or corrected-to-normal vision gave their informed consent prior to

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participating in the experiment, and they were tested at the Department of General Psychology, University of Padova. Nonhuman subjects had been trained previously to use a joystick with their hands to control a cursor on a computer screen (see Evans, Beran, Chan, Klein, & Menzel, 2008; Richardson, Washburn, Hopkins, Savage-Rumbaugh, & Rumbaugh, 1990). They all had participated in numerous previous computerized experiments (e.g., Beran, 2008; Beran, Evans, Klein, & Einstein, 2012; Beran & Parrish, 2012, 2013; Beran & Rumbaugh, 2001; Beran & Smith, 2011). They had continuous access to water and received a daily diet of fruits and vegetables independent of the amount of work they completed on the task, and thus they were not food deprived for the purposes of this experiment. During the task, they could earn additional food through delivery of fruit-flavored pellets (monkeys) or small pieces of food reward, such as banana slices or cereal pieces (chimpanzees), for correct responses. Materials and procedure. All testing was conducted using the Language Research Center’s Computerized Test System. For nonhuman primates, this system consists of a personal computer, digital joystick, color monitor, and pellet dispenser (Evans et al., 2008; Richardson et al., 1990). Subjects manipulated a joystick with their hands to produce isomorphic movements of a small cursor on the computer screen. For monkeys, correct responses to stimuli resulted in the delivery of 45-mg (capuchins) or 94-mg (rhesus) banana-flavored chow pellets (Bio-Serv, Frenchtown, NJ) via a pellet dispenser that was connected to the computer through a digital I/O board (PDISO8A; Keithley Instruments, Cleveland, OH). For chimpanzees, a human experimenter delivered food rewards manually for correct responses. Critically, the experimenter did not view the computer screen during trials, and therefore did not know the trial type, response of the chimpanzee, or any other information that might have led to inadvertent cues to the chimpanzees. The experimenter merely delivered food rewards after hearing the auditory tone that indicated a correct response. The program used in this experiment was written in Visual Basic 6.0. Each subject was tested individually. At the outset of each trial, a light-gray-colored rectangle appeared in the center of the computer screen that was medium gray in background color. At the bottom center of the screen was a small red circle that was the cursor under control of the joystick. Subjects moved the cursor into contact with the light gray rectangle to cause presentation of the next pair of stimuli. These stimuli were presented at left center and right center of the screen, with the cursor directly between them. Each stimulus consisted of an array of 33 dots arranged in a plus-shaped pattern (see Figure 3). Within each stimulus array, each dot was colored black or white. Subjects were rewarded when they selected the array that contained relatively more white dots within it compared with black dots. For correct responses, a melodic chime sound was played. Incorrect responses led to a buzz tone and a 20-s (monkeys) or 8-s (chimpanzees) timeout, during which the screen remained blank. There was a 1-s intertrial interval after either of those outcomes, and then the cursor and the lightgray rectangle appeared to indicate the start of the next trial. The procedure was presented in two phases: a training phase and a test phase. In the training phase, subjects were progressively trained to make relative quantity judgments with increasing difficulty; no Solitaire arrangement was presented in this phase. Only after reaching the learning criterion did they begin the test phase

Figure 3. Three example trials used in Experiment 2. The top row shows an easy discrimination, and the middle row shows a more difficult discrimination. In both cases, the correct choice is the one on the left. The bottom row shows the two arrangements presented on Solitaire trials. Humans typically believe there are more white dots than black dots in the array at left, and the opposite for the array at right, and so on these trials, the expectation is that the experience of this illusion would lead to a bias to select the array at left as being the one with more white dots. The central dot in each row is the cursor that was used to select one of the two arrays. The color version of this figure appears in the online article only.

