A Stroop-like effect in color-naming of color-word lexigrams by a chimpanzee (Pan troglodyte)

The Stroop effect (J. R. Stroop, 1935) reflects the difficulty in ignoring irrelevant, but automatically processed, semantic information that is inherent in certain stimuli. With humans, researchers have found this effect when they asked participants to name the color of the letters that make up a word that is incongruent with that color. The authors tested a chimpanzee that had learned to associate geometric symbols called lexigrams with specific colors. When the chimpanzee had to make different responses that depended on the color of stimuli presented to her, she showed a Stroop-like effect when researchers presented to her the previously learned symbols for colors in incongruent font colors. Her accuracy performance was significantly poorer with these stimuli than with congruent color-referent lexigrams, noncolor-referent lexigrams, and nonlexigram stimuli, although there were not any significant differences in response latency. The authors' results demonstrated color-word interference in a Stroop task with a nonhuman animal.     

Chimpanzees (Pan troglodytes) remember the location of a hidden food item after altering their orientation to a spatial array

Two chimpanzees (Pan troglodytes) had a direct view of an experimenter placing a food item beneath one of several cups within a horizontal spatial array. The chimpanzees then were required to move around the spatial array, shifting their orientation to the array by 180 degrees . Both chimpanzees remembered the location of the food item. In the next experiment, a visual barrier was placed between the chimpanzees and the spatial array after the food item had been hidden to prevent visual tracking of the location of the object during the chimpanzees' movement. One chimpanzee remembered the location of the hidden item in this variation. These results demonstrate another capacity for spatial memory in this species that complements data indicating chimpanzee spatial memory for invisible displacements, array rotations, and array transpositions.     

Tactics to obtain a hidden food item in chimpanzee pairs (Pan troglodytes)

Five dyads of chimpanzees were tested in a competitive situation, as a pilot study to examine chimpanzees' understanding of conspecifics' knowledge. A human experimenter baited one of five containers in an outdoor enclosure. Chimpanzee A (witness) could see where the food was hidden, while chimpanzee B (witness-of-witness) could not see the baited place but could observe the chimpanzee A watching the food being hidden. Then the two were released into the enclosure. This procedure was repeated for a certain number of days along with a control condition in which neither could see the baited location. The witness-of-witness developed tactics to forestall the witness in two pairs. The witness misled the witness-of-witness by taking a route to an empty container in several cases. These episodes might represent examples of deception. Tactics and counter-tactics thus developed through the interaction between the witness and the witness-of-witness, illustrating the high social intelligence of chimpanzees. An examination of the changes in tactics suggests a possibility that the witness-of-witness understands the witness's knowledge of the location of hidden food. Accepted after revision: 22 May 2001 Electronic Publication     

Hiding valuables: A questionnaire study of mnemonically risky behavior

A survey investigated incidences where individuals intentionally hid objects but later forgot the item's location. Surveys from 111 younger and 99 older adults yielded the following prototypical scenario: the person hid the valuable 1 year ago to prevent theft, and waited 1 month before an initial 30-minute search, which included looking in dresser drawers plus 1.5 other locations. The person made three additional searches (total searches = 4) for the missing valuable over a 2 week period. Three-quarters of the hidden valuables were eventually found (usually by accident) about 1 month after hiding the object. About half of the time, the person had already searched the hiding area. Older adults typically hid jewellery from thieves, while younger adults hid money from friends and relatives. Older adults searched more often for longer periods of time for the hidden object, and took longer to eventually find the object, compared with younger adults.     

Development of sex differences in spatial memory.

Males typically perform better than females on tests of spatial abilities. However, Silverman and Eals (1992) found that females can surpass males on certain tasks, in particular recall of object location. This study investigated the age at which females begin having greater recall in object location. Recall was tested with object arrays (sets of line drawings of common objects) using samples of 80 adults (M age = 21 yr.) and 139 children at 3 age groups. Adult data replicated earlier findings, indicating significantly greater recall by women. The samples of children at different ages--4 yr. (n = 43), 10-11 yr. (n = 53), and 15-16 yr. (n = 43)--did not show significant sex differences in this type of recall. The findings suggest that females may surpass males in this spatial ability as adults, apparently due to gradual improvements in spatial memory through childhood and adolescence.     

