The Thatcher illusion in squirrel monkeys (Saimiri sciureus)

Like humans, Old World monkeys are known to use configural face processing to distinguish among individuals. The ability to recognize an individual through the perception of subtle differences in the configuration of facial features plays an important role in social cognition. To test this ability in New World monkeys, this study examined whether squirrel monkeys experience the Thatcher illusion, a measure of face processing ability in which changes in facial features are difficult to detect in an inverted face. In the experiment, the monkeys were required to distinguish between a target face and each of the three kinds of distracter faces whose features were altered to be different from those of the target. For each of the pairs of target and distracter faces, four rotation-based combinations of upright and inverted face presentations were used. The results revealed that when both faces were inverted and the eyes of the distracter face were altered by rotating them at an angle of 180° from those of the target face, the monkeys' discrimination learning was obstructed to a greater extent than it was under the other conditions. Thus, these results suggest that the squirrel monkey does experience the Thatcher illusion. Furthermore, it seems reasonable to assume that squirrel monkeys can utilize information about facial configurations in individual recognition and that this facial configuration information could be useful in their social communications.     

Colour versus quantity as cues in reverse-reward-competent squirrel monkeys (Saimiri sciureus)

To assess the relative salience of colour and quantity cues, squirrel monkeys previously trained to reach for the smaller of two quantities of food in a reverse-reward contingency task received colour discrimination training. After initial failure to discriminate between two colours of dots under a differential reinforcement regime, they learned the task when the S- colour was associated with zero reward. The monkeys then showed good retention on the original reverse-reward task of 1 versus 4 with pairs of dots presented in S+ or S- colours. However, on "mismatch" trials of 1S- versus 4S+ , only 2 of 4 monkeys tested showed a preference--1 monkey chose based on quantity, the other based on colour. Individual differences and the possible roles of overshadowing and blocking are discussed.     

Colour versus quantity as cues in reverse-reward-competent squirrel monkeys (Saimiri sciureus)

To assess the relative salience of colour and quantity cues, squirrel monkeys previously trained to reach for the smaller of two quantities of food in a reverse-reward contingency task received colour discrimination training. After initial failure to discriminate between two colours of dots under a differential reinforcement regime, they learned the task when the S? colour was associated with zero reward. The monkeys then showed good retention on the original reverse-reward task of 1 versus 4 with pairs of dots presented in S + or S? colours. However, on “mismatch” trials of 1S? versus 4S + , only 2 of 4 monkeys tested showed a preference—1 monkey chose based on quantity, the other based on colour. Individual differences and the possible roles of overshadowing and blocking are discussed.     

Training squirrel monkeys (Saimiri sciureus) to deceive: acquisition and analysis of behavior toward cooperative and competitive trainers

Three squirrel monkeys (Saimiri sciureus) learned to reach toward a container that covered food if a cooperative trainer rewarded such reaches by giving the food. A competitive trainer kept any food found, but wrong selections by this trainer were also rewarded. The monkeys initially reached toward the baited container indiscriminately, but gradually and with the aid of color-cued containers, all 3 reliably reached "honestly" and "deceptively" in the presence of the cooperative and competitive trainers, respectively. The monkeys did not appear to take the trainers' knowledge about the location of the food into account, and deception did not occur if food was placed under the normally unbaited container. With additional containers present, monkeys misled the competitive trainer into selecting the unbaited container farthest from the baited one. Although not indicative of mental attribution, the monkeys' behavior suggests awareness of the acquired communicative function of the reaching response.     

Comparative studies of squirrel monkeys (Saimiri sciureus) and titi monkeys (Callicebus moloch): performance on choice tasks in near space

Squirrel monkeys (Saimiri sciureus) and titi monkeys (Callicebus moloch) were studied in tasks involving reaching for food in near space (arm's reach). Although performance by monkeys of the two species differed in several ways familiar from previous studies, the species did not differ in the tendency to adopt a habitual position or limb during reaching. The findings contrast with previous work on spatial preferences in these species in tasks involving movement of the whole body. Together with the results of previous studies on movement patterns in these two species, the findings are placed in a comparative psychological framework of the proximate sources of use of space in nature.     

Reverse-reward learning in squirrel monkeys (Saimiri sciureus): retesting after 5 years, and assessment on qualitative transfer

Seven squirrel monkeys (Saimiri sciureus) previously trained on reverse-reward tasks were presented with the original "1-versus-4" task after a 5-year interval without reverse-reward experience (Experiment 1). None of them reliably selected the smaller food array; however, at around chance level, their performance was superior to when they were first exposed to the task almost 6 years previously, suggesting some long-term memory retention. One naive monkey consistently selected the larger array, as expected. In Experiment 2, trials consisting of 1 versus 1 piece of two qualitatively different types of food were interspersed among familiar 1-versus-4 trials. None of five monkeys tested reliably selected the less-preferred food to get the more preferred food as the reward, and one monkey scored below chance. However, when one piece of low-preference food was paired with four pieces of high-preference food (Experiment 3), all four monkeys tested avoided reaching for the latter and thereby obtained it as the reward; two monkeys obtained perfect scores on these trials. These two monkeys were trained on a specific qualitative reverse-reward pairing and then again tested on new pairings (Experiment 4), but transfer was incomplete. Compound trials that pit quantity against quality in novel ways appear taxing for squirrel monkeys, despite competence in reverse-reward on both dimensions separately.     

