Measuring reward assessment in a semi-naturalistic context: the effects of selective amygdala, orbital frontal or hippocampal lesions

A hippocampo-cortical pathway detects changes in the validity of an action as a predictor of reward

Much research has been dedicated to understanding the psychological and neural bases of goal-directed action, yet the relationship between context and goal-directed action is not well understood. Here, we used excitotoxic lesions, chemogenetics, and circuit-specific manipulations to demonstrate the role of the ventral hippocampus (vHPC) in contextual learning that supports sensitivity to action-outcome contingencies, a hallmark of goal-directed action. We found that chemogenetic inhibition of the ventral, but not dorsal, hippocampus attenuated sensitivity to instrumental contingency degradation. We then tested the hypothesis that this deficit was due to an inability to discern the relative validity of the action compared with the context as a predictor of reward. Using latent inhibition and Pavlovian context conditioning, we confirm that degradation of action-outcome contingencies relies on intact context-outcome learning and show that this learning is dependent on vHPC. Finally, we show that chemogenetic inhibition of vHPC terminals in the medial prefrontal cortex also impairs both instrumental contingency degradation and context-outcome learning. These results implicate a hippocampo-cortical pathway in adapting to changes in instrumental contingencies and indicate that the psychological basis of this deficit is an inability to learn the predictive value of the context. Our findings contribute to a broader understanding of the neural bases of goal-directed action and its contextual regulation.

An evidence review of the association of immune and inflammatory markers with obesity-related eating behaviors

Background

Eating behaviors contribute to disproportionate energy intake and are linked to the development of obesity. Animal studies support the role of inflammatory cytokines and chemokines in the regulation of obesity-related eating behaviors and offer a potential target to combat obesity through the modulation of inflammation. However, more complex eating behaviors are present in humans, and their relationships with immune/inflammation markers are unclear. The present study reviewed current literature to synthesize the evidence on the association of immune/inflammation markers with obesity-related eating behaviors in humans.

Methods

A systematic search of three electronic databases yielded 811 articles, of which 11 met the inclusion criteria.

Results

The majority of the included studies (91%) were either case-control or cross-sectional studies. A variety of immune/inflammation markers and obesity-related eating behaviors have been assessed in the chosen studies. Three out of four studies identified a positive relationship between C-reactive protein (CRP)/high-sensitivity CRP and loss of control eating. Other inflammatory markers that potentially have a positive relationship with obesity-related eating behaviors include fractalkine and fibrinogen. Additionally, immune molecules, including interferon gamma (INF-γ), interleukin (IL)-7, IL-10, and α-melanocyte-stimulating hormone-reactive immunoglobulin G (α-MSH/IgG) immune complex, may have negative associations with obesity-related eating behaviors. However, most findings were identified by single studies.

Conclusion

Limited studies have been conducted in humans. Current evidence indicates a potential bi-directional relationship between inflammatory/immune markers and obesity-related eating behaviors. Additional studies with sophisticated research design and comprehensive theoretical models are warranted to further delineate the relationship between immune/inflammation markers and obesity-related eating behaviors.

Better living through understanding the insula: Why subregions can make all the difference

Insula function is considered critical for many motivated behaviors, with proposed functions ranging from attention, behavioral control, emotional regulation, goal-directed and aversion-resistant responding. Further, the insula is implicated in many neuropsychiatric conditions including substance abuse. More recently, multiple insula subregions have been distinguished based on anatomy, connectivity, and functional contributions. Generally, posterior insula is thought to encode more somatosensory inputs, which integrate with limbic/emotional information in middle insula, that in turn integrate with cognitive processes in anterior insula. Together, these regions provide rapid interoceptive information about the current or predicted situation, facilitating autonomic recruitment and quick, flexible action. Here, we seek to create a robust foundation from which to understand potential subregion differences, and provide direction for future studies. We address subregion differences across humans and rodents, so that the latter's mechanistic interventions can best mesh with clinical relevance of human conditions. We first consider the insula's suggested roles in humans, then compare subregional studies, and finally describe rodent work. One primary goal is to encourage precision in describing insula subregions, since imprecision (e.g. including both posterior and anterior studies when describing insula work) does a disservice to a larger understanding of insula contributions. Additionally, we note that specific task details can greatly impact recruitment of various subregions, requiring care and nuance in design and interpretation of studies. Nonetheless, the central ethological importance of the insula makes continued research to uncover mechanistic, mood, and behavioral contributions of paramount importance and interest. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.

