Asymmetrical hand use in rhesus monkeys (Macaca mulatta) in tactually and visually regulated tasks

Manual preference, performance, and dexterity for bimanual grass-feeding behavior in wild geladas (Theropithecus gelada)

We assessed whether wild geladas, highly specialized terrestrial grass eaters, are lateralized for bimanual grass-plucking behavior. According to the literature, we expected that complex motor movements in grass feeding would favor the emergence of a population-level hand bias in these primates. In addition, we described geladas' manual behavior based on systematic observations of several individuals. Our study group included 28 individuals belonging to a population of free-ranging geladas frequenting the Kundi plateau, Ethiopia. We filmed monkeys while feeding on grass, and hand preference and performance were coded. Geladas performed more plucking movements per second with their left hand (LH) compared to the right one and preferred their LH both to start and finish collection bouts. Also, the rhythmic movements of each hand had a significant tendency toward isochrony. Finally, geladas used forceful pad-to-pad precision grips, in-hand movements, and compound grips to pluck and collect grass blades, considered the most advanced manual skills in primate species. The LH's leading role suggests an advantage of the right hemisphere in regulating geladas' bimanual grass-feeding behavior. The tactile input from the hands and/or rhythmic hand movements might contribute to explaining this pattern of laterality. Our findings highlighted the importance of adopting multiple laterality measures to investigate manual laterality. Moreover, the need to speed up the execution time of manual foraging might be a further important factor in studying the evolution of manual laterality and dexterity in primates.

New bands in the sleep stages of spider monkeys (Ateles geoffroyi): Electroencephalographic correlations and spatial distribution

The study of electroencephalographic (EEG) signals in nonhuman primates has led to important discoveries in neurophysiology and sleep behavior. Several studies have analyzed digital EEG data from primate species with prehensile tails, like the spider monkey, and principal component analysis has led to the identification of new EEG bands and their spatial distribution during sleep and wakefulness in these monkeys. However, the spatial location of the EEG correlations of these new bands during the sleep-wake cycle in the spider monkey has not yet been explored. Thus, the objective of this study was to determine the spatial distribution of EEG correlations in the new bands during wakefulness, rapid eye movement (REM) sleep, and non-REM sleep in this species. EEG signals were obtained from the scalp of six monkeys housed in experimental conditions in a laboratory setting. Regarding the 1-21 Hz band, a significant correlation between left frontal and central regions was recorded during non-REM 2 sleep. In the REM sleep, a significant correlation between these cortical areas was seen in two bands: 1-3 and 3-13 Hz. This reflects a modification of the degree of coupling between the cortical areas studied, associated with the distinct stages of sleep. The intrahemispheric EEG correlation found between left perceptual and motor regions during sleep in the spider monkey could indicate activation of a neural circuit for the processing of environmental information that plays a critical role in monitoring the danger of nocturnal predation.

Handedness for Unimanual Grasping in 564 Great Apes: The Effect on Grip Morphology and a Comparison with Hand Use for a Bimanual Coordinated Task

A number of factors have been proposed to influence within and between species variation in handedness in non-human primates. In the initial study, we assessed the influence of grip morphology on hand use for simple reaching in a sample of 564 great apes including 49 orangutans Pongo pygmaeus, 66 gorillas Gorilla gorilla, 354 chimpanzees Pan troglodytes and 95 bonobos Pan paniscus. Overall, we found a significant right hand bias for reaching. We also found a significant effect of the grip morphology of hand use. Grasping with the thumb and index finger was more prevalent in the right compared to left hand in all four species. There was no significant sex effect on the patterns of handedness. In a subsample of apes, we also compared consistency in hand use for simple reaching with previously published data on a task that measures handedness for bimanual actions. We found that the ratio of subjects with consistent right compared to left hand use was more prevalent in bonobos, chimpanzees and gorillas but not orangutans. However, for all species, the proportion of subjects with inconsistent hand preferences between the tasks was relatively high suggesting some measures may be more sensitive in assessing handedness than others.

The contributions of vision and haptics to reaching and grasping

This review aims to provide a comprehensive outlook on the sensory (visual and haptic) contributions to reaching and grasping. The focus is on studies in developing children, normal, and neuropsychological populations, and in sensory-deprived individuals. Studies have suggested a right-hand/left-hemisphere specialization for visually guided grasping and a left-hand/right-hemisphere specialization for haptically guided object recognition. This poses the interesting possibility that when vision is not available and grasping relies heavily on the haptic system, there is an advantage to use the left hand. We review the evidence for this possibility and dissect the unique contributions of the visual and haptic systems to grasping. We ultimately discuss how the integration of these two sensory modalities shape hand preference.

