Individual differences in the responses of naïve rhesus monkeys to snakes

Attention to Stimuli of Learned versus Innate Biological Value Relies on Separate Neural Systems

The neural bases of attention, a set of neural processes that promote behavioral selection, is a subject of intense investigation. In humans, rewarded cues influence attention, even when those cues are irrelevant to the current task. Because the amygdala plays a role in reward processing, and the activity of amygdala neurons has been linked to spatial attention, we reasoned that the amygdala may be essential for attending to rewarded images. To test this possibility, we used an attentional capture task, which provides a quantitative measure of attentional bias. Specifically, we compared reaction times (RTs) of adult male rhesus monkeys with bilateral amygdala lesions and unoperated controls as they made a saccade away from a high- or low-value rewarded image to a peripheral target. We predicted that: (1) RTs will be longer for high- compared with low-value images, revealing attentional capture by rewarded stimuli; and (2) relative to controls, monkeys with amygdala lesions would exhibit shorter RT for high-value images. For comparison, we assessed the same groups of monkeys for attentional capture by images of predators and conspecifics, categories thought to have innate biological value. In performing the attentional capture task, all monkeys were slowed more by high-value relative to low-value rewarded images. Contrary to our prediction, amygdala lesions failed to disrupt this effect. When presented with images of predators and conspecifics, however, monkeys with amygdala lesions showed significantly diminished attentional capture relative to controls. Thus, separate neural pathways are responsible for allocating attention to stimuli with learned versus innate value.SIGNIFICANCE STATEMENT Valuable objects attract attention. The amygdala is known to contribute to reward processing and the encoding of object reward value. We therefore examined whether the amygdala is necessary for allocating attention to rewarded objects. For comparison, we assessed the amygdala's contribution to attending to objects with innate biological value: predators and conspecifics. We found that the macaque amygdala is necessary for directing attention to images with innate biological value, but not for directing attention to recently learned reward-predictive images. These findings indicate that the amygdala makes selective contributions to attending to valuable objects. The data are relevant to mental health disorders, such as social anxiety disorders and small animal phobias, that arise from biased attention to select categories of objects.

Prefrontal influences on the function of the neural circuitry underlying anxious temperament in primates

Anxious temperament, characterized by heightened behavioral and physiological reactivity to potential threat, is an early childhood risk factor for the later development of stress-related psychopathology. Using a well-validated nonhuman primate model, we tested the hypothesis that the prefrontal cortex (PFC) is critical in regulating the expression of primate anxiety-like behavior, as well as the function of subcortical components of the anxiety-related neural circuit. We performed aspiration lesions of a narrow 'strip' of the posterior orbitofrontal cortex (OFC) intended to disrupt both cortex and axons entering, exiting and coursing through the pOFC, particularly those of the uncinate fasciculus (UF), a white matter tract that courses adjacent to and through this region. The OFC is of particular interest as a potential regulatory region because of its extensive reciprocal connections with amygdala, other subcortical structures and other frontal lobe regions. We validated this lesion method by demonstrating marked lesion-induced decreases in the microstructural integrity of the UF, which contains most of the fibers that connect the ventral PFC with temporal lobe structures as well as with other frontal regions. While the lesions resulted in modest decreases in threat-related behavior, they substantially decreased metabolism in components of the circuit underlying threat processing. These findings provide evidence for the importance of structural connectivity between the PFC and key subcortical structures in regulating the functions of brain regions known to be involved in the adaptive and maladaptive expression of anxiety.