The ever-changing OFC landscape: What neural signals in OFC can tell us about inhibitory control

Despite decades of research on OFC function, the exact function(s) of OFC remain elusive. In recent years, 1 of the earliest hypotheses about OFC function, namely its involvement in inhibitory control, has drifted to the periphery of the functional OFC landscape in favor of theories suggesting a role for OFC in the representation of task or state space. The reasons for this drift are valid, owing in part to the development of more sensitive behavioral approaches, a clear emphasis on cross-species and cross-method comparisons, as well as the elegant integration of reinforcement learning theories. However, recent evidence recording from OFC during the performance of traditional inhibitory control tasks has found new evidence supporting a role for OFC in inhibitory control. While the extent to which these findings can be integrated into existing frameworks is in its infancy, this review seeks to highlight these findings with the goal of providing new insights into function of OFC. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

An insular view of the social decision-making network

Social animals must detect, evaluate and respond to the emotional states of other individuals in their group. A constellation of gestures, vocalizations, and chemosignals enable animals to convey affect and arousal to others in nuanced, multisensory ways. Observers integrate social information with environmental and internal factors to select behavioral responses to others via a process call social decision-making. The Social Decision Making Network (SDMN) is a system of brain structures and neurochemicals that are conserved across species (mammals, reptiles, amphibians, birds) that are the proximal mediators of most social behaviors. However, how sensory information reaches the SDMN to shape behavioral responses during a social encounter is not well known. Here we review the empirical data that demonstrate the necessity of sensory systems in detecting social stimuli, as well as the anatomical connectivity of sensory systems with each node of the SDMN. We conclude that the insular cortex is positioned to link integrated social sensory cues to this network to produce flexible and appropriate behavioral responses to socioemotional cues.

Effects of systemic cholinergic antagonism on reinforcer devaluation in macaques

The capacity to adjust actions based on new information is a vital cognitive function. An animal's ability to adapt behavioral responses according to changes in reward value can be measured using a reinforcer devaluation task, wherein the desirability of a given object is reduced by decreasing the value of the associated food reinforcement. Elements of the neural circuits serving this ability have been studied in both rodents and nonhuman primates. Specifically, the basolateral amygdala, orbitofrontal cortex, nucleus accumbens, and mediodorsal thalamus have each been shown to play a critical role in the process of value updating, required for adaptive goal selection. As these regions receive dense cholinergic input, we investigated whether systemic injections of non-selective nicotinic or muscarinic acetylcholine receptor antagonists, mecamylamine and scopolamine, respectively, would impair performance on a reinforcer devaluation task. Here we demonstrate that in the presence of either a nicotinic or muscarinic antagonist, animals are able to shift their behavioral responses in an appropriate manner, suggesting that disruption of cholinergic neuromodulation is not sufficient to disrupt value updating, and subsequent goal selection, in rhesus macaques.

Orbitofrontal cortex reflects changes in response-outcome contingencies during probabilistic reversal learning