Representation of object location in 6.5-month-old infants

Within a small bounded space, the location of a hidden object can be coded in terms of distance information, general area of hiding, or the boundary of the space. 6.5-month-old infants' use of these three coding strategies was examined using a visual search task. Infants watched as an object was hidden at one of four identical locations. After a short delay (10 s), the object either reappeared at the location where it was hidden (possible event), or reappeared at one of the other three locations (impossible event). Looking behavior was not systematically influenced by the amount of distance the object moved from the original location of hiding or by whether the object was hidden near a boundary. Infants did not appear to code the location of a hidden object in terms of distance information, general area of hiding, or whether it was hidden at a boundary. However, the location of reappearance (i.e., impossible event) did influence looking times. Infants were surprised when the object reappeared at a boundary position that was previously unoccupied. They were not surprised when the object reappeared at a central location. Thus, two factors influenced coding of location: boundary information (but in a different way than specified) and the nature of the change (absence vs. presence of an object). The influence of these two factors on coding of spatial information was discussed.     

Early understanding of perception as a source of knowledge

Two studies investigated preschool children's ability to infer another person's knowledge or ignorance on the basis of that person's recent perceptual experience. In Experiment 1 children were questioned about their own and a puppet's knowledge of a hidden object's color and about their own and the puppet's ability to see the hidden object. Three- and 4-year-olds attributed knowledge and perceptual experience to the person (either themselves or the puppet) who had viewed the hidden object, but not to the person who did not view it. Experiment 2 further investigated 3-year-olds understanding of perception as a source of knowledge. Children were asked to indicate which of two puppets, one who had viewed a hidden object and one who had not, would be able to tell them the object's color. Children chose the correct puppet more often than would be predicted by chance. The results of these experiments suggest that understanding of perception as a source of knowledge is present by the age of 3 years.     

One-year-olds’ understanding of nonverbal gestures directed to a third person

We investigated whether infants comprehend others’ nonverbal communicative intentions directed to a third person, in an ‘overhearing’ context. An experimenter addressed an assistant and indicated a hidden toy's location by either gazing ostensively or pointing to the location for her. In a matched control condition, the experimenter performed similar behaviors (absent-minded gazing and extended index finger) but did not communicate ostensively with the assistant. Infants could then search for the toy. Eighteen-month-old infants were skillful in using both communicative cues to find the hidden object, whereas 14-month-olds performed above chance only with the pointing cue. Neither age group performed above chance in the control condition. This study thus shows that by 14–18 months of age, infants are beginning to monitor and comprehend some aspects of third party interactions.     

Out of Sight Is Not Out of Mind: Developmental Changes in Infants' Understanding of Visual Perception During the Second Year

Three experiments investigated 14-, 18-, and 24- month-old infants' understanding of visual perception. Infants viewed films in which a protagonist was either able to view the location of a hidden object (Visual Access condition) or was blindfolded when the object location was revealed (No Visual Access condition). When requested to find the object, the protagonist pointed either at the correct location or at the incorrect location. Across experiments, 18-month-olds looked longer at the unexpected action (e.g., person pointing at the incorrect location in the Visual Access condition). By 24 month of age, infants could infer the correct search behavior from gaze alone. These findings suggest that by the middle of the second year of life, infants understand the psychological relation between an observer and an object, even if the object is no longer visible.     