with more difficult discriminations, including trials with the Solitaire arrangements. Each session of the training phase consisted of up to three stages that increased the objective difficulty of the quantity judgment. Subjects progressed from one stage to the next when they met a training criterion. For monkeys, this was correct responding in at least 19 of the most recent 25 trials they completed. For chimpanzees, this was correct responding in at least 12 of the most recent 15 trials they completed. In Stage I, the smaller dot quantity in each array ranged from one to three dots, and the larger ranged from 30 to 32 dots. In Stage II, the smaller dot quantity in each array ranged from three to six dots, and the larger ranged from 27 to 30 dots. In Stage III, the smaller dot quantity in each array ranged from five to 10 dots, and the larger ranged from 23 to 28 dots. In all stages, the sum of the two sets always was 33 dots. In all stages, the array with the larger target color was randomly assigned to the left or to the right of the cursor. In the test phase, the smaller dot quantity in each array ranged from five to 16 dots, and the larger ranged from 17 to 28 dots (again, summing always to 33 dots). Thus, the very hardest discriminations were presented here (e.g., 16 vs. 17 dots). The test phase also introduced the Solitaire trials. These were presented with a random probability of 0.15 on each trial. For these trials, one array (randomly positioned to the left or right of the cursor) showed the target color dots (white) more centrally located, whereas the other array showed the target color dots more peripherally located (see Figure 3). On these trials, each array contained 32 dots (16 white, 16 black). Subjects were nondifferentially reinforced for their selections on these trials, with a probability of

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DIFFERENTIAL PERCEPTION OF THE SOLITAIRE ILLUSION

0.50 for reward and a probability of 0.50 for no reward. In both cases, the 1-s intertrial interval began immediately (i.e., there was no timeout). Because subjects worked at their own pace, and for different session durations, the trial counts per session varied, as did the number of sessions completed. To provide comparability within and across species, we tested each subject until comparable numbers of trials had been completed (see Table 1). For nearly all individuals of both monkey species, this required only one test session, and in some cases, two sessions. All chimpanzees required five test sessions. One additional methodological variation was used. Monkeys were tested before chimpanzees, and based on the response-time data that were generated in those sessions, we established that chimpanzees had to respond within 1,500 ms or the trial was terminated. This response time encompassed more than 97% of the responses made by the monkeys. It was critical to establish a rapid response time for the chimpanzees to ensure that they could not attempt to enumerate the stimuli sets given their past experiences in such enumeration tasks (e.g., Beran & Rumbaugh, 2001), and also so that we could use the same criteria for testing adult humans, for whom concerns about the use of counting are clearly justified. Thus, after 1,500 ms without a response, the program treated the trial as an incorrect response and gave the chimpanzees a timeout, in an effort to train them to respond more quickly. However, these trials were not used for any analysis presented hereafter. For the human task, the procedure was similar to that described with nonhuman primates, with a few exceptions. We used a 17-in. LCD color monitor and a cordless joystick (Logitech freedom 2.4=). In addition, no auditory feedback was given, but visual feedback was provided. The word “Correct” in green color or the

Table 1 Sessions and Trials Completed by Each Nonhuman Subject in Experiments 2 and 3 Experiment 2

Capuchin monkeys Griffin Liam Logan Nala Nkima Widget Wren Rhesus monkeys Chewie Gale Han Hank Luke Murph Chimpanzees Lana Mercury Panzee Sherman

Experiment 3

Sessions completed

Trials completed

Sessions completed

Trials completed

1 1 1 1 1 2 1

1,479 2,000 2,000 1,439 2,000 1,517 1,885

3 1 1 1 1 1 1

1,077 1,704 1,681 1,645 1,666 1,279 996

2 1 1 1 2 1

1,573 1,378 1,196 1,666 803 1,163

3 3 3 4 2 3

1,369 1,482 1,429 1,572 1,226 1,336

5 5 5 5

889 1,000 790 982

Note. There were also 12 human participants in Experiment 2. Each of them completed 800 trials in one test session.

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word “Incorrect” in red color appeared within a rectangle placed in the middle of the screen. Timeouts for incorrect responses were shortened to only 4 s. As for the nonhuman primates, in test trials with the Solitaire arrangements, there was nondifferential reinforcement with a 0.50 probability of positive feedback and a 0.50 probability of simply moving to the intertrial interval. Each human participant completed 800 trials in a single session. Before starting the experiment, participants read the following text: You will perform a computer task. You will see a box that says “Start Trial,” and you should select it by using the joystick. Then, two arrays of dots will appear, and you should select one of those. Try to respond quickly and accurately. You may take a short break if you need to, but otherwise please try to complete as many trials as you can.

In this way, the rules for correct responding could only be inferred from the feedback, exactly as happened in the nonhuman primates’ experiment.