In a continuously changing environment, in which behavioral outcomes are rarely certain, animals must be able to learn to integrate feedback from their choices over time and adapt to changing reward contingencies to maintain flexible behavior. The orbitofrontal region of prefrontal cortex (OFC) has been widely implicated as playing a role in the ability to flexibly control behavior. We used a probabilistic reversal learning task to measure rats' behavioral flexibility and its neural basis in the activity of single neurons in OFC. In this task, one lever, designated as 'correct', was rewarded at a high probability (80%) and a second, spatially distinct lever, designated as 'incorrect', was rewarded at a low probability (20%). Once rats reached a learning criterion for reliably selecting the correct lever, reward contingencies of the two levers were switched, and daily sessions were conducted until rats reliably selected the new correct lever. All rats performed the initial Acquisition and subsequent Reversal successfully, with more sessions needed to learn the Reversal. OFC neurons were recorded during five behavioral sessions spanning Acquisition and Reversal learning. The dominant pattern of neural responding in OFC, identified by principal component analysis of the population of neurons recorded, was modulated by reward outcome across behavioral sessions. Generally, activity was higher following rewarded choices than unrewarded. However, there was a correlation between reduced responses to reward following incorrect choices and the establishment of the preference for the correct lever. These results show how signaling by individual OFC neurons may participate in the flexible adaptation of behavior under changing reward contingencies.

Social change and access to a palatable diet produces differences in reward neurochemistry and appetite in female monkeys

Understanding factors that contribute to the etiology of obesity is critical for minimizing the effects of obesity-related adverse physical health outcomes. Emotional eating or the inability to control intake of calorically dense diets (CDD) under conditions of psychosocial stress exposure is a potential risk factor for the development of obesity in people. Decreases in dopamine 2 receptors (D2R) availability have been documented in substance abuse and obesity in humans, as well as animal models of chronic stressor exposure. Social subordination in macaques is a well-established animal model of a chronic psychogenic stressor that results in stress axis dysregulation, attenuated striatal D2R levels, and stress-induced hyperphagia in complex dietary environment. However, it remains unclear how these phenotypes emerge as the stressor becomes chronic during the formation of new social groups. Thus, the goal of the current study was to assess how the imposition of social subordination over a four-month period would affect food intake, socioemotional behavior, and D2R binding potential (D2R-BP) in female rhesus monkeys maintained on a typical laboratory chow diet (LCD) compared with those having a choice between a LCD and a CDD. Results showed that access to a CDD leads to increased total caloric intake and preference for a CDD over a LCD. For the dietary choice condition, females directing less aggression towards group mates during the four-month period, a characteristic of lower social status, consumed progressively more calories over the four-month period than more aggressive females. This relation between agonistic behavior and appetite was not observed for females in LCD-only condition. Finally, decreased D2R-BP in the orbitofrontal cortex was predictive of increased overall caloric intake in all females regardless of dietary environment, suggesting that reduced availability of D2R within the prefrontal cortex is associated with unrestrained eating. Studies are continuing to determine how newly imposed dominance ranks continue to affect reward neurochemistry and appetite over time, and how this is influenced by the dietary environment.

The influence of context on recognition memory in monkeys: effects of hippocampal, parahippocampal and perirhinal lesions

This study further investigated the specific contributions of the medial temporal lobe structures to contextual recognition memory. Monkeys (Macaca mulatta) with either neurotoxic lesions of the hippocampus, aspiration lesions of the perirhinal cortex and parahippocampal areas TH/TF, or sham operations were tested on five conditions of a visual-paired comparison (VPC) task in which 3-dimensional objects were presented over multicolored backgrounds. In two conditions (Conditions 1 and 2: Context-changes), the sample object was presented on a new background during the retention tests, whereas in the three others (Conditions 3-5: No-context-changes) the sample object was presented over its familiar background. Novelty preference scores of control animals were weaker, but still significantly different from chance, in the Context-changes conditions than on the No-context-changes conditions. Animals in the three experimental groups showed strong preference for novelty on the No-context-change conditions, but weaker novelty preference on the Context-change conditions than controls. Thus, animals in all three lesion types had greater difficulty recognizing an object when its background was different from that used during encoding. The data are consistent with the view that the hippocampal formation, areas TH/TF, and perirhinal cortex contribute interactively to contextual memory processes.