Motivation and memory in zebra finch (<Emphasis Type="Italic">Taeniopygia guttata</Emphasis>) foraging behavior

Remembering combinations of information such as what resources have been seen in which locations could play an important role in enhancing individual survival through increased foraging success. To date, there have been relatively few investigations of avian memory involving more than one category of information. This study explored zebra finches’ (Taeniopygia guttata) capacity to recall two categories in combination, namely food-type and spatial location. Birds were trained to remove variously weighted flaps to find two types of food hidden beneath. Memory for food-types and locations was assessed by pre-feeding the birds to satiety on and devaluing one food-type, and then testing the birds’ efficiency at finding the non-devalued food. When allowed one trial to learn locations of two food rewards that were hidden beneath lightly weighted flaps, birds performed better than chance at locating a food reward. However, they did not preferentially search for the non pre-fed food, suggesting that they were unable to recall both food type and location in combination. Zebra finches made fewer errors when tested on the one-trial task using more heavily weighted flaps than with lightly weighted flaps; there was equivocal evidence that they remembered which food type was hidden where on this task. When given repeated exposures to the locations of the two food rewards, finches located a food reward more accurately than on the one-trial tasks, and were also more likely to recall the locations of the different food types. In this foraging paradigm, experience and motivation may have influenced the birds’ performance.     

A simple but powerful test of perseverative search in dogs and toddlers

Perseverative (A-not-B) errors during the search of a hidden object were recently described in both dogs and 10-month-old infants. It was found that ostensive cues indicating a communicative intent of the person who hides the object played a major role in eliciting perseverative errors in both species. However, the employed experimental set-up gave rise to several alternative explanations regarding the source of these errors. Here we present a simplified protocol that eliminates the ambiguities present in the original design. Using five consecutive object hiding events to one of two locations in a fixed order (“AABBA”), we tested adult companion dogs and human children (24 months old). The experimenter performed the hiding actions while giving ostensive cues in each trial and moved the target object to the given location in a straight line. Our results show that in the B trials, both 24-month-old children and dogs could not reliably find the hidden object, and their performance in the first B trials was significantly below that of any of the A trials. These results are the first to show that the tendency for perseverative errors in an ostensive-communicative context is a robust phenomenon among 2-year-old children and dogs, and not the by-product of a topographically elaborate hiding event.     

Infants’ coding of location in continuous space

The ability to code location in continuous space is fundamental to spatial behavior. Existing evidence indicates a robust ability for such coding by 12 months, but systematic evidence on earlier origins is lacking. A series of studies investigated 5-month-olds’ ability to code the location of an object hidden in a sandbox, using a looking-time paradigm. In Experiment 1, after familiarization with a hiding-and-finding sequence at one location, infants looked longer at an object being disclosed from a location 12 inches (30 cm) away than at an object emerging from the hiding location, showing they were able to code location in continuous space. In Experiment 2, infants reacted with greater looking when objects emerged from locations 8 inches (20 cm) away from the hiding location, showing that location coding was more finely grained than could be inferred based on the first study. In Experiment 3, infants were familiarized with an object shown in hiding-and-finding sequences at two different locations. Infants looked longer at objects emerging 12 inches (30 cm) away from the most recent hiding location than to emergence from the other location, showing that infants could code location even when events had previously occurred at each location. In Experiment 4, after familiarization with two objects with different shapes, colors, and sounding characteristics, shown in hiding-and-finding sequences in two locations, infants reacted to location violations as they had in Experiment 3. However, they did not react to object violations, that is, events in which the wrong object emerged from a hiding location. Experiment 5 also found no effect of object violation, even when the infants initially saw the two objects side by side. Spatiotemporal characteristics may play a more central role in early object individuation than they do later, although further study is required.     

Gender differences in object location memory: A meta-analysis

The goal of the present study was to quantify the magnitude of gender differences in object location memory tasks. A total of 123 effect sizes (d) drawn from 36 studies were included in a meta-analysis using a hierarchical approach. Object identity memory (37 effect sizes) and object location memory (86 effect sizes) tasks were analyzed separately. Object identity memory task showed significant gender differences that were homogeneous and in favor of women. For the object location memory tasks, effect sizes had to be partitioned by age (younger than 13, between 13 and 18, older than 18), object type (common, uncommon, gender neutral, geometric, masculine, feminine), scoring method (accuracy, time, distance), and type of measure (recall, recognition) to achieve homogeneity. Significant gender differences in favor of females were obtained in all clusters above the age of 13, with the exception of feminine, uncommon, and gender-neutral objects. Masculine objects and measures of distance produced significant effects in favor of males. Implications of these results for future work and for theoretical interpretations are discussed.     

