Maze runners: monkeys show restricted Arabic numeral summation during computerized two-arm maze performance.

Mazes have been used in many forms to provide compelling results showcasing nonhuman animals' capacities for spatial navigation, planning, and numerical competence. The current study presented computerized two-arm mazes to four rhesus macaques. Using these mazes, we assessed whether the monkeys could maximize rewards by overcoming mild delays in gratification and sum the values of Arabic numerals. Across four test phases, monkeys used a joystick controller to choose one of two maze arms on the screen. Each maze arm contained zero, one or two Arabic numerals, and any numerals in the chosen maze arm provided the monkeys with rewards equivalent to the value of those numerals. When deciding which arm to enter, monkeys had to consider distance to numerals and numeral value. In some tests, gaining the maximum reward required summing the value of two numerals within a given arm. All four monkeys successfully maximized reward when comparing single numerals and when comparing arms that each contained two numerals. However, some biases occurred that were suboptimal: the largest single numeral and the delay of reward (by placing numerals farther into an arm from the start location) sometimes interfered with the monkeys' abilities to optimize. These results indicate that monkeys experience difficulties with inhibition toward single, high valence stimuli in tasks where those stimuli must be considered in relation to overall value when represented by symbolic stimuli such as numerals.

There's "magic" in comparative cognition.

Among the many important empirical and theoretical contributions in her career Clayton and her colleagues advanced the idea that comparative cognition researchers would benefit from considering the role of magic and the techniques of the magician in some areas of cross-species cognitive study. They provided compelling and exciting studies using the techniques of the magician and demonstrated how those affect nonhuman animals that rely on vision, showing that there are similarities and dissimilarities in how susceptible some nonhuman species are to the magician's effects that typically work so well on human observers.

Congratulations to Animal Cognition on its 50th birthday! Some thoughts on the last 50 years of animal cognition research.

In this article, the author reflects on some of the key issues that have arisen in comparative cognition and the role and impact of the journal Animal Cognition through its first 25 years by pretending to look back at this period from the year 2047. Successes within comparative cognition are described and the role that Animal Cognition has played in the growth of comparative cognition are discussed. Concerns are presented about issues that affect the opportunities that researchers have to work with nonhuman species and to produce good comparative cognitive science. Prescriptions for what the author hopes will happen next also are offered all in the lens of a prospectively imagined retrospective on this field.

Assessing aphantasia prevalence and the relation of self-reported imagery abilities and memory task performance.

Aphantasia is the experience of having little to no visual imagery. We assessed the prevalence rate of aphantasia in 5,010 people from the general population of adults in the United States through self-report and responses to two visual imagery scales. The self-reported prevalence rate of aphantasia was 8.9% in this sample. However, not all participants who reported themselves as aphantasic showed low-imagery profiles on the questionnaire scales, and scale prevalence was much lower (1.5%). Self-reported aphantasic individuals reported lower dream frequencies and self-talk and showed poorer memory performance compared to individuals who reported average and high mental imagery. Self-reported aphantasic individuals showed a greater preference for written instruction compared to video instruction for learning a hypothetical new task although there were differences for men and women in this regard. Categorizing aphantasia using a scale measure and relying on self-identification may provide a more consistent picture of who lacks visual imagery.

I'll (not) take that: The reverse-reward contingency task as a test of self-control and inhibition.

While searching for more evidence of quantitative skills in chimpanzees to add to what she already had found, Boysen discovered something else. When training chimpanzees to point at what they would not get, and not pointing at what they would get, none could do this for piles of food items. Even when those items in the pointed-at set were given away to another chimpanzee, and even with experience in the task, failure persisted. This test, the reverse-reward contingency test, has now been used with many species, as a means of assessing inhibitory control and perhaps self-control in animals. Typically, the task is difficult, and only specific manipulations have worked to allow primates to overcome the reversed contingencies. This includes using symbolic stimuli, adding another layer to the story, and more value to the task itself as a measure perhaps of forms of cognitive control in other species. I will discuss some of these empirical results, including from other chimpanzees who were given variations of the task, and how these studies have influenced numerous areas within comparative cognitive science.