The addictive dimensionality of obesity

Our brains are hardwired to respond and seek immediate rewards. Thus, it is not surprising that many people overeat, which in some can result in obesity, whereas others take drugs, which in some can result in addiction. Though food intake and body weight are under homeostatic regulation, when highly palatable food is available, the ability to resist the urge to eat hinges on self-control. There is no homeostatic regulator to check the intake of drugs (including alcohol); thus, regulation of drug consumption is mostly driven by self-control or unwanted effects (i.e., sedation for alcohol). Disruption in both the neurobiological processes that underlie sensitivity to reward and those that underlie inhibitory control can lead to compulsive food intake in some individuals and compulsive drug intake in others. There is increasing evidence that disruption of energy homeostasis can affect the reward circuitry and that overconsumption of rewarding food can lead to changes in the reward circuitry that result in compulsive food intake akin to the phenotype seen with addiction. Addiction research has produced new evidence that hints at significant commonalities between the neural substrates underlying the disease of addiction and at least some forms of obesity. This recognition has spurred a healthy debate to try and ascertain the extent to which these complex and dimensional disorders overlap and whether or not a deeper understanding of the crosstalk between the homeostatic and reward systems will usher in unique opportunities for prevention and treatment of both obesity and drug addiction.

Effects of Selective Neonatal Amygdala Damage on Concurrent Discrimination Learning and Reinforcer Devaluation in Monkeys

OBJECTIVES

The amygdala is known to be a key neural structure in many neuropsychiatric disorders. Primarily known for its involvement in fear regulation, the amygdala also plays a critical role in appetitive flexible decision-making. Yet, its contribution to the development of flexible goal-directed behavior has not been thoroughly examined.

DESIGN

The current study examined flexible decision-making abilities after neonatal amygdala lesions in nonhuman primates using a behavioral paradigm known to measure the flexible monitoring of goal-directed choices in rodents, monkeys, and humans.

METHOD

Rhesus monkeys of both sexes were divided into two groups, a sham-operated control group (N=4) and a group with neonatal neurotoxic amygdala lesions (N=5). Animals received the lesions at 1-2 weeks and were tested at both four and six years of age on a concurrent discrimination reinforcer devaluation task.

RESULTS

Although neonatal amygdala damage spared learning stimulus-reward associations, it severely impaired the ability to flexibly shift object choices away from those items associated with devalued food rewards. The results were similar to those obtained in monkeys that had acquired the same lesions in adulthood.

CONCLUSIONS

Thus, the amygdala is critical for appetitive decision-making, and provide further evidence of little functional sparing after early amygdala insult. The findings are discussed in relation to other behavioral measures on the same animals and to clinical neuropsychiatric disorders.

The role of the anterior cingulate cortex in choices based on reward value and reward contingency

Although several studies have emphasized the role of the anterior cingulate cortex (ACC) in associating actions with reward value, its role in guiding choices on the basis of changes in reward value has not been assessed. Accordingly, we compared rhesus monkeys with ACC lesions and controls on object- and action-based reinforcer devaluation tasks. Monkeys were required to associate an object or an action with one of two reward outcomes, and we assessed the monkey's shift in choices of objects or actions after changes in the value of 1 outcome. No group differences emerged on either task. For comparison, we tested the same monkeys on their ability to make choices guided by reward contingency in object- and action-based reversal learning tasks. Monkeys with ACC lesions were impaired in using rewarded trials to sustain the selection of the correct object during object reversal learning. They were also impaired in using errors to guide choices in action reversal learning. These data indicate that the role of the ACC is not restricted to linking specific actions with reward outcomes, as previously reported. Instead, the data suggest a more general role for the ACC in using information about reward and nonreward to sustain effective choice behavior.