The advantage of objects over images in discrimination and reversal learning by kea, Nestor notabilis

Studies investigating the same paradigm but employing different methods are often directly compared in the literature. One such paradigm used to assess behavioural flexibility in animals is reversal learning. Commonly, these studies require individuals to learn the reward contingency of either solid objects presented on the ground or images presented on a touchscreen. Once learned, these contingencies are swapped. Researchers often refer to trials required to reach learning criteria from different studies, to compare the flexibility of different species, but rarely take methodological differences into account. A direct evaluation of the validity of such comparisons is lacking. To address this latent question, we confronted kea, an alpine parrot species of New Zealand and known for its behavioural flexibility, with a standard reversal learning paradigm on the touchscreen and a standard reversal learning paradigm with solid objects. The kea required significantly more trials to reach criterion in the acquisition and the reversal on the touchscreen. Also, the absolute increase in the number of trials required for the reversal was significantly greater on the touchscreen. This indicates that it is not valid to compare learning speed across studies that do not correspond in the addressed methodology. Taking into account the kea's ecology and explorative nature we discuss stimulus abstraction (limited depth cues and tactile stimulus feedback) and the spatial relation between reward and stimulus on the touchscreen as possible causes for decreased inhibition in this condition. Contrary to the absolute increase in number of trials required for the reversal, the increase in relation to the acquisition was greater with solid objects. This highlights the need for further research on the mechanisms involved causing methodology-dependent differences, some of which we discuss, in order to increase the validity of interpretations across studies and in respect to the subject's ecology.

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We provide a first direct comparison of two standard reversal learning approaches.

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Discrimination and reversal of solid objects seems easier than on the touchscreen.

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This highlights issues when comparing the same paradigm using different approaches.

Comparing human and nonhuman primate handedness: challenges and a modest proposal for consensus

In the past 20-25 years, there have been a number of studies published on handedness in nonhuman primates. The goal of these studies has been to evaluate whether monkeys and apes show patterns of hand preference that resemble the right-handedness found in the human species. The extant findings on handedness in nonhuman primates have revealed inconsistent evidence for population-level handedness within and between species. In this article, I discuss some of the methodological and statistical challenges to comparative studies of handedness in human and nonhuman primates. I further offer a framework for developing some consensus on evaluating the validity of different handedness measures and the characterization of individual hand preferences.

Genetic and environmental contributions to the expression of handedness in chimpanzees (Pan troglodytes)

Most humans are right-handed and, like many behavioral traits, there is good evidence that genetic factors play a role in handedness. Many researchers have argued that non-human animal limb or hand preferences are not under genetic control but instead are determined by random, non-genetic factors. We used quantitative genetic analyses to estimate the genetic and environmental contributions to three measures of chimpanzee handedness. Results revealed significant population-level handedness for two of the three measures-the tube task and manual gestures. Furthermore, significant additive genetic effects for the direction and strength of handedness were found for all three measures, with some modulation due to early social rearing experiences. These findings challenge historical and contemporary views of the mechanisms underlying handedness in non-human animals.

Kinematics of reaching and implications for handedness in rhesus monkey infants

Kinematic studies of reaching in human infants using two-dimensional (2-D) and three-dimensional (3-D) recordings have complemented behavioral studies of infant handedness by providing additional evidence of early right asymmetries. Right hand reaches have been reported to be straighter and smoother than left hand reaches during the first year. Although reaching has been a popular measure of handedness in primates, there has been no systematic comparison of left and right hand reach kinematics. We investigated reaching in infant rhesus monkeys using the 2-D motion analysis software MaxTRAQ Lite+ (Innovision Systems). Linear mixed-effects models revealed that left hand reaches were smoother, but not straighter, than right hand reaches. An early left bias matches previous findings of a left hand preference for reaching in adult rhesus monkeys. Additional work using this kind of kinematic approach will extend our understanding of primate handedness beyond traditional studies measuring only frequency or bouts of hand use.