Nonhuman primates learn adjacent dependencies but fail to learn nonadjacent dependencies in a statistical learning task with a salient cue.

There is ample evidence that humans and nonhuman animals can learn complex statistical regularities presented within various types of input. However, humans outperform their nonhuman primate counterparts when it comes to recognizing relationships that exist across one or several intervening stimuli (nonadjacent dependencies). This is especially true when the two elements in the dependency do not share any perceptual similarity (arbitrary associations). In the present study, we investigated whether manipulating the saliency of the predictive stimulus would enhance nonadjacent dependency learning in nonhuman primates. Rhesus macaques and tufted capuchins engaged in a computerized signal detection task that included sequences that were random in nature, included an adjacent dependency, or included a nonadjacent dependency. We manipulated the saliency of the predictive stimulus, such that the predictor jittered in place on the screen in some grammar blocks, as well as the transitional probability (the likelihood of the stimulus preceding the target to accurately predict the target's appearance) from block to block. Some monkeys evidenced learning of adjacent dependencies by faster response times to targets that followed a predictive stimulus compared to targets that were not preceded by a predictor. However, consistent with the body of evidence that indicates that nonhuman animals' statistical learning mechanisms are not at the same level of sophistication as humans', there was no evidence that monkeys learned nonadjacent dependencies of arbitrary associations, even when the salient cue was present.

A chimpanzee recognizes varied acoustical versions of sine-wave and noise-vocoded speech.

Previous research demonstrated that a language-trained chimpanzee recognized familiar English words in sine-wave and noise-vocoded forms (Heimbauer et al. Curr Biol 21:1210-1214, 2011). However, those results did not provide information regarding processing strategies of the specific acoustic cues to which the chimpanzee may have attended. The current experiments tested this chimpanzee and adult humans using sine-wave and noise-vocoded speech manipulated using specific sine-waves and a different number of noise bands, respectively. Similar to humans tested with the same stimuli, the chimpanzee was more successful identifying sine-wave speech when both SW1 and SW2 were present - the components that are modeled on formants F1 and F2 in the natural speech signal. Results with noise-vocoded speech revealed that the chimpanzee and humans performed best with stimuli that included four or five noise bands, as compared to those with three and two. Overall, amplitude and frequency modulation over time were important for identification of sine-wave and noise-vocoded speech, with further evidence that a nonhuman primate is capable of using top-down processes for speech perception when the signal is altered and incomplete.

Post-event misinformation effects in a language-trained chimpanzee (Pan troglodytes).

Errors of source monitoring are widespread human memory challenges, and our memories are subject to distortion upon the presentation of subsequent misinformation. Less is known about if and when misinformation effects occur in nonhuman species' memory. Here we tested a symbol-trained chimpanzee's recall memory of a hidden food item's identity after a 10-min delay. During this delay, the subject was sometimes (depending on the condition) shown consistent or inconsistent video information about the identity of the food, before being asked to name the item to a second experimenter blind to the reward and condition. Across all conditions, our subject, Sherman, correctly named the food item at above chance levels. In the Inconsistent condition, in which Sherman was shown a video with misleading information, his performance was the worst of all conditions (although accuracy was still high). Interestingly, however, during three of the four trials in this condition in which Sherman made a mistake, he incorrectly named the food item shown during the misleading video information. These results provide evidence that chimpanzees, like humans, may be vulnerable to misinformation effects, even when that misleading information is presented in a different modality (video) than the original live event memory, demonstrating further commonality between human and ape memory systems.

Outcome expectancy and suboptimal risky choice in nonhuman primates.