Preserved stimulus-reward and reversal learning after selective neonatal orbital frontal areas 11/13 or amygdala lesions in monkeys

Neither lesions of orbital frontal (OFC) areas 11/13 nor selective amygdala lesions alter the ability to learn stimulus-reinforcer association and reversal discriminations in adult monkeys. Here, we investigated whether the same conclusion will hold true when the same lesions occur in infancy. Infant rhesus monkeys received sham-operations, neurotoxic amygdala lesions, or aspiration OFC 11/13 lesions at 8-15 days of age and were trained on object discrimination reversal (ODR) tasks. Performance on a single pair (1-Pair) ODR was assessed at the age of 3 months and 3 years, and then animals were tested in a 5-Pair ODR task in which they had to concurrently learn and reverse five discrimination problems. The results indicated that the ability to solve a single-pair discrimination problem followed by six reversals appears to be late maturing in monkeys but is spared following selective lesions of either OFC areas 11/13 or amygdala, even with the use of the more challenging 5-object ODR task. Finally, performance in the 1 and 5-Pair ODR at 3 years was comparable to that following adult-onset lesions, indicating that neither OFC areas 11/13 nor amygdala are critical for the development of reversal learning.

Ventromedial frontal lobe damage disrupts the accuracy, but not the speed, of value-based preference judgments

The ventromedial frontal lobe (VMF) plays a role in decision making, but its precise function remains unclear. Several lines of evidence suggest that VMF is involved in representing the economic value of options. A prior study from our lab has shown that patients with lesions to the VMF are less consistent than controls in making simple preference judgments between stimuli presented in pairs. Here, we followed up that observation in a larger sample, using more sensitive tasks, and examining the category-specificity of this effect. Patients with damage to VMF (N=15) were compared to patients with frontal damage sparing that region (N=8) and to demographically matched healthy control participants (N=23). Five separate preference tasks were administered, requiring subjects to indicate their preference for 12 stimuli presented two at a time, in all possible combinations. Categories included fruits, vegetables, colors, landscapes, and puppies. Choices were analyzed for internal consistency, and decision times were measured. Three control tasks with the same format, but requiring perceptual judgments, were also administered. VMF patients were significantly more erratic than both non-VMF and healthy control participants in their preference judgments across all stimulus categories. However, decision times, and the relationship between decision time and relative value, were similar to that seen in control participants. The groups did not differ in perceptual judgment performance. These findings add further weight to the claim that VMF plays a critical role in simple value-based decision-making under conditions of certainty. This region appears to be necessary for consistent choices across a variety of stimulus categories, supporting the view that human VMF represents the (relative) value of decision options rather generally. That such damage impairs decision 'accuracy' without affecting reaction time has implications for theories of the role of VMF in decision-making, arguing that this region may be critical for linking a particular value to a particular option.

Transient inactivation of orbitofrontal cortex blocks reinforcer devaluation in macaques

The orbitofrontal cortex (OFC) and its interactions with the basolateral amygdala (BLA) are critical for goal-directed behavior, especially for adapting to changes in reward value. Here we used a reinforcer devaluation paradigm to investigate the contribution of OFC to this behavior in four macaques. Subjects that had formed associations between objects and two different primary reinforcers (foods) were presented with choices of objects overlying the two different foods. When one of the two foods was devalued by selective satiation, the subjects shifted their choices toward the objects that represented the nonsated food reward (devaluation effect). Transient inactivation of OFC by infusions of the GABA(A) receptor agonist muscimol into area 13 blocked the devaluation effect: the monkeys did not reduce their selection of objects associated with the devalued food. This effect was observed when OFC was inactivated during both satiation and the choice test, and during the choice test only. This supports our hypothesis that OFC activity is required during the postsatiety object choice period to guide the selection of objects. This finding sharply contrasts with the role of BLA in the same devaluation process (Wellman et al., 2005). Whereas activity in BLA was required during the selective satiation procedure, it was not necessary for guiding the subsequent object choice. Our results are the first to demonstrate that transient inactivation of OFC is sufficient to disrupt the devaluation effect, and to document a role for OFC distinct from that of BLA for the conditioned reinforcer devaluation process in monkeys.