Fast Detection of Snakes and Emotional Faces in the Macaque Amygdala

Primate vision is reported to detect snakes and emotional faces faster than many other tested stimuli. Because the amygdala has been implicated in avoidance and emotional behaviors to biologically relevant stimuli and has neural connections with subcortical nuclei involved with vision, amygdalar neurons would be sensitive to snakes and emotional faces. In this study, neuronal activity in the amygdala was recorded from Japanese macaques (Macaca fuscata) during discrimination of eight categories of visual stimuli including snakes, monkey faces, human faces, carnivores, raptors, non-predators, monkey hands, and simple figures. Of 527 amygdalar neurons, 95 responded to one or more stimuli. Response characteristics of the amygdalar neurons indicated that they were more sensitive to the snakes and emotional faces than other stimuli. Response magnitudes and latencies of amygdalar neurons to snakes and monkey faces were stronger and faster than those to the other categories of stimuli, respectively. Furthermore, response magnitudes to the low pass-filtered snake images were larger than those to scrambled snake images. Finally, analyses of population activity of amygdalar neurons suggest that snakes and emotional faces were represented separately from the other stimuli during the 50–100 ms period from stimulus onset, and neutral faces during the 100–150 ms period. These response characteristics indicate that the amygdala processes fast and coarse visual information from emotional faces and snakes (but not other predators of primates) among the eight categories of the visual stimuli, and suggest that, like anthropoid primate visual systems, the amygdala has been shaped over evolutionary time to detect appearance of potentially threatening stimuli including both emotional faces and snakes, the first of the modern predators of primates.

Prefrontal cortex interactions with the amygdala in primates

This review addresses functional interactions between the primate prefrontal cortex (PFC) and the amygdala, with emphasis on their contributions to behavior and cognition. The interplay between these two telencephalic structures contributes to adaptive behavior and to the evolutionary success of all primate species. In our species, dysfunction in this circuitry creates vulnerabilities to psychopathologies. Here, we describe amygdala-PFC contributions to behaviors that have direct relevance to Darwinian fitness: learned approach and avoidance, foraging, predator defense, and social signaling, which have in common the need for flexibility and sensitivity to specific and rapidly changing contexts. Examples include the prediction of positive outcomes, such as food availability, food desirability, and various social rewards, or of negative outcomes, such as threats of harm from predators or conspecifics. To promote fitness optimally, these stimulus-outcome associations need to be rapidly updated when an associative contingency changes or when the value of a predicted outcome changes. We review evidence from nonhuman primates implicating the PFC, the amygdala, and their functional interactions in these processes, with links to experimental work and clinical findings in humans where possible.

Preferential Neuronal Responses to Snakes in the Monkey Medial Prefrontal Cortex Support an Evolutionary Origin for Ophidiophobia

Ophidiophobia (snake phobia) is one of the most common specific phobias. It has been proposed that specific phobia may have an evolutionary origin, and that attentional bias to specific items may promote the onset of phobia. Noninvasive imaging studies of patients with specific phobia reported that the medial prefrontal cortex (mPFC), especially the rostral part of the anterior cingulate cortex (rACC), and amygdala are activated during the presentation of phobogenic stimuli. We propose that the mPFC-amygdala circuit may be involved in the pathogenesis of phobia. The mPFC receives inputs from the phylogenically old subcortical visual pathway including the superior colliculus, pulvinar, and amygdala, while mPFC neurons are highly sensitive to snakes that are the first modern predator of primates, and discriminate snakes with striking postures from those with non-striking postures. Furthermore, the mPFC has been implicated in the attentional allocation and promotes amygdala-dependent aversive conditioning. These findings suggest that the rACC focuses attention on snakes, and promotes aversive conditioning to snakes, which may lead to anxiety and ophidiophobia.

Damage to Orbitofrontal Areas 12 and 13, but Not Area 14, Results in Blunted Attention and Arousal to Socioemotional Stimuli in Rhesus Macaques

An earlier study in monkeys indicated that lesions to the mid-portion of the ventral orbitofrontal cortex (OFC), including Walker’s areas 11 and 13 (OFC11/13), altered the spontaneous scanning of still pictures of primate faces (neutral and emotional) and the modulation of arousal. Yet, these conclusions were limited by several shortcomings, including the lesion approach, use of static rather than dynamic stimuli, and manual data analyses. To confirm and extend these earlier findings, we compared attention and arousal to social and nonsocial scenes in three groups of rhesus macaques with restricted lesions to one of three OFC areas (OFC12, OFC13, or OFC14) and a sham-operated control group using eye-tracking to capture scanning patterns, focal attention and pupil size. Animals with damage to the lateral OFC areas (OFC12 and OFC13) showed decreased attention specifically to the eyes of negative (threatening) social stimuli and increased arousal (increased pupil diameter) to positive social scenes. In contrast, animals with damage to the ventromedial OFC area (OFC14) displayed no differences in attention or arousal in the presence of social stimuli compared to controls. These findings support the notion that areas of the lateral OFC are critical for directing attention and modulating arousal to emotional social cues. Together with the existence of face-selective neurons in these lateral OFC areas, the data suggest that the lateral OFC may set the stage for multidimensional information processing related to faces and emotion and may be involved in social judgments.