Results In the test phase, Lana timed out on 9.6% of trials with a true difference between dot colors, and on 17.2% of trials with the Solitaire arrangement. Mercury timed out on 2.3% of trials with a true difference between dot colors, and on 1.0% of trials with the Solitaire arrangement. Panzee timed out on 9.3% of trials with a true difference between dot colors, and on 12.3% of trials with the Solitaire arrangement. Sherman timed out on 2.1% of trials with a true difference between dot colors, and on 5.3% of trials with the Solitaire arrangement. All species performed very well in discriminating true relative differences in target dot quantities. Figure 4 presents the performance of each species for all trials except Solitaire trials in the test phase, as a function of the difference in the number of the two colors of dots in the arrays. For all species, there was a significant positive correlation of this difference and the percentage of correct responses: capuchin monkeys, r(10) ⫽ 0.80, p ⫽ .002; rhesus monkeys, r(10) ⫽ 0.84, p ⫽ .001; chimpanzees, r(10) ⫽ 0.93, p ⬍ .001; humans, r(10) ⫽ 0.655, p ⫽ .021. On the whole, all species could discern small differences between the arrays, and thus could be expected to seek such differences when presented with the Solitaire pattern. Figure 5 presents the percentage of choices of the Solitaire pattern with target colored dots more centrally located for each species. The data are presented as successive blocks of 20 such presentations of the Solitaire patterns, so that the early choice biases (if they existed) are shown, as well as the trends over time to perceive (or not) the illusion. For each of these blocks, we compared performance to the 50% chance level that would indicate no bias to choose either arrangement, using a two-tailed binomial test, with p ⬍ .05 indicating a statistically significant bias. All human participants showed a statistically significant bias for the pattern in which white target items were centrally located (hereafter called the Solitaire arrangement) in nearly all trial blocks (73 out of 77 blocks). Thus, humans clearly showed evidence of the Solitaire illusion early and throughout the experiment. With the exception of one trial block for Panzee, none of the chimpanzees showed a statistically significant bias for or against

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Figure 4. Mean performance as a function of the difference between colored dots (black and white) within each array in test phase of Experiment 2. Smaller differences indicate objectively more difficult trials. Error bars indicate 95% confidence intervals.

the Solitaire arrangement, thereby showing no evidence of the illusion in this experiment. Two of six rhesus monkeys (Gale and Han) showed early and consistent biases to choose the Solitaire arrangement. Chewie showed no bias early, but a bias by the end of the experiment. Murph showed no bias throughout. Hank and Luke, however, showed a bias for the opposite arrangement for most trial blocks. Thus, evidence for the Solitaire illusion in rhesus monkeys was inconsistent but present. One capuchin monkey (Logan) showed a consistent and strong bias to choose the Solitaire arrangement. One monkey (Griffin) showed a consistent but weaker bias in a similar direction. Wren and Nkima showed an occasional bias, although not from the earliest trials. Liam and Widget showed only rare biased choices of the Solitaire arrangement, and Nala showed no biases. Thus, the evidence for the Solitaire illusion in capuchin monkeys was also inconsistent but present. We further assessed the performance of each individual to better determine whether any biases for or against the Solitaire arrangement could have been the result of chance responding. To do this, we generated a Monte Carlo simulation that randomly selected one of the two arrangements on each trial for the same number of trial blocks as was completed by a given participant. This simulation was run 100 times for each subject, and we tallied the number of

those runs in which the simulation produced at least as many above-chance blocks of 20 trials as had been produced by the real participant. If this number exceeded zero, we concluded that the participant’s overall pattern of responding did not indicate a true preference for the Solitaire illusion, as it could be generated by this simulation with a random response pattern. Among the human participants, a significant preference for the Solitaire arrangement occurred in four of six blocks for one participant, in five of six blocks for one participant, in six of six blocks for five participants, in six of seven blocks for one participant, and in seven and seven blocks for four participants. These patterns were never recreated by the simulation in any of its runs. For the chimpanzees, only Panzee ever exceeded chance levels of preference for the Solitaire arrangement in any block. She did this in one out of three blocks, and the simulation indicated that one block above chance out of three occurred in 25 of the 100 runs of the simulation. Thus, Panzee’s performance likely indicated no true preference for the Solitaire arrangement. None of the other chimpanzees ever showed a bias, and so no simulation of their data was performed. For the rhesus monkeys, Chewie, Gale, Han, and Murph showed at least one block in which performance in choosing the Solitaire arrangement exceeded chance levels. The simulation showed that for Murph this pattern (one block in nine of above-chance selec-