Neurobiology of economic choice: a good-based model

Traditionally the object of economic theory and experimental psychology, economic choice recently became a lively research focus in systems neuroscience. Here I summarize the emerging results and propose a unifying model of how economic choice might function at the neural level. Economic choice entails comparing options that vary on multiple dimensions. Hence, while choosing, individuals integrate different determinants into a subjective value; decisions are then made by comparing values. According to the good-based model, the values of different goods are computed independently of one another, which implies transitivity. Values are not learned as such, but rather computed at the time of choice. Most importantly, values are compared within the space of goods, independent of the sensorimotor contingencies of choice. Evidence from neurophysiology, imaging, and lesion studies indicates that abstract representations of value exist in the orbitofrontal and ventromedial prefrontal cortices. The computation and comparison of values may thus take place within these regions.

Contribution of the amygdala, but not orbitofrontal or medial prefrontal cortices, to the expression of flavour preferences in marmoset monkeys

The development of food preferences contributes to a balanced diet, and involves both innate and learnt factors. By associating flavour cues with the reinforcing properties of the food (i.e. postingestive nutrient cues and innately preferred tastes, such as sweetness), animals acquire individual preferences. How the brain codes and guides selection when the subject has to choose between different palatable foods is little understood. To investigate this issue, we trained common marmoset monkeys (Callithrix jacchus) to respond to abstract visual patterns on a touch-sensitive computer screen to gain access to four different flavoured juices. After preferences were stable, animals received excitotoxic lesions of either the amygdala, the orbitofrontal cortex or the medial prefrontal cortex. Neither the orbitofrontal nor the medial prefrontal cortex lesions affected pre-surgery-expressed flavour preferences or the expression of preferences for novel flavours post-surgery. In contrast, amygdala lesions caused a shift in the preferences for juices expressed pre-surgery such that, post-surgery, juices were chosen according to their overall carbohydrate (simple sugars) content or 'sweetness'. Subsequent tests revealed that amygdala-lesioned animals only expressed juice preferences if they differed in 'sweetness'. Unlike controls, orbitofrontal cortex-lesioned and medial prefrontal cortex-lesioned animals, they were unable to display preferences between juices matched for 'sweetness' i.e. 5% sucrose solutions aromatised with different essential oils. The most parsimonious explanation is that the amygdala contributes to the expression of food preferences based on learnt cues but not those based on an innate preference for sweetness.

Ventromedial frontal lobe damage disrupts value maximization in humans

Recent work in neuroeconomics has shown that regions in orbitofrontal and medial prefrontal cortex encode the subjective value of different options during choice. However, these electrophysiological and neuroimaging studies cannot demonstrate whether such signals are necessary for value-maximizing choices. Here we used a paradigm developed in experimental economics to empirically measure and quantify violations of utility theory in humans with damage to the ventromedial frontal lobe (VMF). We show that people with such damage are more likely to make choices that violate the generalized axiom of revealed preference, which is the one necessary and sufficient condition for choices to be consistent with value maximization. These results demonstrate that the VMF plays a critical role in value-maximizing choice.

Obesity-mediated inflammation may damage the brain circuit that regulates food intake

Adiposity is associated with chronic low-grade systemic inflammation and increased inflammation in the hypothalamus, a key structure in feeding behavior. It remains unknown whether inflammation impacts other brain structures that regulate feeding behavior. We studied 44 overweight/obese and 19 lean individuals with MRI and plasma fibrinogen levels (marker of inflammation). We performed MRI-based segmentations of the medial and lateral orbitofrontal cortex (OFC) and hippocampal volumes. Gray matter (GM) volumes were adjusted for head size variability. We conducted logistic and hierarchical regressions to assess the association between fibrinogen levels and brain volumetric data. Using diffusion tensor imaging (DTI), we created apparent diffusion coefficient (ADC) maps and conducted voxelwise correlational analyses. Fibrinogen concentrations were higher among the overweight/obese (t[61] = -2.33, P = 0.023). Lateral OFC associated together with fibrinogen correctly classified those with excess of weight (accuracy = 76.2%, sensitivity = 95.5%, and specificity=31.6%). The lateral OFC volumes of overweight/obese were negatively associated with fibrinogen (r = -0.37, P = 0.016) and after accounting for age, hypertension, waist/hip ratio and lipid and sugar levels, fibrinogen significantly explained an additional 9% of the variance in the lateral OFC volume (β = -0.348, ΔR(2) = 0.093, ΔF P = 0.046). Among overweight/obese the associations between GM ADC and fibrinogen were significantly positive (P < 0.001) in the left and right amygdala and the right parietal region. Among lean individuals these associations were negative and located in the left prefrontal, the right parietal and the left occipital lobes. This is the first study to report that adiposity-related inflammation may reduce the integrity of some of the brain structures involved in reward and feeding behaviors.

