On the limits of the matching concept in monkeys (Cebus apella).

Two cebus monkeys, with many years of experience matching a variety of static visual stimuli (forms and colors) within a standard matching-to-sample paradigm, were trained to press a left lever when a pair of displayed static stimuli were the same and to press a right lever when they were different. After learning the same/different task, the monkeys were tested for transfer to dynamic visual stimuli (flashing versus steady green disks), with which they had no previous experience. Both failed to transfer to the dynamic stimuli. A third monkey, also with massive past experience matching static visual stimuli, was tested for transfer to the dynamic stimuli within our standard matching paradigm, and it, too, failed. All 3 subjects were unable to reach a moderate acquisition criterion despite as many as 52 sessions of training with the dynamic stimuli. These results provide further evidence that, in monkeys, the matching (or identity) concept has a very limited reach; they consequently do not support the view held by some theorists that an abstract matching concept based on physical similarity is a general endowment of animals.     

A reversed Ebbinghaus–Titchener illusion in bantams (Gallus gallus domesticus)

A disk surrounded by smaller disks looks larger, and one surrounded by larger disks looks smaller than reality. This visual illusion, called the Ebbinghaus–Titchener illusion, remains one of the strongest and most robust illusions induced by contrast with the surrounding stimuli in humans. In the present study, we asked whether bantams would perceive this illusion. We trained three bantams to classify six diameters of target disks surrounded by inducer disks of a constant diameter into “small” or “large”. In the test that followed, the diameters of the inducer disks were systematically changed. The results showed that the Ebbinghaus–Titchener figures also induce a strong illusion in bantams, but in the other direction, that is, bantams perceive a target disk surrounded by smaller disks to be smaller than it really is and vice versa. Possible confounding factors, such as the gap between target disk and inducer disks and the weighted sum of surface of these figural elements, could not account for the subjects’ biased responses. Taken together with the pigeon study by Nakamura et al. (J Exp Psychol Anim Behav Process 34:375–387 2008), these results show that bantams as well as pigeons perceive an illusion induced by assimilation effects, not by contrast ones, for the Ebbinghaus–Titchener types of illusory figures. Perhaps perceptual processes underlying such illusory perception (i.e., lack of contrast effects) shown in bantams and pigeons may be partly shared among other avian species.     

Formation of the sameness-difference concept by Japanese monkeys from a small number of color stimuli

Japanese monkeys were trained to form the sameness-difference concept. In Experiment 1, four monkeys were trained with two colors to discriminate matching stimulus pairs from nonmatching pairs by reinforcing only lever-pressing responses to matching pairs with a variable-interval schedule. Three monkeys showed successful transfer of this discrimination to two new colors, thus demonstrating that some Japanese monkeys are able to form this relational concept from a minimum number of stimuli. In Experiment 2, two monkeys were trained, in a Yes/No procedure with three colors, to press one lever under matching pairs and another lever under nonmatching pairs. Poor transfer performances to three new colors suggest that simultaneously establishing two different response patterns to matching and nonmatching pairs is ineffective in forming the concept. In Experiment 3, the amount of transfer to three new colors after mastering a standard three-color matching-to-sample task was compared with that of a modified task in which correct responses were reinforced with a within-trial variable-interval schedule. All three monkeys showed greater transfer with the modified procedure. The results suggest that the variable-interval schedule adopted within trials is effective in forming the sameness-difference concept.     

Rhesus macaques (Macaca mulatta) categorize unknown conspecifics according to their dominance relations

The authors trained 3 adult male rhesus macaques (Macaca mulatta) to categorize pairs of unknown conspecifics presented in a video according to the dominance status of the videotaped monkeys. The subjects were trained to choose the dominant monkey for a category of films (e.g., films showing 1 monkey chasing another); then, new films were presented involving different conspecifics, and the monkeys' first responses to this new category of behavior (e.g., monkeys fighting) were taken as evidence of transfer. Two subjects were able to generalize categorical judgments of dominance to new films involving new behaviors. These findings seem to indicate that monkeys can use abstract social concepts and are aware of the social relationships within their group.     