Animals will favor a risky option when a stimulus signaling reward bridges the choice and the outcome. The present experiments investigated signal-induced risky choices and reward-outcome expectations in rhesus and capuchin monkeys. Risky choice was assessed by preference for a large-probabilistic reward over a modest-certain reward. Outcome expectancy was assessed by providing a truncation-response to shorten the delay period. In Experiment 1 both species generally favored the risky option compared to a safe option when the outcomes were signaled and generally shortened the delays except when a signaled-loss stimulus was presented. The use of the delay-truncation response suggested that the monkeys were sensitive to the information conveyed by the stimulus. Experiments 2 and 3 were designed to investigate whether the delay-truncation response used by capuchin monkeys was strategically used reflecting explicit decision-making versus a conditioned response to reward stimuli. A perceptual judgment task was included and the selective use of the delay-truncation response on unsignaled correct trials may suggest the involvement of metacognitive processes. The capuchin monkeys generally truncated the delays except under conditions where reward would not be expected (risky-loss or incorrect-judgment). When the outcomes were unsignaled during the delay some capuchin monkeys were less likely to truncate the delay following an incorrect task response. Overall, the monkeys: (1) made more risky choices when the outcomes were signaled - consistent with gambling-like behavior. (2) selectively truncated the unsignaled delays when rewards could be anticipated (even when metacognitive-like awareness guided anticipation) - suggesting that delay truncation responses reflect explicit outcome expectancy.

Words matter: Reflections on language projects with chimpanzees and their implications.

Beginning in the 1960s the first systematic projects dedicated to testing whether great apes could acquire some aspects of human language were conducted. The ape subjects demonstrated remarkable capacities to learn and use elements of either sign language or an artificial language. The results from research across several laboratories drew a mixture of excitement and skepticism, and critiques and debates have ensued since the earliest reports were published. This continues today. Terrace (2019, Nim: A chimpanzee who learned sign language. New York, NY: Columbia University Press) repeats many of the same points made decades earlier, and has added some additional critiques. That scientists hold different perspectives on what to conclude from ape language studies is expected. However, any conclusion one draws should be based upon available evidence, which we outline in this review. We also address the critiques offered by Terrace (2019), including the stance that apes cannot understand or use words. Focusing on symbol use by chimpanzees and bonobos we describe evidence that argues for understanding of words, including capacities for declarative communication and intersubjectivity found in these apes. We conclude that the many decades of research using a variety of symbol systems challenges the absolutist position that chimpanzees and bonobos cannot learn language or understand the concept of a word.

Linear numerosity illusions in capuchin monkeys (Sapajus apella), rhesus macaques (Macaca mulatta), and humans (Homo sapiens).

Numerosity illusions emerge when the stimuli in one set are overestimated or underestimated relative to the number (or quantity) of stimuli in another set. In the case of multi-item arrays, individual items that form a better Gestalt are more readily grouped, leading to overestimation by human adults and children. As an example, the Solitaire illusion emerges when dots forming a central cluster (cross-pattern) are overestimated relative to the same number of dots on the periphery of the array. Although this illusion is robustly experienced by human adults, previous studies have produced weaker illusory results for young children, chimpanzees, rhesus macaques, capuchin monkeys, and guppies. In the current study, we presented nonhuman primates with other linear arrangements of stimuli from Frith and Frith's (Percept Psychoph 11:409-410, 1972) original paper with human participants that included the Solitaire illusion. Capuchin monkeys, rhesus macaques, and human adults learned to quantify black and white dots that were presented within intermingled arrays, responding on the basis of the more numerous dot colors. Humans perceived the various illusions similar to the original findings of Frith and Frith (1972), validating the current comparative design; however, there was no evidence of illusory susceptibility in either species of monkey. These results are considered in light of illusion susceptibility among primates as well as considering the role of numerical discrimination abilities and perceptual processing mode on illusion emergence.

Correction to: Linear numerosity illusions in capuchin monkeys (Sapajus apella), rhesus macaques (Macaca mulatta), and humans (Homo sapiens).

In the original publication, values of last three rows in Table 1 and Table 2 were incorrectly published.

Exploring the Jastrow Illusion in Humans ( Homo sapiens), Rhesus Monkeys ( Macaca mulatta), and Capuchin Monkeys ( Sapajus apella).