Dysregulation of behavioral and autonomic responses to emotional and social stimuli following bidirectional pharmacological manipulation of the basolateral amygdala in macaques

The amygdala is a key component of the neural circuits mediating the processing and response to emotionally salient stimuli. Amygdala lesions dysregulate social interactions, responses to fearful stimuli, and autonomic functions. In rodents, the basolateral and central nuclei of the amygdala have divergent roles in behavioral control. However, few studies have selectively examined these nuclei in the primate brain. Moreover, the majority of non-human primate studies have employed lesions, which only allow for unidirectional manipulation of amygdala activity. Thus, the effects of amygdala disinhibition on behavior in the primate are unknown. To address this gap, we pharmacologically inhibited by muscimol or disinhibited by bicuculline methiodide the basolateral complex of the amygdala (BLA; lateral, basal, and accessory basal) in nine awake, behaving male rhesus macaques (Macaca mulatta). We examined the effects of amygdala manipulation on: (1) behavioral responses to taxidermy snakes and social stimuli, (2) food competition and social interaction in dyads, (3) autonomic arousal as measured by cardiovascular response, and (4) prepulse inhibition of the acoustic startle (PPI) response. All modalities were impacted by pharmacological inhibition and/or disinhibition. Amygdala inhibition decreased fear responses to snake stimuli, increased examination of social stimuli, reduced competitive reward-seeking in dominant animals, decreased heart rate, and increased PPI response. Amygdala disinhibition restored fearful response after habituation to snakes, reduced competitive reward-seeking behavior in dominant animals, and lowered heart rate. Thus, both hypoactivity and hyperactivity of the basolateral amygdala can lead to dysregulated behavior, suggesting that a narrow range of activity is necessary for normal functions.

Increased attention to snake images in cynomolgus monkeys: an eye-tracking study

Previous studies have revealed faster detection of snake images in humans and non-human primates (NHPs), suggesting automatic detection of evolutionary fear-relevant stimuli. Furthermore, human studies have indicated that general fear-relevance rather than evolutionary relevance is more effective at capturing attention. However, the issue remains unclarified in NHPs. Thus, in the present study, we explored the attentional features of laboratory-reared monkeys to evolutionary and general fear-relevant stimuli (e.g., images of snakes, capturing gloves). Eye-tracking technology was utilized to assess attentional features as it can provide more accurate latency and variables of viewing duration and frequency compared with visual search task (VST) and response latency adopted in previous studies. In addition, those with autism spectrum disorder (ASD) show abnormal attention to threatening stimuli, including snake images. Rett syndrome (RTT) is considered a subcategory of ASD due to the display of autistic features. However, the attentional features of RTT patients or animal models to such stimuli remain unclear. Therefore, we also investigated the issue in MECP2 gene-edited RTT monkeys. The influence of different cognitive loads on attention was further explored by presenting one, two, or four images to increase stimulus complexity. The eye-tracking results revealed no significant differences between RTT and control monkeys, who all presented increased viewing (duration and frequency) of snake images but not of aversive stimuli compared with control images, thus suggesting attentional preference for evolutionary rather than general fear-relevant visual stimuli. Moreover, the preference was only revealed in visual tasks composed of two or four images, suggesting its cognitive-load dependency.

Lesion Studies in Contemporary Neuroscience

Studies of humans with focal brain damage and non-human animals with experimentally induced brain lesions have provided pivotal insights into the neural basis of behavior. As the repertoire of neural manipulation and recording techniques expands, the utility of studying permanent brain lesions bears re-examination. Studies on the effects of permanent lesions provide vital data about brain function that are distinct from those of reversible manipulations. Focusing on work carried out in humans and nonhuman primates, we address the inferential strengths and limitations of lesion studies, recent methodological developments, the integration of this approach with other methods, and the clinical and ecological relevance of this research. We argue that lesion studies are essential to the rigorous assessment of neuroscience theories.