Anticipation of future events in squirrel monkeys (Saimiri sciureus) and rats (Rattus norvegicus): tests of the Bischof-Kohler hypothesis

The Bischof-Kohler hypothesis holds that nonhuman animals cannot anticipate a future event and take appropriate action when that event involves satisfaction of a need not currently experienced. Tests of the Bischof-Kohler hypothesis were performed with squirrel monkeys (Saimiri sciureus) and rats (Rattus norvegicus). In experimental trials with both species, a nonthirsty animal had its water bottle removed and then chose between a smaller and larger quantity of food. Consumption of the food induced thirst. Choice of the smaller quantity led to the return of the water bottle sooner than choice of the larger quantity. Monkeys reversed their baseline preference for the larger quantity of food when the experimental contingencies were introduced, but rats continued to prefer the larger amount. Although the rat findings support the Bischof-Kohler hypothesis, the monkey findings challenge it.     

Alternative response training, differential reinforcement of other behavior, and extinction in squirrel monkeys (Saimiri sciureus)

In Experiment I, (a) extinction, (b) extinction plus reinforcement of a discrete alternative response, and (c) differential reinforcement of other behavior were each correlated with a different stimulus in a three-component multiple schedule. The alternative-response procedure more rapidly and completely suppressed behavior than did differential reinforcement of other behavior. Differential reinforcement of other behavior was slightly more effective than extinction alone. In Experiment II, reinforcement of specific alternative behavior during extinction and differential reinforcement of other behavior were used in two components, while one component continued to provide reinforcement for the original response. Once again, the alternative-response procedure was most effective in reducing responding as long as it remained in effect. However, the responding partially recovered when reinforcement for competing behavior was discontinued. In general, responding was less readily reduced by differential reinforcement of other behavior than by the specific alternative-response procedure.     

Repeated acquisition and discrimination reversal in the squirrel monkey (Saimiri sciureus)

Repeated acquisition and discrimination reversal tasks are often used to examine behavioral relations of, respectively, learning and cognitive flexibility. Surprisingly, despite their frequent use in cognitive neuroscience and behavioral pharmacology, variables that control performance under these two tasks have not been widely studied. The present studies were conducted to directly investigate the controlling variables in nonhuman primates. Squirrel monkeys were trained with a touchscreen variant of the repeated acquisition task in which a novel pair of S⁺/S^? stimuli was presented daily. Subjects learned to discriminate the two stimuli (acquisition) and, subsequently, with the contingencies switched (reversal). Results indicate that rates of both acquisition and reversal learning increased across successive sessions, but that rate of reversal learning remained slower than acquisition learning, i.e., more trials were needed for mastery. Subsequent experiments showed this difference between the rate of learning novel discriminations and reversal was reliable for at least 5 days between acquisition and reversal and notwithstanding the interpolation of additional discriminations. Experimental analysis of the S⁺/S^? elements of the tasks revealed that the difference in the rate of learning could not be attributed to a relatively aversive quality of the S^? or to a relatively appetitive quality of the S⁺, but, rather, to contextual control by the S⁺/S^? stimulus complex. Thus, if either element (S⁺ or S^?) of the stimulus complex was replaced by a novel stimulus, the rate of acquisition approximated that expected with a novel stimulus pair. These results improve our understanding of fundamental features of discrimination acquisition and reversal.     

Number-related discrimination and summation by squirrel monkeys (Saimiri sciureus sciureus and S. boliviensus boliviensus) on the basis of the number of sides of polygons

In Experiment 1, with the number of sides or angles of irregular polygons as cues, programmed training, and a 90% correct criterion (36 of 40), 2 squirrel monkeys' (Saimiri sciureus sciureus and S. boliviensus boliviensus) best performances were to discriminate heptagons from octagons, a 3rd's best was hexagons from heptagons, and a 4th's best was pentagons from heptagons. In Experiment 2, on most trials 2 polygons on one or both discriminanda had to be summed to determine which discrimination had the total fewer sides. Only 1 monkey met criterion (27 of 30) on the 2 tasks, 6 vs. 8 and 7 vs. 8 sides, but the other 3 performed better than chance on the 6 vs. 8 task. We conclude that previous studies of animals' discrimination of polygons in terms of complexity were minimally relevant to this work, and counting and subitizing were rejected in favor of a prototype-matching process to explain our monkeys' performances.     

Auditory sensitivity and equal loudness in the squirrel monkey (Saimiri sciureus).