In the Jastrow size illusion, two vertically stacked but offset stimuli of identical size are misperceived such that the bottom stimulus is overestimated relative to the top stimulus due to their spatial layout. In this study, we explored whether nonhuman primates perceive this geometric illusion in the same manner as humans. Human adults, rhesus macaques, and capuchin monkeys were presented with a computerized size discrimination task including Jastrow illusion probe trials. Consistent with previous results, humans perceived the illusory stimuli, validating the current experimental approach. Adults selected the bottom figure as larger in illusion trials with identical shapes, and performance was facilitated in trials with a true size difference when the larger figure was positioned at bottom. Monkeys performed very well in trials with a true size difference including difficult discriminations (5% difference in stimuli size), but they did not show evidence of the Jastrow illusion. They were indifferent between top and bottom stimuli in the illusory arrangement, showing no evidence of a human-like (or reversed) bias. These results are considered in light of differences in perceptual processing across primates and in comparison to previous comparative studies of the Jastrow and other size illusions.

Rhesus monkeys (Macaca mulatta) remember agency information from past events and integrate this knowledge with spatial and temporal features in working memory.

The purpose of the present study was to examine whether rhesus monkeys remember information about their own agency-along with spatial, temporal and contextual properties-from a previously experienced event. In Experiment 1, rhesus monkeys (n = 4) used symbols to reliably indicate whether they had performed or observed an event on a computer screen. In Experiment 2, naïve and experienced monkeys (n = 8) reported agency information when stringent controls for perceptual and proprioceptive cues were included. In Experiment 3, five of the monkeys completed a task in which they reported agency information along with spatial and temporal features of events. Two monkeys performed this agency discrimination when they could not anticipate which memory test they would receive. There was also evidence that these features were integrated in memory. Implications of this research are discussed in relation to working memory, episodic memory and self-awareness in nonhuman animals.

Visual artificial grammar learning by rhesus macaques (Macaca mulatta): exploring the role of grammar complexity and sequence length.

Humans and nonhuman primates can learn about the organization of stimuli in the environment using implicit sequential pattern learning capabilities. However, most previous artificial grammar learning studies with nonhuman primates have involved relatively simple grammars and short input sequences. The goal in the current experiments was to assess the learning capabilities of monkeys on an artificial grammar-learning task that was more complex than most others previously used with nonhumans. Three experiments were conducted using a joystick-based, symmetrical-response serial reaction time task in which two monkeys were exposed to grammar-generated sequences at sequence lengths of four in Experiment 1, six in Experiment 2, and eight in Experiment 3. Over time, the monkeys came to respond faster to the sequences generated from the artificial grammar compared to random versions. In a subsequent generalization phase, subjects generalized their knowledge to novel sequences, responding significantly faster to novel instances of sequences produced using the familiar grammar compared to those constructed using an unfamiliar grammar. These results reveal that rhesus monkeys can learn and generalize the statistical structure inherent in an artificial grammar that is as complex as some used with humans, for sequences up to eight items long. These findings are discussed in relation to whether or not rhesus macaques and other primate species possess implicit sequence learning abilities that are similar to those that humans draw upon to learn natural language grammar.

Simians in the Shape School: A comparative study of executive attention.

Executive functions (EF) have been studied extensively in children and adults. However, EF tasks for young children can be difficult to administer and interpret. Espy (1997, Developmental Neuropsychology, 13, 495-499) designed the Shape School task to measure inhibition and switching in preschool-aged children. Shape School presents cartoon-like characters that children must flexibly name by their color, their shape, or both, depending on cues that indicate the appropriate rule. Shape School has been found to be age sensitive as well as predictive of performance on other EF tasks. We presented a computerized analogue of Shape School to seven rhesus macaques. Monkeys were trained to categorize characters by color or shape, or to inhibit this response, depending on whether the characters had eyes open, eyes closed, or wore hats. Monkeys performed above chance on the inhibition and switching components of the task. Long runs of a single classification rule and long runs of noninhibition trials had no significant impact on performance when the rule changed or inhibition was required. This nonverbal adaptation of Shape School can measure EF in nonhuman animals and could be used in conjunction with other EF tasks to provide a clearer picture of both human and nonhuman executive functions.