Heightened Defensive Responses Following Subtotal Lesions of Macaque Orbitofrontal Cortex

Anxiety disorders are characterized by excessive attention to threat. Several brain areas, including the orbitofrontal cortex (OFC), have been associated with threat processing, with more recent work implicating specialized roles for the medial and lateral subregions of the OFC in mediating specific symptoms of anxiety disorders. Virtually no causal work, however, has evaluated the role of these OFC subregions in regulating behavioral responses under threat. To address this gap, we compared male rhesus monkeys (Macaca mulatta) with bilateral excitotoxic lesions restricted to either the lateral OFC (lOFC), targeting Walker's areas 11 and 13, or the medial OFC (mOFC), targeting Walker's area 14, to a group of unoperated controls on behavioral responses to the presentation of a fake rubber snake, fake spider, and neutral stimuli. Both lesion groups showed heightened defensive and reduced approach responses, accompanied by longer latencies to retrieve a food reward, in the presence of the threatening stimuli. Compared to unoperated controls, the mOFC lesion group also showed longer latencies to reach for rewards and a greater proportion of defensive responses (e.g., piloerection) in the presence of neutral stimuli. Thus, monkeys with mOFC lesions displayed a greater tendency to express defensive responses even in the absence of threat. Overall, our data reveal that both the mOFC and lOFC contribute to the attenuation of defensive responses. Notably, these findings, obtained following selective, excitotoxic lesions of the OFC, are diametrically opposed to the effects of aspiration lesions of OFC observed in macaques.SIGNIFICANCE STATEMENT Engaging in adaptive defensive responses under threat promotes biological fitness. The orbitofrontal cortex (OFC) has been implicated in regulating defensive responses to threat, with distinct subregions likely playing different roles. Here we tested the effects of excitotoxic damage restricted to either the lateral or medial subdivisions of the OFC in rhesus macaques. We found significantly heightened defense and reduced approach responses to threatening stimuli in both lesion groups. While lateral OFC lesions led to an increase in defense responses to the threatening stimuli, medial OFC lesions produced increases in defense responses to both threatening and neutral stimuli. Our findings provide insights into the neural regulation of defensive responses to threat and inform the etiology and treatment of anxiety disorders in humans.

Assessing anxiety in nonhuman primates

Anxiety can be broadly described as a psychological state in which normally innocuous environmental stimuli trigger negative emotional expectations. Human anxiety disorders are multidimensional and may be organic or acquired, situational or pervasive. The broad ranging nature of the anxiety phenotype speaks to the need for models that identify its various components and root causes to develop effective clinical treatments. The cross-species comparative approach to modeling anxiety disorders in animals aims to understand mechanisms that both contribute to and modulate anxiety. Nonhuman primate models provide an important bridge from nonprimate model systems because of the complexity of nonhuman primates' biobehavioral capacities and their commonalities with human emotion. The broad goal of this review is to provide an overview of various procedures available to study anxiety in the nonhuman primate, with a focus on the behavioral aspects of anxiety. Commonly used methods covered in this review include assessing animals in their home environment or in response to an ethologically relevant threat, associative conditioning and startle response tests, and cognitive bias tests. We also discuss how these procedures can help veterinarians and researchers care for captive nonhuman primates.