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.

Re-valuing the amygdala

Recent advances indicate that the amygdala represents valence: a general appetitive/aversive affective characteristic that bears similarity to the neuroeconomic concept of value. Neurophysiological studies show that individual amygdala neurons respond differentially to a range of stimuli with positive or negative affective significance. Meanwhile, increasingly specific lesion/inactivation studies reveal that the amygdala is necessary for processes--for example, fear extinction and reinforcer devaluation--that involve updating representations of value. Furthermore, recent neuroimaging studies suggest that the human amygdala mediates performance on many reward-based decision-making tasks. The encoding of affective significance by the amygdala might be best described as a representation of state value-a representation that is useful for coordinating physiological, behavioral, and cognitive responses in an affective/emotional context.

Selective aspiration or neurotoxic lesions of orbital frontal areas 11 and 13 spared monkeys' performance on the object discrimination reversal task

Damage to the orbital frontal cortex (OFC) has long been associated with reversal learning deficits in several species. In monkeys, this impairment follows lesions that include several OFC subfields. However, the different connectional patterns of OFC subfields together with neuroimaging data in humans have suggested that specific OFC areas play distinctive roles in processing information necessary to guide behavior (Kringelbach and Rolls, 2004; Barbas, 2007; Price, 2007). More specifically, areas 11 and 13 contribute to a sensory network, whereas medial areas 10, 14, and 25 are heavily connected to a visceromotor network. To examine the contribution of areas 11 and 13 to reversal learning, we tested monkeys with selective damage to these two OFC areas on two versions of the ODR task using either one or five discrimination problems. We compared their performance with that of sham-operated controls and of animals with neurotoxic amygdala lesions, which served as operated controls. Neither damage to areas 11 and 13 nor damage to the amygdala affected performance on the ODR tasks. The results indicate that areas 11 and 13 do not critically contribute to reversal learning and that adjacent damage to OFC subfields (10, 12, 14, and 25) could account for the ODR deficits found in earlier lesion studies. This sparing of reversal learning will be discussed in relation to deficits found in the same animals on tasks that measure behavioral modulation when relative value of affective (positive and negative) stimuli was manipulated.

Impact of amygdala, orbital frontal, or hippocampal lesions on threat avoidance and emotional reactivity in nonhuman primates

The authors measured the effects of bilateral amygdaloid, orbital frontal, or hippocampal lesions on emotional reactivity and passive avoidance in rhesus monkeys (Macaca mulatta). Animals were presented with 8 neutral or 8 aversive objects, each paired with a highly preferred food reward. Sham-operated control animals displayed heightened defensive behaviors and typically would not approach or retrieve the food when paired with a potential predator (coiled rubber snake), 2 conditioned aversive stimuli for laboratory-housed monkeys (a capture net and leather handling gloves), and 1 object displaying a threatening social signal (direct eye contact from a human-like doll). Animals with amygdala lesions, but not hippocampal or orbital frontal lesions, showed less tension-related behaviors and diminished passive avoidance of the rubber snake and its matched neutral item (a coiled piece of hose) relative to control animals. All operated groups displayed normal patterns of behavior toward conditioned and socially aversive objects. These results expand our understanding of how the primate brain evaluates reward and threat, and indicate a highly specialized role for the amygdala in mediating passive avoidance and emotional reactivity to potentially life-threatening stimuli.