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DIFFERENTIAL PERCEPTION OF THE SOLITAIRE ILLUSION

Figure 5. Percentage of choices of the Solitaire arrangement (more centrally located target colored dots) for each species in test phase of Experiment 2. Data are presented in successive blocks of 20 trials in which the Solitaire arrangements were presented, one with the target colored dots centrally located, and one with the nontarget colored dots centrally located. The two horizontal lines indicate the levels at which performance is significantly higher than chance (top line) or significantly lower than chance (bottom) in choosing the more centrally arranged dot pattern (p ⬍ .05, binomial test). For chimpanzees, timed-out trials were not included in the analysis.

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tion of the Solitaire arrangement) occurred in 23 out of 100 runs of the simulation. Thus, Murph’s performance likely indicated no preference for the Solitaire arrangement. However, Chewie showed a preference in four out of 10 blocks, and the simulation never generated a comparable result pattern. The same was true for Gale’s performance (nine out of 10 blocks indicated a preference) and Han’s performance (seven out of eight blocks indicated a preference), neither of which was ever recreated by the simulation in any run. It should be noted also that two monkeys (Hank and Luke) showed a bias against the Solitaire arrangement. For the capuchin monkeys, Nala never showed a preference for the Solitaire arrangement. Liam showed a preference in two out of 14 blocks, but this pattern was generated by the simulation in four of its runs. Widget showed a preference in two out of 11 blocks, but this pattern was generated by the simulation in three of its runs. Thus, these monkeys likely did not show a true preference for the Solitaire arrangement. However, the remaining four monkeys did. Griffin showed a preference for the Solitaire arrangement in seven out of 10 blocks, Wren showed a preference in seven out of 14 blocks, Nkima showed a preference in 11 out of 15 blocks, and Logan showed a preference in 12 out of 14 blocks. These patterns were never recreated by the simulation in any of its runs.

Discussion This experiment was designed to assess the Solitaire illusion among four primate species via a computerized task using comparable procedures between species, including similar training and testing procedures. The quantity discrimination performances of all species were in line with that reported in many past studies with these species (e.g., Beran, 2001, 2004, 2008; Beran & Beran, 2004; Brannon & Terrace, 2000; Cantlon & Brannon, 2007; Evans, Beran, Harris & Rice, 2009). In the presence of the Solitaire patterns, chimpanzees did not show any bias, supporting the conclusion of Experiment 1 that this illusion does not easily emerge in chimpanzees as it does in humans. As the methodologies of the two experiments differed in many respects—spontaneous choice test (Experiment 1)/training procedure (Experiment 2), food items as stimuli (Experiment 1)/dots as stimuli (Experiment 2), limited number of trials (Experiment 1)/large number of trials (Experiment 2), long presentation time (Experiment 1)/short presentation time (Experiment)—we conclude that the lack of misperception of the Solitaire pattern in chimpanzees likely reflects a general phenomenon of chimpanzees’ visual system, instead of being the by-product of motivational factors or other limits related to a specific procedure. In addition, human participants showed a clear misperception of numerosity when tested in comparable conditions in Experiment 2. In this sense, the results of this experiment point toward the existence of a discontinuity in the perceptual mechanisms underlying the Solitaire illusion between humans and chimpanzees. The data from the monkeys were less clear. Three rhesus monkeys seemed to exhibit a humanlike perception of the Solitaire pattern. However, two other monkeys showed an opposite bias, and the remaining monkey did not show any bias. In short, evidence for the illusion was inconsistent in rhesus monkeys, although these data perhaps provide a glimpse of the Solitaire illusion in this species. The data from capuchin monkeys appear to be complicated, too. Four subjects out of seven showed the same bias reported in humans, and no capuchin monkeys showed an opposite bias.