Monkey pulvinar neurons fire differentially to snake postures

There is growing evidence from both behavioral and neurophysiological approaches that primates are able to rapidly discriminate visually between snakes and innocuous stimuli. Recent behavioral evidence suggests that primates are also able to discriminate the level of threat posed by snakes, by responding more intensely to a snake model poised to strike than to snake models in coiled or sinusoidal postures (Etting and Isbell 2014). In the present study, we examine the potential for an underlying neurological basis for this ability. Previous research indicated that the pulvinar is highly sensitive to snake images. We thus recorded pulvinar neurons in Japanese macaques (Macaca fuscata) while they viewed photos of snakes in striking and non-striking postures in a delayed non-matching to sample (DNMS) task. Of 821 neurons recorded, 78 visually responsive neurons were tested with the all snake images. We found that pulvinar neurons in the medial and dorsolateral pulvinar responded more strongly to snakes in threat displays poised to strike than snakes in non-threat-displaying postures with no significant difference in response latencies. A multidimensional scaling analysis of the 78 visually responsive neurons indicated that threat-displaying and non-threat-displaying snakes were separated into two different clusters in the first epoch of 50 ms after stimulus onset, suggesting bottom-up visual information processing. These results indicate that pulvinar neurons in primates discriminate between poised to strike from those in non-threat-displaying postures. This neuronal ability likely facilitates behavioral discrimination and has clear adaptive value. Our results are thus consistent with the Snake Detection Theory, which posits that snakes were instrumental in the evolution of primate visual systems.

Differential behavior patterns in cynomolgus monkey Macaca fascicularis in home cage in response to human gaze

BACKGROUND

Non-human primates, when encountering human beings, show wariness and alertness. These behaviors differ when there is direct human gaze vs. when human averts his gaze.

METHODS

We observed cynomolgus monkey in their home cage and studied their behaviors in response to human gaze. Four behaviors were analyzed: opening mouth, staring at observer, agitated activity, and approaching observer.

RESULTS

Three behaviors appeared to be sensitive to human gaze between when the human observer gazed at the monkey and when the human observer looked away. Individual animals also displayed subpatterns of responses to human gaze.

CONCLUSIONS

These results indicate that, even in their home cage, monkeys display a heightened level of awareness when gazed upon by a human observer, suggesting that human gaze may elicit emotional reactions. Further, under the human gaze, distinct behavioral subpatterns were apparent within the monkey cohort in our study, indicative of subgroups within the cohort.

Individual differences in behavioral and cardiovascular reactivity to emotive stimuli and their relationship to cognitive flexibility in a primate model of trait anxiety

High trait anxiety is a risk factor for the development of anxiety disorders. Like the disorders themselves high trait anxiety has marked phenotypic variation at the level of symptomatology and neural circuits, suggesting that there may be different symptoms and distinct neural circuits associated with risk for these disorders. To address these issues, it is essential to develop reliable animal models of trait anxiety in a non-human primate whose brain bears structural and functional similarity to humans. The present study investigated individual variation in responsivity to fearful and anxiety provoking stimuli in the common marmoset monkey. Seven out of 27 animals failed to display discriminative, conditioned cardiovascular and behavioral responses on an auditory fear discrimination task, similar to that seen in high anxious humans and rodents. Their heightened emotionality to a rubber snake was consistent with the hypothesis that they were high in trait-like anxiety. Evidence for phenotypic variation in the high anxiety group was provided by the finding that discrimination failure was predicted early in conditioning by either hyper-vigilant scanning to the cues or a reduction in blood pressure to the context, i.e., test apparatus. Given that high trait anxiety in humans can be associated with altered prefrontal cognitive functioning and previously we implicated the marmoset anterior orbitofrontal (antOFC) and ventrolateral prefrontal cortex (vlPFC) in negative emotion regulation, we also tested the marmosets on two tests of cognitive flexibility differentially dependent on these two regions. While the high anxious group did not differ overall in their perseverative performance, the two distinct phenotypes were differentially correlated with reduced perseverative responding on the OFC- and vlPFC-dependent flexibility tests. Together, this study provides a new model of trait anxiety in marmosets amenable to analysis of phenotypic variation and neural circuitry.

Nonhuman primate models of social behavior and cocaine abuse

RATIONALE

Although cocaine is often abused in social situations, very few animal studies examine the effects of cocaine in the context of social behavior.

OBJECTIVES

This review highlights studies investigating the behavioral effects of cocaine in the context of social housing conditions using nonhuman primates. In addition, this review presents recent findings examining the effects of self-administering cocaine on social behavior and the effects of manipulations hypothesized to be stressful or enriching on the interactions between cocaine reinforcement and social rank. The following dependent variables are examined: (1) cocaine-induced changes in social behavior and (2) cocaine self-administration in cynomolgus monkeys of varying social ranks. The independent variables examined include several environmental and pharmacological manipulations.