Contingent aftereffects: lateral interactions between color and motion

A randomly dotted yellow disk was rotated at a speed of 5 rpm, alternating in direction every 10 sec. Its change in direction of rotation was paired with a change in surround color, which was either red or green. After 15 min of exposure, observers reported vivid motion aftereffects contingent on the color of both the stationary disk and the surround, even though during adaptation only motion or color was associated with either alone. In further experiments, it was established that a change in color (or direction of motion) of the disk could be associated with a change in direction of motion (or color) of the surround. Such lateral effects were found even when a wide (5 degree) annulus was introduced between the disk and the surround during adaptation and testing. Furthermore, the aftereffects generalized to the annulus, which was not associated with either color or motion during adaptation. However, when the disk alone was adapted to color and motion, no generalization to the surround was found (and vice versa), suggesting that the effects are not produced by adaptation of large receptive fields or by scatter of light within the eye. The results appear to conflict with the ideas that contingent aftereffects are confined to the adapted area of the retina and that they are built up by links between single-duty neurones, and with an extreme view of the segregation of color and motion early in human vision.     

Sequential planning in rhesus monkeys (<Emphasis Type="Italic">Macaca mulatta</Emphasis>)

In the current study, we examined the planning abilities of rhesus monkeys (Macaca mulatta) by training them on a five-item list composed of coloured photographs and then testing them on switch and mask trials. In contrast to previous studies where monkeys made responses using a joystick, in the current study, monkeys made responses directly to a touch screen. On switch trials, after a response to the first list item, the on-screen positions of two list items were exchanged. Performance on trials in which the second and third list items were exchanged was poorer compared to normal (non-switch) trials for all subjects. When the third and fourth items were exchanged, however, only one subject continued to show performance deficits. On mask trials, following a response to the first item, the remaining items were covered by opaque white squares. When two items were masked, all four subjects responded to each masked item at a level significantly above chance. When three items were masked, however, only one subjected was able to respond to all three masked items at a level significantly above chance. The results of the present study indicate that three of our four monkeys planned one response ahead while a single monkey planned two responses ahead. The significance of these findings is discussed in relation to previous studies on planning in chimpanzees and monkeys.     

Effect of introducing novel configurational cues on a learned color discrimination in squirrel monkeys.

The study explored the effect of introducing novel cues on a learned color discrimination. 6 squirrel monkeys were initially trained to discriminate between plaques covered with green and brown construction paper. In a series of transfer tests, these color cues were then presented in 6 different patterns and varied in 4 sizes. The results indicate that some patterns will produce more disruption than others. Furthermore, reducing the size of the cues may also lead to a decrement in performance level. The outcome corresponds well with those of studies which show that contextual variables influence the acquisition of discriminative responses.     

Visually guided facial actions in rhesus monkeys

We trained 2 monkeys to display facial actions in response to corresponding arbitrary visual cues. Each monkey executed the task successfully, and each displayed two different facial actions corresponding to either hand-sign or color cues. More than 90% of the responses were correct for each monkey. These results provide evidence that monkeys can execute facial actions in response to conditioned visual cues in the absence of social context. These data suggest that facial actions of monkeys are flexible enough for use in further laboratory investigations—for example, in studies on the neural mechanisms underlying the execution of actions.     

How chimpanzees (<Emphasis Type="Italic">Pan troglodytes</Emphasis>) perform in a modified emotional Stroop task

The emotional Stroop task is an experimental paradigm developed to study the relationship between emotion and cognition. Human participants required to identify the color of words typically respond more slowly to negative than to neutral words (emotional Stroop effect). Here we investigated whether chimpanzees (Pan troglodytes) would show a comparable effect. Using a touch screen, eight chimpanzees were trained to choose between two simultaneously presented stimuli based on color (two identical images with differently colored frames). In Experiment 1, the images within the color frames were shapes that were either of the same color as the surrounding frame or of the alternative color. Subjects made fewer errors and responded faster when shapes were of the same color as the frame surrounding them than when they were not, evidencing that embedded images affected target selection. Experiment 2, a modified version of the emotional Stroop task, presented subjects with four different categories of novel images: three categories of pictures of humans (veterinarian, caretaker, and stranger), and control stimuli showing a white square. Because visits by the veterinarian that include anaesthetization can be stressful for subjects, we expected impaired performance in trials presenting images of the veterinarian. For the first session, we found correct responses to be indeed slower in trials of this category. This effect was more pronounced for subjects whose last anaesthetization experience was more recent, indicating that emotional valence caused the slowdown. We propose our modified emotional Stroop task as a simple method to explore which emotional stimuli affect cognitive performance in nonhuman primates.     