The effects of the type of reinforcer on auditory sensitivity and equal-loudness data were determined in the squirrel monkey. The monkeys, restrained and provided with earphones, were conditioned to depress and hold a bar down in the presence of a stimulus light and then to terminate the holding response after onset of a tone. In Experiment 1, the specified behavior sequence postponed electric shock; in Experiment 2, a food reinforcer was dependent on bar release during the tone. The shape of the auditory sensitivity function and the acuity level at each frequency were the same for the two procedures. The audible frequency range extended from below 0.125 kHz (lowest frequency used) to 46 kHz. Sensitivity was maximum at 8 kHz. Latency of bar release following tone onset served as the basic data for constructing a family of equal-loudness contours. The type of reinforcer appeared not to be a determinant of either the shape of individual loudness contours or the pattern of family of equal-loudness functions. At the lower sound-pressure levels, the equal-loudness contours closely paralleled the threshold curves. At more-intense levels, the contours tended to flatten and depend less on frequency.     

Numerity of a socially housed hamadryas baboon (Papio hamadryas) and a socially housed squirrel monkey (Saimiri sciureus)

To extend a study conducted by E. M. Brannon and H. S. Terrace (1998, 2000), the authors trained 1 hamadryas baboon (Papio hamadryas) and 1 squirrel monkey (Saimiri sciureus) to respond to stimuli representing the numerosities 1-4 in ascending order. When tested with novel stimuli of the same numerosities, both subjects' performance appeared to be based on the numerical attributes of stimuli. Subjects were then tested on their ability to order pairs of numerosities derived from the values 1-9. Both subjects successfully ordered pairs that included the untrained numerosities 5-9 regardless of the total surface area of numerosities. Accuracy and latency of responding also showed numerical distance and magnitude effects. Numerosity was a salient cue to both subjects, suggesting that these 2 families of primates perceive ordinal relations. The outcome shows that cognitive studies of this type can be effectively conducted with socially housed animals.     

Conditional discrimination with conceptual simultaneous and successive cues in the squirrel monkey (Saimiri sciureus)

At the end of several stages of training, 4 squirrel monkeys met an 87% criterion for choosing correctly between exemplars of the concept "sameness" and the concept "difference," the simultaneous cues, as a function of having been cued by an exemplar either of the concept "triangularity" (cued sameness) or "heptagonality" (cued difference), the successive cues, which were presented in random order. In addition, the best monkey met criterion when the exemplars of sameness and difference were presented 16 s after the withdrawal of the exemplar of triangularity or heptagonality, and the other monkeys performed successfully with shorter delays. The results are discussed in terms of (a) "working memory," (b) the significance for the evolution of behavior of investigating the conceptual capacities of animals, and (c) implications for language in nonhuman animals.     

Attention to combined attention in new world monkeys (Cebus apella, Saimiri sciureus)

Co-orientation by capuchin (Cebus apella) and squirrel (Saimiri sciureus) monkeys in response to familiar humans abruptly switching the direction of their visual attention was recorded. Co-orientation occurred more frequently overall in capuchins than squirrel monkeys. Capuchins showed a tendency to habituate within trials involving consecutive attention switches performed by 2 different people, whereas squirrel monkeys co-oriented more when the 2nd attention switch was by a 2nd actor. These results suggest variable attention-processing abilities in New World monkeys, including differences in summation of attention by others.     

Slow wave changes in amygdala to visual, auditory, and social stimuli following lesions of the inferior temporal cortex in squirrel monkey (Saimiri sciureus)

Radiotelemetry of slow wave activity of the amygdala was recorded under a variety of conditions. Power, and the percentage of power in the delta band, increased in response to stimulation. Recordings of monkey vocalizations and slides of ethologically relevant, natural objects produced a greater increase in power than did control stimuli. The responses to auditory stimuli increased when these stimuli were presented in an unrestrained, group setting, yet the responses to the vocalizations remained greater than those following control stimuli. Both the natural auditory and visual stimuli produced a reliable hierarchy with regard to the magnitude of response. Following lesions of inferior temporal cortex, these two hierarchies are disrupted, especially in the auditory domain. Further, these same stimuli, when presented after the lesion, produced a decrease, rather than an increase, in power. Nevertheless, the power recorded from the natural stimuli was still greater than that recorded from control stimuli in that the former produced less of a decrease in power, following the lesion, than did the latter. These data, in conjunction with a parallel report on evoked potentials in the amygdala, before and after cortical lesions, lead us to conclude that sensory information, particularly auditory, available to the amygdala, following the lesion, is substantially the same, and that it is the interpretation of this information, by the amygdala, which is altered by the cortical lesion.     

Measurement of social dominance in squirrel monkeys

Several laboratory methods for measuring social dominance in squirrel monkeys are described.     

Squirrel monkeys (Saimiri sciureus) choose smaller food arrays: long-term retention, choice with nonpreferred food, and transposition

Squirrel monkeys (Saimiri sciureus) that had learned to reach toward 1 piece of food instead of 4 in a reverse-reward contingency were tested after an 8-month delay with no intervening relevant experiences. All monkeys except 1 continued to show inhibitory control by reliably reaching toward the smaller quantity, most of them doing so within 2 sessions. Performance was maintained when a low-preference food replaced prized foods as arrays and rewards. When the quantities 1 and 4 were replaced with different ones, there was strong evidence of transposition at group level, although individual differences in bias toward the smaller quantities became apparent. Individual differences in mastering the original task more than 8 months previously were quite stable, suggesting robustness in the operations required for this form of self-control.