Measuring and modeling the interaction among reward size, delay to reward, and satiation level on motivation in monkeys

Motivation is usually inferred from the likelihood or the intensity with which behavior is carried out. It is sensitive to external factors (e.g., the identity, amount, and timing of a rewarding outcome) and internal factors (e.g., hunger or thirst). We trained macaque monkeys to perform a nonchoice instrumental task (a sequential red-green color discrimination) while manipulating two external factors: reward size and delay-to-reward. We also inferred the state of one internal factor, level of satiation, by monitoring the accumulated reward. A visual cue indicated the forthcoming reward size and delay-to-reward in each trial. The fraction of trials completed correctly by the monkeys increased linearly with reward size and was hyperbolically discounted by delay-to-reward duration, relations that are similar to those found in free operant and choice tasks. The fraction of correct trials also decreased progressively as a function of the satiation level. Similar (albeit noiser) relations were obtained for reaction times. The combined effect of reward size, delay-to-reward, and satiation level on the proportion of correct trials is well described as a multiplication of the effects of the single factors when each factor is examined alone. These results provide a quantitative account of the interaction of external and internal factors on instrumental behavior, and allow us to extend the concept of subjective value of a rewarding outcome, usually confined to external factors, to account also for slow changes in the internal drive of the subject.

Behavioral and hormonal reactivity to threat: effects of selective amygdala, hippocampal or orbital frontal lesions in monkeys

We compared the effects of bilateral amygdala, hippocampal or orbital frontal cortex lesions on emotional and hormonal reactivity in rhesus monkeys (Macaca mulatta). Experiment 1 measured behavioral reactivity to an unfamiliar human intruder before and after surgery. Animals with amygdala lesions demonstrated decreases in one passive defensive behavior (freezing), whereas animals with hippocampal lesions showed decreases in a more stimulus-directed defensive behavior (tooth grinding). Orbital frontal cortex lesions also reduced these two defensive behaviors, as well as decreased cage-shaking dominance displays. Animals with amygdala, hippocampal or sham lesions also demonstrated increased tension-related behaviors after surgery, but those with orbital frontal lesions did not. Finally, all three lesions diminished the operated animals' ability to modulate tension-related behaviors depending on the magnitude of threat posed by the human intruder. Experiment 2 measured circulating levels of cortisol and testosterone when a subset of these same animals was at rest and following physical restraint, temporary isolation, exposure to threatening objects and social interactions with an unfamiliar conspecific. None of the lesions impacted on testosterone levels in any condition. Amygdala or orbital frontal lesions blunted cortisol reactivity during isolation from peers, but not during any other condition. Hippocampal lesions did not alter circulating levels of cortisol under any condition. These results indicate that the amygdala, hippocampus and orbital frontal cortex play distinct, yet complimentary roles in coordinating emotional and hormonal reactivity to threat.

Bilateral neurotoxic amygdala lesions in rhesus monkeys (Macaca mulatta): consistent pattern of behavior across different social contexts

Although the amygdala has been repeatedly implicated in normal primate social behavior, great variability exists in the specific social and nonsocial behavioral changes observed in nonhuman primates with bilateral amygdala lesions. One plausible explanation pertains to differences in social context. This study measured the social behavior of amygdala-lesioned and unoperated rhesus monkeys (Macaca mulatta) in 2 contexts. Monkeys interacted in 4-member social groups over 32 test days. They were previously assessed in pairs (N. J. Emery et al., 2001) and were therefore familiar with each other at the beginning of this study. Across the 2 contexts, amygdala lesions produced a highly consistent pattern of social behavior. Operated monkeys engaged in more affiliative social interactions with control partners than did controls. In the course of their interactions, amygdala-lesioned monkeys also displayed an earlier decrease in nervous and fearful personality qualities than did controls. The increased exploration and sexual behavior recorded for amygdala-lesioned monkeys in pairs was not found in the 4-member groups. The authors concluded that the amygdala contributes to social inhibition and that this function transcends various social contexts.