CONCLUSIONS

The studies reviewed here indicate that several variables can differentially affect cocaine self-administration when studied in a social context, rather than in individually housed animals. These variables include the social rank and sex of the individual, drug history, the nature of the "fear"-inducing manipulation, and the reliability of cortisol as an appropriate measure of "stress." While the inclusion of socially housed animals necessitates larger sample sizes, animal models incorporating social behavior are more homologous to the human condition and should be implemented when possible.

Individual differences in the effects of environmental stimuli on cocaine choice in socially housed male cynomolgus monkeys

RATIONALE

Studies in laboratory animals have demonstrated an influence of environmentally derived stress and enrichment on the reinforcing effects of stimulants.

OBJECTIVE

To characterize the effects of acute exposure to ethologically valid environmental stimuli on the reinforcing strength of cocaine relative to food in socially housed monkeys.

MATERIALS AND METHODS

Choice between cocaine and food was assessed in subsets of 16 socially housed (4/pen) male cynomolgus monkeys immediately after the following manipulations: (1) treats placed in home cage, (2) a 10-min exposure to a rubber snake, or (3) 3 to 7 days of living in a larger environment without cage mates.

RESULTS

Placing treats in the home cage shifted the cocaine dose-response curve to the left in five monkeys tested and to the right in 4 of 12 animals. The rubber snake significantly shifted the cocaine choice curve to the left in dominant monkeys. Exposure to an enlarged environment decreased cocaine choice in 9 of 15 monkeys; this effect was transient and not related to social rank. Repeated testing did not affect cocaine choice.

CONCLUSIONS

Brief exposure to environmental events hypothesized to be stressors or enrichment altered cocaine choice, although not all individuals were affected and the effects were transient. Importantly, the data suggest that implementing positive changes in the environment produced effects that are clinically desirable. Understanding the behavioral and neurobiological mechanisms mediating sensitivity to environmental events in socially housed animals will lead to better treatment strategies for drug addiction.

Neonatal amygdala lesions alter responsiveness to objects in juvenile macaques

The amygdala is widely recognized to play a central role in emotional processing. In nonhuman primates, the amygdala appears to be critical for generating appropriate behavioral responses in emotionally salient contexts. One common finding is that macaque monkeys that receive amygdala lesions as adults are behaviorally uninhibited in the presence of potentially dangerous objects. While control animals avoid these objects, amygdala-lesioned animals readily interact with them. Despite a large literature documenting the role of the amygdala in emotional processing in adult rhesus macaques, little research has assessed the role of the amygdala across the macaque neurodevelopmental trajectory. We assessed the behavioral responses of 3-year-old (juvenile) rhesus macaques that received bilateral ibotenic acid lesions of the amygdala or hippocampus at 2 weeks of age. Animals were presented with salient objects known to produce robust fear-related responses in macaques (e.g., snakes and reptile-like objects), mammal-like objects that included animal-like features (e.g., eyes and mouths) but not reptile-like features (e.g., scales), and non-animal objects. The visual complexity of objects was scaled to vary the objects' salience. In contrast to control and hippocampus-lesioned animals, amygdala-lesioned animals were uninhibited in the presence of potentially dangerous objects. They readily retrieved food rewards placed near these objects and physically explored the objects. Furthermore, while control and hippocampus-lesioned animals differentiated between levels of object complexity, amygdala-lesioned animals did not. Taken together, these findings suggest that early damage to the amygdala, like damage sustained during adulthood, permanently compromises emotional processing.