Representation of serial order in pigeons (Columba livia)

In Experiment 1, 2 groups of pigeons were trained to respond to either a 4-item (A→B→C→D) or 5-item (A→B→C→D→E) list. After learning their respective list, half of the subjects were trained on a positive pair with reinforcement provided when pairs were responded to in the order true to that of the original sequence (4-item: B→C; 5-item: B→D). The remaining subjects were trained on a negative pair with reinforcement provided for responding to the pairs in the order opposite to that learned in the original sequence (4-item: C→B; 5-item: D→B). Subjects in the positive pair condition learned their respective pair faster than did subjects in the negative pair condition. In Experiment 2, after reaching criterion on a 4-item list, subjects received 16 BC probe trials spread across 4 sessions of training. Subjects performed significantly above chance on the probe trials. The performance of our subjects in Experiments 1 and 2 demonstrates that, similar to monkeys, pigeons form a representation of the lists that they learn.     

Multisensory synesthetic interactions in the speeded classification of visual size

In the present study, we attempted to demonstrate a synesthetic relationship between auditory frequency and visual size. In Experiment 1, participants performed a speeded visual size discrimination task in which they had to judge whether a variable-sized disk was bigger or smaller than a standard reference disk. A task-irrelevant sound that was either synesthetically congruent with the relative size of the disk (e.g., a low-frequency sound presented with a bigger disk) or synesthetically incongruent with it (e.g., a low-frequency sound presented with a smaller disk) was sometimes presented together with the variable disk. Reaction times were shorter in the synesthetically congruent condition than in the incongruent condition. Verbal labeling and semantic mediation interpretations of this interaction were explored in Experiment 2, in which high- and low-frequency sounds were presented in separate blocks of trials, and in Experiment 3, in which the tones were replaced by the spoken words "high" and "low." Response priming/bias explanations were ruled out in Experiment 4, in which a synesthetic congruency effect was still reported even when participants made same-versus-different discrimination responses regarding the relative sizes of the two disks. Taken together, these results provide the first empirical demonstration that the relative frequency of an irrelevant sound can influence the speed with which participants judge the size of visual stimuli when the sound varies on a trial-by-trial basis along a synesthetically compatible dimension. The possible cognitive bases for this synesthetic association are also discussed.     

Postsaccadic processing of the retinal image during picture scanning

Eye movements were monitored while observers inspected photographs of natural scenes. At the end of each saccade (i.e., at the beginning of each period of steady fixation), the stimulus was replaced for a certain period of time by a uniform field (Experiment 1) or a blurred version of the stimulus scene (Experiment 2). Total fixation duration was measured as a function of the duration of the initial uniform field or the blurred image that followed the saccade. It was found that fixation duration increased proportionally with the duration of the initial replacement field, even for durations as short as 25 msec. These results suggest that the visual system uses information on the retina right after each saccade is completed and that the blurred, low-resolution information used in Experiment 2 (cutoff frequency of 0.8 cpd) is not sufficient for the requirements of picture processing in this task.     

Instrumentation of a self-luminous fixation annulus

A simple and inexpensive way of constructing a fixation annulus is described. Such a device is important for experiments in which a fixation spot in the test field area is undesirable or when subjects’ experimental tasks preclude their manipulation of a fixation dot brightness control. A luminous compound is inserted into transparent plastic tubing which can be shaped and inserted into a groove milled to fit the tube’s diameter and shape. Annulus luminance is in the scotopic range. Repeated measurement showed that there was no brightness loss over time. Ways of further attenuating brightness are discussed.     

Working Memory for Temporal and Nontemporal Events in Monkeys

This is the first report that introduces appropriate behavioral tasks for monkeys for investigations of working memory for temporal and nontemporal events. Using several behavioral tests, the study also shows how temporal information is coded during retention intervals in the tasks. Each of three monkeys was trained with two working memory tasks: delayed matching-to-sample of stimulus duration (DMS-D) and delayed matching-to-sample of stimulus color (DMS-C). The two tasks employed an identical apparatus and responses and differed only in the temporal and nontemporal attribute of the stimuli to be retained for correct performance. When a retention interval between the sample and comparison stimuli was prolonged, the monkeys made more incorrect responses to short samples in the DMS-C task, suggesting “trace decay” of memory for short stimuli. However, the same monkeys showed no such increase in incorrect responses to short samples in the DMS-D task, suggesting active coding of temporal information, that is, the length of stimulus duration, during the retention interval. When variable lengths of samples were presented with a fixed retention interval, the monkeys made more incorrect responses when length differences between short and long samples were small in the DMS-D task, but not in the DMS-C task. This suggests that the codes of working memory retained in the DMS-D task were not absolute (analogical) but rather were relative (categorical) and related to differences in the duration of the samples.     

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.     