Spontaneous symbol acquisition and communicative use by pygmy chimpanzees (Pan paniscus)

Two pygmy chimpanzees (Pan paniscus) have spontaneously begun to use symbols to communicate with people. In contrast to common chimpanzees (Pan troglodytes) using the same communicative system, the pygmy chimpanzees did not need explicit training in order to form referential symbol-object associations. Instead, they acquired symbols by observing others use these symbols in daily communications with them. In addition, the pygmy chimpanzees have begun to comprehend spoken English words and can readily identify lexigrams upon hearing the spoken words. By contrast, common chimpanzees who received similar exposure to spoken English are unable to do so. The older pygmy chimpanzee has begun to form requests of the form agent-verb-recipient in which he is neither the agent nor the recipient. By contrast, similarly aged common chimpanzees limited their requests to simple verbs, in which the agent was always presumed to be the addressee and the chimpanzee itself was always the recipient, thus they had no need to indicate a specific agent or recipient. These results suggest that these pygmy chimpanzees exhibit symbolic and auditory perceptual skills that are distinctly different from those of common chimpanzees.     

Infants' search for visible objects: implications for the interpretation of early search errors

This study examined the error patterns of 9-month-old infants searching for hidden objects and objects that were visible within a container. Although errors occurred in both conditions, there were important differences between them. When the object was hidden, infants showed significant perseveration in that they searched more often at the object's previous hiding place than at a control location. When the object was visible, however, they made fewer errors and the errors they did make were as likely to be to the control location as to the previous hiding place. These results suggest that infants' errors in searching for a visible object reflect lapses of attention rather than systematic misunderstandings of objects or space and so are not incompatible with an information-processing account of early search.     

Cues that chimpanzees do and do not use to find hidden objects

Chimpanzees follow conspecific and human gaze direction reliably in some situations, but very few chimpanzees reliably use gaze direction or other communicative signals to locate hidden food in the object-choice task. Three studies aimed at exploring factors that affect chimpanzee performance in this task are reported. In the first study, vocalizations and other noises facilitated the performance of some chimpanzees (only a minority). In the second study, various behavioral cues were given in which a human experimenter either touched, approached, or actually lifted and looked under the container where the food was hidden. Each of these cues led to enhanced performance for only a very few individuals. In the third study – a replication with some methodological improvements of a previous experiment – chimpanzees were confronted with two experimenters giving conflicting cues about the location of the hidden food, with one of them (the knower) having witnessed the hiding process and the other (the guesser) not. In the crucial test in which a third experimenter did the hiding, no chimpanzee found the food at above chance levels. Overall, in all three studies, by far the best performers were two individuals who had been raised in infancy by humans. It thus seems that while chimpanzees are very good at “behavior reading” of various sorts, including gaze following, they do not understand the communicative intentions (informative intentions) behind the looking and gesturing of others – with the possible exception of enculturated chimpanzees, who still do not understand the differential significance of looking and gesturing done by people who have different knowledge about states of affairs in the world. Received: 8 November 1999 / Accepted after revision: 24 January 2000     

Testing for symmetry in the conditional discriminations of language-trained chimpanzees

If subjects are taught to match Stimulus A to B and then, without further training, match B to A, they have passed a test of symmetry. It has been suggested that non-humans' lack of success on symmetry tests might be overcome by giving them a history of symmetry exemplar training, that is, by directly teaching a large number of conditional relations (e.g., AB, CD, EF,...) and also directly training the "reverse" of these relations (e.g., BA, DC, FE,...). The chimpanzee subjects of the present study, Sherman, Austin, and Lana, had already received extensive symmetry exemplar training as a result of attempts to teach a selection-based language system of lexigrams. The present study systematically subjected 2 of these chimps (Sherman and Lana), for the first time, to standard symmetry tests in controlled conditions. Both chimps failed the tests, even when their correct responses on test trials were reinforced. The findings do not support the exemplar training hypothesis, and cast doubt upon whether the chimps can pass tests of stimulus equivalence.     