Selective changes in foraging behavior following bilateral neurotoxic amygdala lesions in rhesus monkeys

Across a variety of species, the amygdala appears to play a key role in the detection and avoidance of potential dangers (e.g., unfamiliar social partners, novel objects or contexts, potential predators, etc.). For many species, seeking out appropriate food sources and avoiding novel, distasteful or potentially tainted food is also a daily concern. Amygdala damage in nonhuman primates has been linked to increased willingness to select unfamiliar or unpalatable foods, as well as inedible items that intact animals typically reject. However, such findings have not always been consistent and have typically been observed in relatively restrictive, laboratory-based testing contexts. We evaluated the food choices of six adult male rhesus monkeys (Macaca mulatta) with bilateral, neurotoxic amygdala lesions and six age- and experienced-matched unoperated control animals. Each animal was able to forage freely in a large enclosure stocked with five preferred and five nonpreferred foods that changed locations each day. While both groups quickly selected palatable foods, monkeys with amygdala lesions consistently selected unpalatable foods that the unoperated control animals generally avoided. Even after repeated presentations of the unpalatable foods, the amygdala-lesioned monkeys failed to change their initial pattern of diminished avoidance. These results are consistent with a general role for the amygdala in danger detection and prevention of harm in the presence of novel or noxious stimuli, regardless of whether such stimuli are conspecifics, predators, objects or foods.

Distinct contributions of the amygdala and hippocampus to fear expression

The present study attempted to distinguish the independent contributions of the amygdala and hippocampus to fear expression. Rhesus monkeys (Macaca mulatta) with bilateral excitotoxic amygdala lesions (n = 4), bilateral excitotoxic hippocampal lesions (n = 8) and unoperated controls (n = 9) were allowed to reach over a neutral junk object or fear-provoking stimulus (i.e., a rubber snake or a jumping rubber spider) to retrieve a food reward. Monkeys were exposed to each stimulus for 30 s. On each trial we recorded the monkey's latency to retrieve the food reward and scored their whole-body reactions to the object. Confirming previous work we found that, relative to controls, both operated groups showed shorter food-retrieval latencies and exhibited fewer defensive and more approach behaviors when exposed to the fear-provoking stimuli. However, only monkeys with amygdala lesions showed an abnormal, excessive visual interest in the snake and spider. By contrast, monkeys with hippocampal lesions displayed behaviors that were unrelated to the presence of the fear stimuli, thereby indicating a lack of interest in, and emotional reactivity towards, the snake and spider. These data show that the hippocampus and amygdala contribute independently to the overall expression of defensive responses.

The amygdala, reward and emotion

Recent research provides new insights into amygdala contributions to positive emotion and reward. Studies of neuronal activity in the monkey amygdala and of autonomic responses mediated by the monkey amygdala show that, contrary to a widely held view, the amygdala is just as important for processing positive reward and reinforcement as it is for negative. In addition, neuropsychological studies reveal that the amygdala is essential for only a fraction of what might be considered 'stimulus-reward processing', and that the neural substrates for emotion and reward are partially nonoverlapping. Finally, evidence suggests that two systems within the amygdala, operating in parallel, enable reward-predicting cues to influence behavior; one mediates a general, arousing effect of reward and the other links the sensory properties of reward to emotion.

Genetic modulation of cognitive flexibility and socioemotional behavior in rhesus monkeys

In human and nonhuman primates, structural variants of the gene encoding the serotonin transporter [5-hydroxytryptamine transporter (5-HTT)] affect the transcription and functional efficacy of 5-HTT. Prior work has shown that structural variants differentially affect function of the amygdala and ventromedial prefrontal cortex (VMPFC), regions important for the regulation and expression of emotion. However, relatively little is known about the impact of 5-HTT allelic variants on cognition. To address this question, we tested rhesus monkeys carrying orthologous structural variants of 5-HTT on a battery of tasks that assess cognitive flexibility, reward processing, and emotion. Here we show that rhesus monkeys carrying two copies of the short allele (SS) of the rhesus 5-HTT gene-linked polymorphic region (rh5-HTTLPR) show significantly reduced cognitive flexibility as measured by two tasks in the battery: object discrimination reversal learning and instrumental extinction. Monkeys with the SS genotype also displayed alterations in socioemotional behavior. Genotype variation was not related to visual perceptual abilities, valuation of food rewards, or the ability to express a wide range of defensive responses. Although emotional alterations associated with 5-HTT variation have been described as the primary phenotype, the present study reports differences in at least one type of cognitive flexibility, which has not been described previously. Because behaviors modulated by the 5-HTTLPR are a subset of those dependent on the VMPFC, analysis of structural and functional correlates of gene variation in this region may inform the nature of the genetic modulation of cognition.