Handedness is associated with immune functioning and behavioural reactivity in rhesus macaques

In the present study we examined the relationship among handedness, immune functioning, and behavioural reactivity in rhesus macaques. We used the absolute number of CD4+ (T-helper) and CD8+ (T-suppressor) cells as dependent measures of immune functioning. We derived reactivity profiles from behavioural responses to a threat, and hand preference profiles from a quadrupedal food-reaching test. The results indicate positive correlations between the frequency of right versus left hand reaches and the absolute number of CD4+ cells, and between the frequency of right versus left hand reaches and the degree of human-directed aggression in response to an invasive threat. Immune measures were not associated with the strength of hand preference. These results are consistent with and extend previous findings obtained with rodents to nonhuman primates and provide further support for the view that behavioural lateralisation is associated with immune functioning and behavioural reactivity.

Manual exploratory procedures and asymmetries for a haptic search task: a comparison between capuchins (Cebus apella) and humans

Left-hand preferences in monkeys (capuchins, macaques) for haptic discriminations have been taken to indicate that the right hemisphere is more involved than the left hemisphere in solving these tasks. We confirmed this left-hand bias in a group of 21 capuchins haptically searching for sunflower seeds located in crevices in 12 different objects (Lacreuse & Fragaszy, 1996). In an attempt to specify the relations of the left-hand bias to hand performance and hemispheric lateralisation, we analysed the manual exploratory procedures used by the subjects when exploring the objects, and compared them to those of humans confronted with the same task. All the hand-movement patterns displayed by humans were also observed in capuchins, but humans performed exhaustive explorations, whereas capuchins restricted their haptic investigations to limited portions of the objects. Both species adopted several ''exploratory procedures'' to investigate particular objects. Although capuchins showed a left-hand preference to perform the task, finer analyses of haptic exploratory procedures revealed no difference in the way the left and right hands explored the objects, nor in the efficiency with which each hand solved the task. Nor was any measurable difference in manual exploratory procedure and efficiency found in humans. The discussion emphasises the need to complement measures of manual preference with finer indices of hemispheric lateralisation, for a better understanding of functional asymmetries in primates.

Studying primate learning in group contexts: Tests of social foraging, response to novelty, and cooperative problem solving

Learning commonly refers to the modification of behavior through experience, whereby an animal gains information about stimulus-response contingencies from interacting with its physical environment. Social learning, on the other hand, occurs when the same information originates, not from the animal's personal experience, but from the actions of others. Socially biased learning is the 'collective outcome of interacting physical, social, and individual factors' [D. Fragaszy, E. Visalberghi, Learn. Behav. 32 (2004) 24-35.] (see p. 24). Mounting interest in animal social learning has brought with it certain innovations in animal testing procedures. Variants of the observer-demonstrator and cooperation paradigms, for instance, have been used widely in captive settings to examine the transmission or coordination of behavior, respectively, between two animals. Relatively few studies, however, have examined social learning in more complex group settings and even fewer have manipulated the social environment to empirically test the effect of group dynamics on problem solving. The present paper outlines procedures for group testing captive non-human primates, in spacious arenas, to evaluate the social modulation of learning and performance. These methods are illustrated in the context of (1) naturalistic social foraging problems, modeled after traditional visual discrimination paradigms, (2) response to novel objects and novel extractive foraging tasks, and (3) cooperative problem solving. Each example showcases the benefits of experimentally manipulating social context to compare an animal's performance in intact groups (or even pairs) against its performance under different social circumstances. Broader application of group testing procedures and manipulation of group composition promise to provide meaningful insight into socially biased learning.

Hand preference in unimanual and bimanual tasks and postural effect on manual laterality in captive red-capped mangabeys (Cercocebus torquatus torquatus)

Hand preference in 11 captive red-capped mangabeys (Cercocebus torquatus torquatus) was examined under different conditions: a free situation during spontaneous food processing, three different postural conditions (brachiating, and bipedal and tripedal standing), and a situation involving bimanual processing. Generally, individual laterality was found regardless of the task and behavior involved. However, the number of monkeys with hand preferences and the strength of the preference increased with the complexity of the tasks. The monkeys exhibited a significantly higher and positive mean manual preference index (HI) when they were hanging than when they were quadrupedal or sitting. The strength of manual preference (ABS-HI) was in turn higher when the monkeys were hanging or bipedal than when they were quadrupedal. The strength of manual preference was higher for both the bimanual and experimental tasks than for unimanual tasks and spontaneous activities. Although our sample was too small to allow us to make any generalizations concerning lateral preferences in red-capped mangabeys, we propose some hypotheses about the influence of posture stability and task complexity.