Within-session reversal learning in rhesus macaques (Macaca mulatta).

In a midsession reversal (MSR) task, animals are typically presented with a simple, simultaneous discrimination (S1+, S2-) where contingencies are reversed (S1-, S2+) half-way through each session. This paradigm creates multiple, relevant cues that can aid in maximizing overall reinforcement. Recent research has shown that pigeons show systematic anticipatory and perseverative errors across the session, which increase as a function of proximity to the reversal trial. This behavior has been theorized to indicate primary control by temporal cues across the session, instead of the cues provided by recent reinforcement history that appear to control behavior shown by humans. Rats, however, appear to be guided by recent reinforcement history when tested in an operant context, thereby demonstrating behavior that parallels that seen in humans, but they appear to be guided by temporal cues when tested in an open-field apparatus, showing behavior more akin to that seen in pigeons. We tested rhesus macaques (Macaca mulatta) on the MSR with a computerized simultaneous visual discrimination to assess whether they would show errors indicative of control by time or by recent reinforcement history. When a single reversal point occurred midsession, rhesus macaques showed no anticipation of the reversal and a similar level of perseveration to rats tested in an operant setting. Nearly identical results also were observed when the monkeys were trained with a single, variable reversal point or with multiple, variable reversal points within a session. These results indicate that temporal cues are not guiding response flexibility in rhesus macaque visual discrimination.

Gambling in rhesus macaques (Macaca mulatta): The effect of cues signaling risky choice outcomes.

Preference for a larger-variable "risky" option over a smaller-reliable "safe" option often depends upon the likelihood that the risky option will deliver a sufficiently sized reward to have an equivalent or superior expected value. However, preference for the risky option has been shown to increase under conditions where informative stimuli signaling the outcome of a risky choice is included between the choice and the outcome and this risk-prone preference persists even when the risky option has a lower expected value than the alternative safe option. In the present study, rhesus macaques chose between a risky option and a safe option across two experimental phases to determine whether the outcome signal affected the degree of preference for the risky option. Overall, six out of seven macaques showed a greater preference for the risky option in the signaled condition than in the unsignaled condition. The macaques' risky choices were sensitive to the expected value of the risky option and the signaled condition produced a general increase in risky choices independently of the expected value of the risky outcome. Overall, these results are consistent with those obtained with other animals, and this may relate to a process where animals show a biased preference for "good news." This process may model some of the relevant factors that explain the psychology of gambling in humans.

The number sense is neither last resort nor of primary import.

Leibovich et al. argue that evidence for an innate sense of number in children and animals may instead reflect the processing of continuous magnitude properties. However, some comparative research highlights responding on the basis of numerosity when non-numerical confounds are controlled. Future comparative tests might evaluate how early experience with continuous magnitudes affects the development of a sense of number.

Trading up: chimpanzees (Pan troglodytes) show self-control through their exchange behavior.

Self-control is defined as the ability or capacity to obtain an objectively more valuable outcome rather than an objectively less valuable outcome though tolerating a longer delay or a greater effort requirement (or both) in obtaining that more valuable outcome. A number of tests have been devised to assess self-control in non-human animals, including exchange tasks. In this study, three chimpanzees (Pan troglodytes) participated in a delay of gratification task that required food exchange as the behavioral response that reflected self-control. The chimpanzees were offered opportunities to inhibit eating and instead exchange a currently possessed food item for a different (and sometimes better) item, often needing to exchange several food items before obtaining the highest valued reward. We manipulated reward type, reward size, reward visibility, delay to exchange, and location of the highest valued reward in the sequence of exchange events to compare performance within the same individuals. The chimpanzees successfully traded until obtaining the best item in most cases, although there were individual differences among participants in some variations of the test. These results support the idea that self-control is robust in chimpanzees even in contexts in which they perhaps anticipate future rewards and sustain delay of gratification until they can obtain the ultimately most valuable item.