The reversed work‐ethic effect: Monkeys avoid stimuli associated with high‐effort1

We investigated how different numbers of responses prior to the presentation of a stimulus influenced preferences for the stimulus in three Japanese monkeys (Macaca fuscata). Monkeys were trained for two kinds of simultaneous discriminations that followed a high‐effort or a low‐effort response. In probe test trials, the stimulus that followed the high‐effort and the stimulus that followed the low‐effort response during discriminations were presented simultaneously. All monkeys chose stimuli that in training followed a low‐effort response. This result suggested that monkeys avoided stimuli that followed high‐effort responses because of the association between those stimuli and the preceding greater effort. The present result is consistent with our previous study conducted with humans, but contradicts analogous studies carried out with pigeons that reported either opposite results (i.e. pigeons preferred the stimuli after high‐effort) or non‐biased preferences. The differences between mammals and birds are discussed in terms of the lack of a successive negative contrast effect in birds.     

Fast Responses of the Human Hand to Changes in Target Position

If a target toward which an individual moves his hand suddenly moves, he adjusts the movement of his hand accordingly. Does he use visual information on the target's velocity to anticipate where he will reach the target? These questions were addressed in the present study. Subjects (N = 6 in each of 4 experiments) were instructed to hit a disk with a rod as soon as it appeared on a screen. Trajectories of the hand toward stationary disks were compared with those toward disks that jumped leftward or rightward as soon as the subject's hand started moving toward the screen, and with those in which either the disk or the background started moving leftward or rightward. About 110 ms after the disk was suddenly displaced, the moving hand was diverted in the direction of the perturbation. When the background moved, the disk's perceived position shifted in the direction in which the background was moving, but the disk appeared to be moving in the opposite direction. When hitting such disks, subjects adjusted their movement in accordance with the perceived position, rather than moving their hand in the direction of the perceived motion in anticipation of the disk's future displacement. Thus, subjects did not use the perceived velocity to anticipate where they would reach the target but responded only to the change in position     

Development of visual attention: a stereoscopic view

Five-year-olds, 8-year-olds, and adults (20 each) tachistoscopically viewed random-dot stereograms containing either uniform depth or an elevated disk whose perimeter fell 1.5 degrees, 2.5 degrees, or 4 degrees beyond fixation. When subjects were forewarned as to the size of the disk to look for (precued trials), signal detection improved (d' increased) for all three groups, and 5-year-olds improved the most. Without forewarning (self-cued trials), the 5-year-olds performed more poorly than the other two groups, which performed alike. Although young children are able to allocate their visual attention selectively, they do not appear to deploy it as quickly or as effectively as their elders when the focus of attention is theirs to choose.     

Resurgence of derived stimulus relations.

Resurgence has been shown in human and nonhuman operant behavior, but not in derived relational responses. The present study examined this issue. Twenty-three undergraduates were trained to make conditional discriminations in a three-choice matching-to-sample paradigm. The training resulted in three equivalence classes, each consisting of four arbitrarily configured visual stimuli. The same 12 stimuli were then reorganized, and the conditional discrimination training was repeated such that three new classes were possible. In a subsequent test of derived relations, most subjects showed response patterns that were consistent with the altered conditional discriminations. Subjects were then exposed to conditional discrimination trials under extinction. Most subjects continued to respond consistently with the most recently reinforced conditional discrimination trials. During the next phase, subjects were exposed to symmetry and equivalence trials. Responses consistent with the most recent training produced feedback saying that the responses were incorrect, whereas other responses produced no feedback. Most subjects showed a resurgence of responding that was consistent with their earlier training. Finally, subjects were exposed to conditional discrimination trials carried out in extinction. Most subjects continued to show a resurgence of responding that was consistent with their early training.     

Attention and cue-producing behavior in the monkey

Explicit cue-producing responses were employed to study attending behavior in the monkey. The subjects learned a discrimination based on compound stimuli, a vertical bar embedded on a red ground versus a horizontal bar on a green ground. On some trials only one of the two stimulus components was presented (red versus green or vertical versus horizontal bar), and the animals had the option of responding on the basis of the component presented or transforming it to the compound discriminanda by means of a cue-producing response. Analysis of the choice and cue-producing response behavior showed that (a) both monkeys acquired the discrimination between the compound cues solely on the basis of the color component, (b) mastery of this discrimination did not confer any "habit loading" (discriminative control) on the bar component, and (c) the monkey may prefer to respond on the basis of one component (color) even though it is capable of using both components equally effectively.