Hand preferences for unimanual and coordinated bimanual tasks in baboons (Papio anubis)

This study examined hand preference in baboons in a sample of 94 subjects for a unimanual task and in a sample of 104 subjects for a bimanual task. For the unimanual task, handedness was assessed by observing simple reaching for grains. For the bimanual task, tubes lined with peanut butter inside were presented to the baboons. The hand and the finger used to remove peanut butter were recorded. Population-level right-handedness was found for the bimanual but not the unimanual task. In addition, test-retest correlations showed consistency in hand use across time for the coordinated bimanual task but not the simple reaching task. No significant effects of age and sex on the direction and strength of hand preferences were found for either task. These are the first evidences of population-level handedness in baboons and the results are discussed in the context of evolutionary theories of cerebral dominance.

Left hand preferences in capuchins (Cebus apella): role of spatial demands in manual activity

Sixteen capuchins (Cebus apella) participated in four tasks that differed in their exploration demands and availability of visual cues. The two explorative tasks required the monkeys to discover sunflower seeds hidden in crevices in objects of various shapes, with vision (Haptic-Visual task) or without vision (Haptic task). Two other tasks required the capuchins to grasp sunflower seeds directly on a flat support, with vision (Visual reaching) or without vision (Tactual reaching). The presence or absence of exploration demands had a significant effect on the direction of hand preferences. The group displayed greater left hand preferences for the Haptic and Haptic-Visual tasks than for the two reaching tasks. The strength of manual preferences did not differ significantly among the four tasks. These findings suggest that the manipulo-spatial demands of a task are of particular importance for the expression of left hand bias in a population of capuchins. It is argued that left hand preferences for the haptic tasks may reflect a right hemisphere specialisation to integrate the spatial and motor components of action.

Manual laterality and task complexity in De Brazza's monkey (Cercopithecus neglectus)

We investigated manual specialisation of six Brazza monkeys, both under structured experimental conditions and while they engaged in unconstrained natural behaviour. Five tasks that varied in complexity, degree of visual guidance, and bimanual coordination were used in the experimental condition. Individual preferences were revealed for each task, and in the case of simple tasks the position of a monkey relative to the task device was shown to impact on manual preference. Strength of laterality and performance were not found to be related. Task complexity influenced strength, but not direction, of manual preference. The small number of animals precluded generalisation to population preferences but the large number of observations and choice of tasks can serve as model for the study of manual preferences in primates.

A Meta-Analysis of Primate Hand Preferences, Particularly for Reaching

P. F. MacNeilage, M. G. Studdert-Kennedy, and B. Lindblom (1987) proposed a progression for handedness in primates that was supposed to account for the evolution of a right bias in human handedness. To test this proposal, the authors performed meta-analyses on 62 studies that provided individual data (representing 31 species: 9 prosimians, 6 New World monkeys, 10 Old World monkeys, 2 lesser apes, and 4 greater apes), of the 118 studies of primate handedness published since 1987. Although evidence of a population-level left-handed bias for prosimians and Old World monkeys supports P. F. MacNeilage et al., the data from apes, New World monkeys, and individual species of prosimians and New World monkeys do not. Something other than primate handedness may have been the evolutionary precursor of the right bias in hand-use distribution among hominids.

Grasping behavior in tufted capuchin monkeys (Cebus apella): Grip types and manual laterality for picking up a small food item

This study investigates prehension in 20 tufted capuchins (Cebus apella) in a reaching task requiring individuals to grasp a small food item fixed to a tray. The aim was twofold: 1) to describe capuchins' grasping techniques in detail, focusing on digit movements and on different areas of contact between the grasping fingers; and 2) to assess the relationship between grip types and manual laterality in this species. Capuchins picked up small food items using a wide variety of grips. In particular, 16 precision grip variants and 4 power grip variants were identified. The most frequently used precision grip involved the distal lateral areas of the thumb and the index finger, while the most preferred kind of power grip involved the thumb and the palm, with the thumb being enclosed by the other fingers. Immature capuchins picked up small food items using power grips more often than precision grips, while adult individuals exhibited no significant preference for either grip type. The analysis performed on the time capuchins took to grasp the food and withdraw it from the tray hole revealed that 1) precision grips were as efficient as power grips; 2) for precision grips, the left hand was faster than the right hand; and 3) for power grips, both hands were equally quick. Hand preference analysis, based on the frequency for the use of either hand for grasping actions, revealed no significant hand bias at group level. Likewise, there was no significant relationship between grip type and hand preference.

