Elisabeth A. Murray

Understanding the organization and function of thalamic pulvinar projections to the amygdala is of interest due to the proposal that this projection provides the amygdala with short-latency visual sensory input that eludes conscious awareness. However, most reports in primates have emphasized a projection from the multimodal medial pulvinar—a pulvinar division unique to primates—versus projections from visual pulvinar divisions (inferior or lateral). Further, reports in other closely related species such as tree shrews and rodents have yielded inconsistent results relative to primates when homology is considered. In these species, subdivisions of the lateral posterior/pulvinar complex, which are homologous to the visual inferior pulvinar of primates, project to the amygdala. Such a difference in pulvino-amygdala connections across these closely related species would be surprising. However, modern methods that reveal subdivisions of the pulvinar were lacking in previous anatomical studies of primate pulvino-amygdala connections. To better understand whether a major shift in pulvino-amygdala projections is truly present across species, we reevaluated the locations of pulvinar neurons projecting to the amygdala in rhesus monkeys ( Macaca mulatta ) using robust anatomical markers for delineating divisions of the pulvinar, and specifically highlighting the border between the medial and inferior pulvinar. Our findings show definitively that pulvino-amygdala projections in macaques share both conserved, via the visual inferior pulvinar, and novel, via the medial pulvinar, profiles. Further, our data provide a refinement in the available routes via which visual information could reach the amygdala, one that includes the inferior pulvinar nucleus of the thalamus.

Altered arousal is characteristic of many mental health disorders, including major depressive disorder (MDD). Several studies link neural activity in the orbitofrontal cortex (OFC) with anticipation of reward, including anticipatory sympathetic arousal, which is blunted in MDD. We therefore studied acquisition and consolidation of appetitive pavlovian memories in two groups of adult male rhesus monkeys: unoperated controls ( N = 4) and those with selective neurotoxic lesions of OFC ( N = 4). The dependent measure was conditioned sympathetic arousal as indexed by differential pupil dilation, and the key comparison was dilation in response to a visual cue that predicted the delivery of a large fluid reward (CS+) versus a cue that predicted no reward (CS−). Control procedures ruled out global effects of the lesion on pupil dilation. All four unoperated controls and all four monkeys with OFC lesions acquired a conditioned increase in the pupil size in response to the CS+. However, three of the four monkeys in the lesion group failed to consolidate the memory underlying this response. In contrast to this impairment, monkeys with OFC lesions acquired and consolidated an operant visual discrimination for the same reward and did so at the same rate as controls. These findings point to a specialized role of OFC in consolidating memories underlying positive affective responses, which further implicates OFC dysfunction in the blunted positive affect characteristic of MDD and suggests therapeutic approaches involving enhanced consolidation and/or reconsolidation of associative memories.

In 1978, Mort Mishkin published a landmark paper describing a monkey model of H.M.'s dense, global amnesia. It depended on a combined removal of the amygdala and hippocampus (the A + H lesion) and a memory test called delayed nonmatching‐to‐sample (DNMS). My first project examined whether the impairment Mishkin had found in visual memory generalized to tactual stimuli. However, to gain access to the hippocampus and amygdala with 1980s surgical methods, we had to remove the underlying cortex. When we were able to test the effects of bilateral removal of that underlying cortex (the entorhinal and perirhinal cortex, or “rhinal cortex” for short) we obtained a dramatic result. This so‐called “control” lesion caused a profound impairment on the DNMS task. A few years later, excitotoxic A + H lesions, which left the rhinal cortex intact, confirmed that removal of the cortical “impediments” had caused the entire memory impairment that Mishkin had observed. These results: (1) forced a reconsideration of the monkey model of global anterograde amnesia; (2) spurred study of the independent contributions of the amygdala, hippocampus, and perirhinal cortex to cognition; and (3) led to the realization that the DNMS task did not test the kinds of memory that H.M. lost after his surgery.