Experiment 3 The results from the rhesus monkeys and capuchin monkeys reported in Experiment 2 are worth noting, as they might suggest a humanlike perception of the Solitaire illusion. However, given the variability in the monkeys’ performances, the possibility existed that the bias reported in some cases was an anomaly. Thus, we replicated Experiment 2 with rhesus monkeys and capuchin monkeys to determine if the Solitaire illusion would persist with additional testing.

Method Subjects. The same rhesus monkeys (N ⫽ 6) and the same capuchin monkeys (N ⫽ 7) were tested as in Experiment 2. Materials and procedure. The task and procedure was identical to Experiment 2, with one exception. Now, the dots were colored blue and red rather than white and black, and the monkeys were rewarded when they selected the array that had more blue dots. Each monkey completed one session with the same progression of testing as in Experiment 2.

Results Figure 6A presents the performance of both species in the test phase for all trials except Solitaire trials, as a function of the difference in the number of the two colors of dots in the arrays. For both species, there was a significant positive correlation of this difference and the percentage of correct responses: capuchin monkeys, r(10) ⫽ 0.77, p ⫽ .003; rhesus monkeys, r(10) ⫽ 0.73, p ⫽ .008. Figures 6B and 6C present the percentage of choices of the Solitaire pattern with target colored dots more centrally located for, respectively, rhesus monkeys and capuchin monkeys. The data again are presented as successive blocks of 20 such presentations of the Solitaire patterns, and we again compared performance to the 50% chance level that would indicate no bias to choose either arrangement, using a two-tailed binomial test, with p ⬍ .05 indicating a statistically significant bias. Two rhesus monkeys (Han and Murph) showed a significant bias to choose the Solitaire arrangement. Hank showed one block in which performance in choosing the Solitaire arrangement exceeded chance levels. One monkey (Chewie) showed a significant bias for the alternate arrangement, and two monkeys (Gale, and Luke) showed no biases. Thus, the evidence for the Solitaire illusion in most rhesus monkeys was weak. However, we did assess the performance of Han and Murph using the Monte Carlo simulation and found that their choice patterns were never recreated by the simulation in any of its runs. Hank’s performance, with one block of preference out of 10, was recreated by the simulation in 21 of its 100 runs, and therefore indicated that his performance did not reflect a true preference. None of the capuchin monkeys showed a significant bias for the Solitaire arrangement. In fact, three monkeys (Liam, Logan, and Nkima) often showed a significant bias for the opposite arrangement. Thus, there was no evidence of the Solitaire illusion in any capuchin monkeys in this experiment.

Discussion Experiment 3 was designed to investigate the robustness of the monkeys’ results reported in the previous experiment. As in Experi-

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Figure 6. (A) Mean performance as a function of the difference between colored dots (blue and red) within each array in test phase of Experiment 3. Smaller differences indicate objectively more difficult trials. Error bars indicate 95% confidence intervals. Percentage of choices of the Solitaire arrangement (more centrally located target colored dots) in test phase of Experiment 3 in rhesus monkeys (B) and capuchin monkeys (C). Data are presented in successive blocks of 20 trials in which the Solitaire arrangements were presented. The two horizontal lines indicate the levels at which performance is significantly higher than chance (top line) or significantly lower than chance (bottom) in choosing the more centrally arranged dot pattern (p ⬍ .05; binomial test).

ment 2, both species learned to discriminate between quantities in training trials even with reduced numerical distances, thus confirming the validity of the training procedure to assess fine numerical discrimination in these species. However, when presented with Solitaire arrangements the pattern of data often contradicted that obtained in Experiment 2. Rhesus monkeys that seemed to perceive the Solitaire illusion in the previous experiment did not show the same evidence

with the novel set of stimuli; similarly, those capuchin monkeys that seemed to have a humanlike perception in Experiment 2 did not show the same bias here. Only one monkey (Han) out of 13 showed a bias in selecting the centrally located target items in both experiments. Interestingly, two subjects— one rhesus monkey (Chewie) and one capuchin monkey (Logan)—reversed their preference compared with what was reported

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in Experiment 2. We can only speculate on why they reversed the bias in two experiments that were identical, with the only exception of stimulus color. Our best explanation is that these subjects, and perhaps other monkeys, were highly sensitive to early exposures of the Solitaire patterns in terms of response outcomes. Those monkeys may have learned that those unique patterns were never punished for either selection, and this may have led to biases for one arrangement over the other, not because of misperceived quantity judgments, but simply because these monkeys learned that these trial types were different from those with true quantity differences in dot color, and therefore settled on just choosing one arrangement when Solitaire trials appeared. Regardless of the exact reason underlying the reversed bias exhibited by some subjects, Experiment 3 largely failed to replicate the results of Experiment 2 in monkeys. Taken together, the data from these two experiments together align with our previous results reported in chimpanzees and do not encourage the hypothesis of a similar highly robust perception of the Solitaire illusion between humans and monkeys.