Effects of estradiol and aging on fine manual performance in female rhesus monkeys

Aging is characterized by a progressive deterioration of motor function related to dysfunctions of the nigrostriatal system. Because estrogen has been reported to protect dopaminergic neurons and to improve the motor deficits associated with Parkinson's disease, we hypothesized that it would partially reverse the age-related decline of motor function in normal aging. We tested the effects of estrogen treatment and withdrawal on fine motor performance in five aged (21-24 years old) and five young (6-9 years old) ovariectomized female rhesus monkeys. The tests required the monkeys to use each hand to retrieve a Life Saver candy from metal rods bent in shapes of different complexity. Monkeys were tested twice a week for 8 consecutive weeks, during treatment with placebo or ethinyl estradiol (EE(2)) in alternating 14-day blocks. Each behavioral test was videotaped and subsequently scored for the duration and the success of the first trial on each shape. Both groups of monkeys improved rapidly with practice in speed and success of retrieval. The older monkeys were slower but as successful as the young monkeys in retrieving the candy. The left hand was faster than the right hand for both the aged and young females. We failed to detect any effect of EE(2) treatment on speed or success of retrieval in either group. These results confirm the slowing of fine motor performance with aging in female rhesus monkeys. They also indicate that estradiol, at least as administered in this study, does not benefit fine manual performance.

Skeletal biology, functional asymmetry and the origins of "Handedness"

The past decade has witnessed the (re)emergence of a debate as to whether handedness is apomorphic within hominins. There are both qualitative and quantitative arguments, some which draw non-human primates into the handed sphere and others which exclude them. Ultimate questions concern origins of structural asymmetry of both brain and body and lateralized behaviours with implications for tool use and language. Lateralization is thus an important realm of phylogenetic study, and archaeologists and psychologists alike have sought to identify handedness within material culture. However, hand preference for tool manufacture and use among extant non-human primates, such as Cebus and Pan, suggests that the archaeological record may well be mute regarding the origins of laterality. In this paper, an argument is put forward positioning skeletal biology as a viable approach to the handedness origins issue. Behaviour is a mediator of the complementary processes of geometric modelling (change in size and shape) and histological remodelling (disuse osteopenia; microfracture repair); therefore, directional asymmetry in the pattern of skeletal modelling and remodelling is a putative signal of lateralized activity.

Leading-limb preferences in marmosets (Callithrix jacchus): Walking, leaping and landing

The leading-limb preferences of 17 common marmosets (Callithrix jacchus) when they initiated and terminated locomotion were determined from video recordings of the subjects walking on, leaping off, and landing on a plexiglas platform. In a majority of sequences, 11 of 17 subjects landed with the right hand and foot contacting the plexiglas substrate before the left hand and foot. Of the 17 subjects, 8 initiated leaping with the right side of the body, whereas only 3 subjects significantly preferred to initiate leaping with the left hand and foot. Leading-limb preferences displayed during landing were positively correlated with those displayed for leaping and walking, although only two subjects displayed significant leading-limb preferences when walking. By contrast, hand preferences for initiating and terminating locomotion were not related to hand preferences for food holding; 16 of 17 marmosets displayed strong and significant hand preferences for food holding, even though many did not show leading-limb preferences for walking, leaping, or landing. These data seem to suggest that the right arm is stronger in marmosets. The right arm is used initially to decelerate the body and absorb the impact of contact with the landing substrate. The tendency for the right arm to be stronger is balanced by a tendency for the left leg to be stronger (providing the force needed for leaping).

Song perception in the European starling: hemispheric specialisation and individual variations

Hemispheric specialisation for speech in humans has been well documented. The lateralisation for song production observed in songbirds is reminiscent of this hemispheric dominance. In order to investigate whether song perception is also lateralised, we made multiunit recordings of the neuronal activity in the field L of starlings during the presentation of species-specific and artificial non-specific sounds. We observed a systematic stronger activation in one hemisphere than in the other one during the playback of species-specific sounds, with inter-subject variability in the predominant hemisphere for song perception. Such an asymmetry was not observed for artificial non-specific sounds. Thus, our results suggest that, at least at the individual level, the two hemispheres of the starlings' brain perceive and process conspecific signals differently.