Comparison of the effects of bilateral orbital prefrontal cortex lesions and amygdala lesions on emotional responses in rhesus monkeys

The present study examines the effects of bilateral orbital prefrontal cortex (PFo) lesions on monkeys' emotional responses in two different contexts: in the presence of a rubber snake and in the presence of a human intruder. For comparison, we also assessed the responses of rhesus monkeys with selective amygdala lesions on these same tasks. Monkeys with PFo lesions, like those with amygdala lesions, displayed blunted emotional responses to the fake snake. Unlike monkeys with amygdala lesions, however, monkeys with PFo lesions displayed more mild aggression than controls in the presence of a human intruder. The findings support the idea that the PFo helps integrate sensory signals in the service of choosing among competing responses. In addition, they point to a divergence of the roles of the PFo and amygdala in responding to a social stimulus, the human intruder.

Attentional Interference Effects of Emotional Pictures: Threat, Negativity, or Arousal?

Attentional interference arising from emotional pictures was examined. Participants had to ignore emotional pictures while solving math problems (Study 1, N = 126) or detecting the location of a line (Study 2, N = 60). Data analyses tested predictions of 3 theories. Evolutionary threat theory predicts interference by snake pictures. Categorical negativity theory predicts interference by negative pictures regardless of their intensity. According to arousal theory, arousal level predicts interference effects. The results supported arousal theory, with the most arousing pictures (strong unpleasant pictures, oppositesex models) producing the strongest interference. The findings are interpreted in the context of process models of emotions that postulate an initial relevance check before further processing of valence and other appraisal dimensions.

The role of the central nucleus of the amygdala in mediating fear and anxiety in the primate

Numerous studies demonstrate that the rhesus monkey is an excellent species with which to investigate mechanisms underlying human emotion and psychopathology. To examine the role of the central nucleus of the amygdala (CeA) in mediating the behavioral and physiological responses associated with fear and anxiety, we used rhesus monkeys to assess the effects of excitotoxic lesions of the CeA. Behavioral and physiological responses of nine monkeys with bilateral CeA destruction (ranging from 46 to 98%) were compared with five animals with asymmetric lesions (42-86.5% destruction on the most affected side) and with 16 unoperated controls. Results suggest that similar to rodent species, the primate CeA plays a role in mediating fear- and anxiety-related behavioral and endocrine responses. Compared with controls and the asymmetric-lesion group, bilaterally lesioned monkeys displayed significantly less fear-related behavior when exposed to a snake and less freezing behavior when confronted by a human intruder. In addition, bilaterally lesioned monkeys had decreased levels of CSF corticotrophin-releasing factor (CRF), and both lesioned groups had decreased plasma ACTH concentrations. In contrast to these findings, patterns of asymmetric frontal brain electrical activity, as assessed by regional scalp EEG, did not significantly differ between control and lesioned monkeys. These findings suggest that in primates, the CeA is involved in mediating fear- and anxiety-related behavioral and pituitary-adrenal responses as well as in modulating brain CRF activity.

Combined unilateral lesions of the amygdala and orbital prefrontal cortex impair affective processing in rhesus monkeys

The amygdala and orbital prefrontal cortex (PFo) interact as part of a system for affective processing. To assess whether there is a hemispheric functional specialization for the processing of emotion or reward or both in nonhuman primates, rhesus monkeys (Macaca mulatta) with combined lesions of the amygdala and PFo in one hemisphere, either left or right, were compared with unoperated controls on a battery of tasks that tax affective processing, including two tasks that tax reward processing and two that assess emotional reactions. Although the two operated groups did not differ from each other, monkeys with unilateral lesions, left and right, showed altered reward-processing abilities as evidenced by attenuated reinforcer devaluation effects and an impairment in object reversal learning relative to controls. In addition, both operated groups showed blunted emotional reactions to a rubber snake. By contrast, monkeys with unilateral lesions did not differ from controls in their responses to an unfamiliar human (human "intruder"). Although the results provide no support for a hemispheric specialization of function, they yield the novel finding that unilateral lesions of the amygdala-orbitofrontal cortical circuit in monkeys are sufficient to significantly disrupt affective processing.