General Discussion Given that nonhuman animals often fall prey to the same perceptual illusions that humans experience, we asked if chimpanzees, rhesus monkeys, and capuchin monkeys might perceive the Solitaire illusion as humans do. In three experiments, we compared the performance of human and nonhuman primates in tasks requiring them to select which array contained the larger number of target items. As expected, the performances of all species were very accurate in the control trials of each experiment in which true numerical differences existed between the two arrays. In contrast, in the Solitaire trials, chimpanzees (Experiments 1 and 2) randomly selected between the Solitaire array patterns. Rhesus monkeys (Experiments 2 and 3) and capuchin monkeys (Experiments 2 and 3) showed more variable patterns of responding, with only one animal consistently showing the illusory response pattern in both experiments. Other monkeys sometimes showed a bias, but in terms of the group performances, monkeys were not comparable with adult humans, whose own data nicely matched those in previous literature in which the illusion was robustly perceived (Frith & Frith, 1972). Because the four species were tested in comparable conditions (no explicit instructions, same stimuli, same presentation time, and the same motor response), our results support the idea of a potential difference in the perception of the Solitaire pattern, although the mixed pattern of data reported with monkeys requires cautious statements and further investigation before concluding that monkeys do not perceive this illusion. The reasons for this difference between humans and other primates are, at present, unclear. Frith and Frith (1972) suggested that perception of the Solitaire illusion may be determined by the grouping of individual elements, such that contiguous or proximal elements form a better Gestalt, thus appearing more numerous. The possibility exists that chimpanzees, rhesus monkeys, and capuchin monkeys have a reduced sensibility to these grouping cues compared with humans, and thus a reduced sensitivity to the Solitaire illusion. Alternative methodological issues might be advanced to explain our results. The use of different stimuli might elicit the Solitaire illusion among nonhuman primates. For example, higher density and higher interconnectedness among individual items might better facilitate percep-

tual grouping, thus favoring a better Gestalt among the centrally clustered items. However, there was very little physical separation among the dots used in Experiment 2 (see Figure 3), and therefore stimuli were highly dense and interconnected. In addition, the similar performance of chimpanzees in Experiments 1 and 2 (in which stimuli have very different density and interconnectedness) discourages the idea that the types of stimuli have significantly affected the null results reported in nonhuman primates. Perhaps less task-sophisticated subjects would show evidence of the Solitaire illusion. All of our subjects had participated in previous cognitive perceptual tasks, some of which involved other visual illusions (e.g., Agrillo et al., 2014; Beran, 2006; Beran & Parrish, 2013; Parrish & Beran, 2014a, 2014b). Interindividual variance observed in monkeys, for instance, might be partially ascribed to the different types of experiments that those monkeys experienced in the past. However, visual illusions investigated in the last years with our subjects had different visual features compared with the Solitaire illusion; in this sense, we think that it is unlikely that subjects’ previous experiences might have strongly influenced the performance in test trials, but this remains to be tested. In this test of the Solitaire illusion in nonhuman animals, we saw only limited (monkeys) or no (chimpanzees) evidence that those animals misperceived quantities the way that humans do. However, we cannot draw a firm conclusion as whether the existence of this illusion is unique to humans. Further studies in nonhuman species are also necessary to assess whether the lack of bias here reported is unique to these apes and presumably monkeys or instead represents a wider phenomenon shared by other nonhuman primates and, more broadly, other nonhuman animals. Until those data are available, the difference reported here between human and nonhuman primates is intriguing and suggestive of the existence of an evolutionary discontinuity between humans and nonhuman primates in the perceptual biases affecting the precision of numerical judgments in the Solitaire illusion.

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Received January 30, 2014 Revision received June 17, 2014 Accepted June 23, 2014 䡲