Plasma cortisol is associated with handedness in infant rhesus monkeys

In this research we examined the relationship between plasma cortisol and handedness in infant rhesus macaques (Macaca mulatta). Specifically, we tested the hypothesis that stress functioning is related to hemispheric specialization and manifested in a positive correlation between cortisol levels and the frequency of right- versus left- hand use. We found a significant positive relationship between cortisol levels sampled at ages 1 and 3 months and lateral bias toward greater use of the right hand versus left hand sampled between ages 4 and 11 months. Further, we found a significant negative relationship between cortisol sampled at age 5 months and strength of lateral bias (independent of direction). These data suggest an early developmental influence of stress functioning on hemispheric specialization for manual control in infant monkeys.

Manual laterality in haptic and visual reaching tasks by tufted capuchin monkeys (Cebus apella). An association between hand preference and hand accuracy for food discrimination

Manual laterality was examined in 26 tufted capuchins (Cebus apella) in three tasks differing in their sensorimotor demands and the availability of visual cues. The Haptic discrimination task required the monkeys to discriminate haptically between two pumpkin seeds and two tinfoil items stuck into a tray inside an opaque box. The other two tasks required the monkeys to reach for two pumpkin seeds stuck into the tray within a transparent box with vision (Visually guided reaching task) or without vision (Visual-Tactual reaching task) during reaching. A significant group-level left hand bias was found for food retrieval in both the Haptic discrimination and Visual-Tactual tasks, and a significant group-level right hand bias in the Visually guided reaching task. The strength of hand preferences did not differ among the tasks. It was found that the accuracy of food recognition in the Haptic discrimination task was greater for the left than the right hand. The results suggest that the differences in the manipulo-spatial requirements of the tasks and in the availability of visual cues can variously affect manual laterality in capuchins. The left-hand preferences for the Haptic discrimination and Visual-Tactual tasks as well as the left-hand advantage for food discrimination may reflect a greater involvement of the right hemisphere in processing haptic information.

Development of hand preferences in marmosets (Callithrix jacchus) and effects of aging

The authors investigated the development of unimanual hand use and hand preferences during feeding in 15 marmosets (Callithrix jacchus), ages birth to 51-70 months. Bimanual hand use was common at 1-2 months, but by 5-8 months unimanual holding had developed and so had significant hand preferences. Half of the marmosets preferred to pick up and take food to the mouth with the left hand, and half preferred the right hand. Individuals maintained the same hand preference at all ages examined. Significant relationships were also found between the postures adopted during feeding and the direction of hand preferences displayed by juvenile marmosets. There was a positive correlation between increased suspension and increased left-hand preference, and a negative correlation between increased feeding in a tripedal posture and increased left-hand preference. These results are discussed in terms of motor development and hemispheric specialization.

Laterality of tail resting posture in three species of New World primates

A variety of mammalian species including prosimian and simian primates wrap their tails around their bodies as a means of thermoregulation and for reasons of comfort during resting or sleep. Adopting such a resting posture requires an animal to move its tail either to the right or to the left of the midline of its body, and thus to perform a lateralized behavior. The purpose of this study was to assess the occurrence of lateral biases in tail resting posture in three species of New World primates. Twenty squirrel monkeys, spider monkeys, and howler monkeys, respectively, were observed and data on tail resting posture were collected and analyzed. The results demonstrate (1) that individual squirrel monkeys and spider monkeys exhibit highly significant lateral biases in tail resting posture; (2) a lack of a lateral bias at the group level; (3) that howler monkeys fail to show side preferences in tail wrapping; (4) a lack of sex differences in this behavior in all three species; and (5) a lack of significant correlations between preferred side of tail resting posture and preferred side of hand use in simple reaching tasks which had been assessed with a subset of animals in previous studies. Thus, the present study provides evidence for a behavioral asymmetry which is well-known to occur in rats but has not been described so far in nonhuman primates, and which might offer an additional approach to the investigation of the mechanisms underlying functional cerebral asymmetries.