This special focus article was prepared to honor the memory of our National Institutes of Health colleague, friend, and mentor Leslie G. Ungerleider, who passed away in December 2020, and is based on a presentation given at a symposium held in her honor at the National Institutes of Health in September 2022. In this article, we describe an extension of Leslie Ungerleider's influential work on the object analyzer pathway in which the inferior temporal visual cortex interacts with the amygdala, and then discuss a broader role for the amygdala in stimulus–outcome associative learning in humans and nonhuman primates. We summarize extant data from our and others' laboratories regarding two distinct frontal–amygdala circuits that subserve nonsocial and social valuation processes. Both neuropsychological and neurophysiological data suggest a role for the OFC in nonsocial valuation and the ACC in social valuation. More recent evidence supports the possibility that the amygdala functions in conjunction with these frontal regions to subserve these distinct, complex valuation processes. We emphasize the dynamic nature of valuation processes and advocate for additional research on amygdala–frontal interactions in these domains.

Deciding whether to forego immediate rewards or explore new opportunities is a key component of flexible behavior and is critical for the survival of the species. Although previous studies have shown that different cortical and subcortical areas, including the amygdala and ventral striatum (VS), are implicated in representing the immediate (exploitative) and future (explorative) value of choices, the effect of the motor system used to make choices has not been examined. Here, we tested male rhesus macaques with amygdala or VS lesions on two versions of a three-arm bandit task where choices were registered with either a saccade or an arm movement. In both tasks we presented the monkeys with explore–exploit tradeoffs by periodically replacing familiar options with novel options that had unknown reward probabilities. We found that monkeys explored more with saccades but showed better learning with arm movements. VS lesions caused the monkeys to be more explorative with arm movements and less explorative with saccades, although this may have been due to an overall decrease in performance. VS lesions affected the monkeys’ ability to learn novel stimulus-reward associations in both tasks, while after amygdala lesions this effect was stronger when choices were made with saccades. Further, on average, VS and amygdala lesions reduced the monkeys’ ability to choose better options only when choices were made with a saccade. These results show that learning reward value associations to manage explore–exploit behaviors is motor system dependent and they further define the contributions of amygdala and VS to reinforcement learning.

Dual-process accounts of item recognition posit two memory processes: slow but detailed recollection, and quick but vague familiarity. It has been proposed, based on prior rodent work, that the amygdala is critical for the familiarity aspect of item recognition. Here, we evaluated this proposal in male rhesus monkeys (Macaca mulatta) with selective bilateral excitotoxic amygdala damage. We used four established visual memory tests designed to assess different aspects of familiarity, all administered on touchscreen computers. Specifically, we assessed monkeys’ tendencies to make low-latency false alarms, to make false alarms to recently seen lures, to produce curvilinear ROC curves, and to discriminate stimuli based on repetition across days. Three of the four tests showed no familiarity impairment and the fourth was explained by a deficit in reward processing. Consistent with this, amygdala damage did produce an anticipated deficit in reward processing in a three-arm-bandit gambling task, verifying the effectiveness of the lesions. Together, these results contradict prior rodent work and suggest that the amygdala is not critical for the familiarity aspect of item recognition.

The hippocampus and perirhinal cortex are both broadly implicated in memory; nevertheless, their relative contributions to visual item recognition and location memory remain disputed. Neuropsychological studies in nonhuman primates that examine memory function after selective damage to medial temporal lobe structures report various levels of memory impairment—ranging from minor deficits to profound amnesia. The discrepancies in published findings have complicated efforts to determine the exact magnitude of visual item recognition and location memory impairments following damage to the hippocampus and/or perirhinal cortex. To provide the most accurate estimate to date of the overall effect size, we use meta‐analytic techniques on data aggregated from 26 publications that assessed visual item recognition and/or location memory in nonhuman primates with and without selective neurotoxic lesions of the hippocampus or perirhinal cortex. We estimated the overall effect size, evaluated the relation between lesion extent and effect size, and investigated factors that may account for between‐study variation. Grouping studies by lesion target and testing method, separate meta‐analyses were conducted. One meta‐analysis indicated that impairments on tests of visual item recognition were larger after lesions of perirhinal cortex than after lesions of the hippocampus. A separate meta‐analysis showed that performance on tests of location memory was severely impaired by lesions of the hippocampus. For the most part, meta‐regressions indicated that greater impairment corresponds with greater lesion extent; paradoxically, however, more extensive hippocampal lesions predicted smaller impairments on tests of visual item recognition. We conclude the perirhinal cortex makes a larger contribution than the hippocampus to visual item recognition, and the hippocampus predominately contributes to spatial navigation.

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 STATEMENTValuable 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.