Walking through doorways causes forgetting: recall

The aim of the current study was to explore how the location updating effect is affected when people are tested using recall rather than recognition, which is what has been done in prior work. Differences in the memory processes involved with these two tasks lead to predictions for two different patterns of data. In Experiment 1, memory was tested by having participants recall the single object they were carrying or had just put down, whereas in Experiment 2, people sometimes needed to recall both objects. It was found that, unlike recognition test performance, a similar location updating effect was found for both Associated (what was currently being carried) and Dissociated (what was recently set down) objects. Moreover, when both objects were correctly recalled, there was a bias to remember them in the order that they were encountered. Finally, if only one object was correctly recalled, it was the Associated object that was currently being carried. Overall, these results are consistent with the Event Horizon Model of event cognition.     

Effects of arm weight on C-not-B task performance: implications for the motor inhibitory deficit account of search failures

Toddlers have been found to fail on a three-location search task involving the invisible displacements of an object, namely the C-not-B task. In this task, a child is shown the experimenter's hand that contains a toy. The toy then successively disappears under the three cloths (A, B, then C). The examiner silently releases the toy under the second cloth (B). The hidden object makes a bump in the B cloth that covers it. Young children emit a strong bias toward the last cloth that the experimenter's hand passes under, and this has been labeled the C-not-B error. One possible explanation for toddlers' failures in the C-not-B task is that children lack the motor inhibitory mechanisms. To test this hypothesis, the robustness of the C-not-B error was tested, in a first experiment, against variations in body parameters. By putting additional weights on the arm, the C-not-B error was reduced substantially and the C-not-B task had a higher rate of success. Indeed, in contrast to control participants, who ignored a visual clue indicating the correct location of the hidden object and reached for the last location of the experimenter's hand, the participants with arm weights initiated their reaching movements by using the visual clue. The findings from the second control group indicate that the dramatic increase in successful performance by children with arm weights is not merely a consequence of the focus on the attention to arm movements. The motion of the experimenter's hand in space appears to have made the task difficult because toddlers had no problems inferring that a lump under a cloth indicates the existence of an object without actually having watched an object be hidden there, as demonstrated in a second experiment. The findings are consistent with the hypothesis that C-not-B task content activates a prepotent motor response that preempts full consideration of a visual clue indicating the correct location of the hidden object. We propose that the success in the C-not-B task of toddlers with additional arm weights could result from a disruption of automatic hand movement that is triggered by sensory signals, namely salient features of the C-not-B task.     

Prospective memory in a language-trained chimpanzee (Pan troglodytes)

Prospective memory involves the encoding, retention, and implementation of an intended future action. Although humans show many forms of prospective memory, less is known about the future oriented processes of nonhuman animals, or their ability to use prospective memory. In this experiment, a chimpanzee named Panzee, who had learned to associate geometric forms called lexigrams with real-world referents, was given a prospective memory test. Panzee selected between two foods the one she wanted to receive more immediately. That food was scattered in an outdoor yard where she could forage for it. Also outdoors were lexigram tokens, one of which represented the food item that remained indoors throughout a 30 min period, and that could be obtained if Panzee brought in the token that matched that food item. After foraging for the selected food item, Panzee consistently remembered to retrieve and return the correct token when food was available indoors, whereas on control trials involving no indoor food she rarely returned a token. This indicated that Panzee encoded information relevant to the future action of token retrieval after extended delays for one type of food, even when a more immediately preferred food was available.