Hand preferences in unimanual and bimanual feeding by wild vervet monkeys (Cercopithecus aethiops)

Lateral preference was examined in spontaneous feeding actions in 2 troops of wild vervet monkeys (Cercopithecus aethiops). Processing of 4 foods (termites, leaf shoots, sugarcane, and fruit) was studied. Actions included unimanual reaching to moving objects, operating from an unstable posture, and coordinated bimanual processing. Between 19 and 31 subjects were available, according to the task. In 2 tasks, laterality of 2 independent stages was measured separately, giving 6 measures in all. On 4 of these measures, most monkeys were ambipreferent, and only a few showed significant hand preferences. Only for termite feeding and detaching material from fruits did the majority show significant lateralization; no tasks elicited exclusive use of 1 hand. Preference appeared labile, because in 2 tasks, population trends reversed with increasing age. No population trends to left or right were found; instead, these monkeys showed ambilaterality, with lateralization associated with task complexity.

Eye Preferences in Common Marmosets (Callithrix jacchus): Influence of Age, Stimulus, and Hand Preference

Eye preferences of the common marmoset ( Callithrix jacchus ) were examined, taking into account age, arousal, and hand preference. Monocular eye use for looking through a small hole at a stimulus was recorded. Of 21 marmosets, 20 displayed right-eye preferences for viewing a piece of familiar food. In subjects tested at 3-8, 12, 15-18, and 22 months, eye preferences were consistent across age. A group bias, indicative of right-eyedness, was also found for viewing other stimuli. The stimuli included a watch, mirror, model of a beetle, and the experimenter's hand. However, when the marmosets viewed a threatening stimulus, a model resembling two rearing snakes, they displayed increased arousal (indicated by longer duration between monocular viewing events and increased incidence of aroused vocalisations) and the eye preferences shifted away from a preference for the right eye to either no preference or a left-eye preference. No relationship between eye preference and hand preference for holding food was found. Therefore, we suggest that eye preferences may reflect hemispheric specialisations for perceptual processing, according to the emotional valence of the stimulus.

Hand preference in infant rhesus macaques (Macaca mulatta)

In this research we examined hand preference in infant rhesus macaques (Macaca mulatta). The subjects were 20 Macaca mulatta, each aged between 4 and 11 months. We assessed hand preference using both a unimanual reaching task and a bimanual coordination task. In the unimanual reaching task, we presented subjects with raisins and noted which hand the animals used to retrieve the food. In the bimanual coordination task, we presented the same subjects with plastic tubes filled with raisin paste and noted which hand the animals used to hold the tubes and which hand the animals used to remove the food. We noted a population-level bias toward use of the left hand for both tasks. These results suggest early right hemisphere advantage for reaching and bimanual coordination in Macaca mulatta, although we acknowledge that this issue needs to be examined more directly through neuroimaging procedures such as Positron Emission Tomography (PET) or functional Magnetic Resonance Imaging (fMRI). We speculate that early right hemisphere advantage contributes to differential patterns of hand preference development for unimanual and bimanual action, and that the development of hand preference for bimanual coordination is related to the emergence of hemispheric specialization for processing species-specific vocalizations.

Haptic discrimination in capuchin monkeys (Cebus apella): evidence of manual specialization

Two experiments investigated the effects of haptic and visual discrimination on hand preference in 22 brown capuchin monkeys (Cebus apella). The percentage of left-handed subjects in Experiment 1 were 63.6%, 45.5%, and 18.2% for haptic, bipedal, and quadrupedal reaching, respectively. In Experiment 2, the haptic demands of the task were manipulated by using additional food types and another tactile medium. Left-hand preferences were further strengthened when reaching into water compared to pineshavings in Experiment 1. Reaching with no tactile interference resulted in equal numbers of lateralized and nonlateralized subjects. These results show that when reaching demands the use of haptic cues, as opposed to visual ones, monkeys shift towards greater left hand use. This is consistent with what is known about right hemisphere superiority for haptic discrimination in humans.

Manual laterality in spider monkeys (Ateles geoffroyi) solving visually and tactually guided food-reaching tasks

The purpose of this study was to assess the occurrence of manual laterality in Ateles geoffroyi and in particular to test the influence of visual and tactual control on preferential hand use. Thirteen spider monkeys were presented with three different food-reaching tasks and assessed for hand preferences with a minimum of 100 reaches per animal. In all three tasks a significant lateral bias towards use of the left hand at the group level was found. Performance in visually and tactually guided reaching tasks did not differ with regard to direction or degree of manual laterality. Males and females showed no significant differences in performance with the exception that males used their mouths to retrieve food when the task permitted, significantly more often than females. The results suggest a possible right-hemisphere specialization in the spider monkey for both visually and tactually guided reaching.