Silicon Foreign Body in the Cerebrum of a Rhesus Macaque (Macaca mulatta).

A male rhesus macaque with a cephalic chamber implant for neurophysiology recording presented with hemiparesis affecting the left thoracic and pelvic limbs at approximately 5 wk after craniotomy surgery. MRI indicated a 1×2-cm ovoid cerebrocortical cystic lesion immediately subjacent to the right hemisphere craniotomy and recording chamber. Transdural aspiration of sterile transudate and resultant decompression resolved the hemiparesis, and follow-up MRI at 1 mo indicated resolution of the lesion. Subsequently, necropsy at study end revealed a cerebrocortical foreign body composed of silicon. The atypically slow cure rate of the lot of silicon used and the unique recording chamber configuration were underlying factors that contributed to the formation of this foreign body. To our knowledge, this report is the first description of iatrogenic intracerebral foreign body in a macaque.

New perspectives on the neurophysiology of primate amygdala emerging from the study of naturalistic social behaviors.

A major challenge of primate neurophysiology, particularly in the domain of social neuroscience, is to adopt more natural behaviors without compromising the ability to relate patterns of neural activity to specific actions or sensory inputs. Traditional approaches have identified neural activity patterns in the amygdala in response to simplified versions of social stimuli such as static images of faces. As a departure from this reduced approach, single images of faces were replaced with arrays of images or videos of conspecifics. These stimuli elicited more natural behaviors and new types of neural responses: (1) attention-gated responses to faces, (2) selective responses to eye contact, and (3) selective responses to touch and somatosensory feedback during the production of facial expressions. An additional advance toward more natural social behaviors in the laboratory was the implementation of dyadic social interactions. Under these conditions, neurons encoded similarly rewards that monkeys delivered to self and to their social partner. These findings reinforce the value of bringing natural, ethologically valid, behavioral tasks under neurophysiological scrutiny. WIREs Cogn Sci 2018, 9:e1449. doi: 10.1002/wcs.1449 This article is categorized under: Psychology > Emotion and Motivation Neuroscience > Cognition Neuroscience > Physiology.

Tactile Stimulation of the Face and the Production of Facial Expressions Activate Neurons in the Primate Amygdala.

The majority of neurophysiological studies that have explored the role of the primate amygdala in the evaluation of social signals have relied on visual stimuli such as images of facial expressions. Vision, however, is not the only sensory modality that carries social signals. Both humans and nonhuman primates exchange emotionally meaningful social signals through touch. Indeed, social grooming in nonhuman primates and caressing touch in humans is critical for building lasting and reassuring social bonds. To determine the role of the amygdala in processing touch, we recorded the responses of single neurons in the macaque amygdala while we applied tactile stimuli to the face. We found that one-third of the recorded neurons responded to tactile stimulation. Although we recorded exclusively from the right amygdala, the receptive fields of 98% of the neurons were bilateral. A fraction of these tactile neurons were monitored during the production of facial expressions and during facial movements elicited occasionally by touch stimuli. Firing rates arising during the production of facial expressions were similar to those elicited by tactile stimulation. In a subset of cells, combining tactile stimulation with facial movement further augmented the firing rates. This suggests that tactile neurons in the amygdala receive input from skin mechanoceptors that are activated by touch and by compressions and stretches of the facial skin during the contraction of the underlying muscles. Tactile neurons in the amygdala may play a role in extracting the valence of touch stimuli and/or monitoring the facial expressions of self during social interactions.

Neurons in the monkey amygdala detect eye contact during naturalistic social interactions.

Primates explore the visual world through eye-movement sequences. Saccades bring details of interest into the fovea, while fixations stabilize the image. During natural vision, social primates direct their gaze at the eyes of others to communicate their own emotions and intentions and to gather information about the mental states of others. Direct gaze is an integral part of facial expressions that signals cooperation or conflict over resources and social status. Despite the great importance of making and breaking eye contact in the behavioral repertoire of primates, little is known about the neural substrates that support these behaviors. Here we show that the monkey amygdala contains neurons that respond selectively to fixations on the eyes of others and to eye contact. These "eye cells" share several features with the canonical, visually responsive neurons in the monkey amygdala; however, they respond to the eyes only when they fall within the fovea of the viewer, either as a result of a deliberate saccade or as eyes move into the fovea of the viewer during a fixation intended to explore a different feature. The presence of eyes in peripheral vision fails to activate the eye cells. These findings link the primate amygdala to eye movements involved in the exploration and selection of details in visual scenes that contain socially and emotionally salient features.

Videos of conspecifics elicit interactive looking patterns and facial expressions in monkeys.

A broader understanding of the neural basis of social behavior in primates requires the use of species-specific stimuli that elicit spontaneous, but reproducible and tractable behaviors. In this context of natural behaviors, individual variation can further inform about the factors that influence social interactions. To approximate natural social interactions similar to those documented by field studies, we used unedited video footage to induce in viewer monkeys spontaneous facial expressions and looking patterns in the laboratory setting. Three adult male monkeys (Macaca mulatta), previously behaviorally and genetically (5-HTTLPR) characterized, were monitored while they watched 10 s video segments depicting unfamiliar monkeys (movie monkeys) displaying affiliative, neutral, and aggressive behaviors. The gaze and head orientation of the movie monkeys alternated between "averted" and "directed" at the viewer. The viewers were not reinforced for watching the movies, thus their looking patterns indicated their interest and social engagement with the stimuli. The behavior of the movie monkey accounted for differences in the looking patterns and facial expressions displayed by the viewers. We also found multiple significant differences in the behavior of the viewers that correlated with their interest in these stimuli. These socially relevant dynamic stimuli elicited spontaneous social behaviors, such as eye-contact induced reciprocation of facial expression, gaze aversion, and gaze following, that were previously not observed in response to static images. This approach opens a unique opportunity to understanding the mechanisms that trigger spontaneous social behaviors in humans and nonhuman primates.

Response characteristics of basolateral and centromedial neurons in the primate amygdala.

Based on cellular architecture and connectivity, the main nuclei of the primate amygdala are divided in two clusters: basolateral (BL) and centromedial (CM). These anatomical features suggest a functional division of labor among the nuclei. The BL nuclei are thought to be involved primarily in evaluating the emotional significance or context-dependent relevance of all stimuli, including social signals such as facial expressions. The CM nuclei appear to be involved in allocating attention to stimuli of high significance and in initiating situation-appropriate autonomic responses. The goal of this study was to determine how this division of labor manifests in the response properties of neurons recorded from these two nuclear groups. We recorded the activity of 454 single neurons from identified nuclear sites in three monkeys trained to perform an image-viewing task. The task required orienting and attending to cues that predicted trial progression and viewing images with broadly varying emotional content. The two populations of neurons showed large overlaps in neurophysiological properties. We found, however, that CM neurons show higher firing and less regular spiking patterns than BL neurons. Furthermore, neurons in the CM nuclei were more likely to respond to task events (fixation, image on, image off), whereas neurons in the BL nuclei were more likely to respond selectively to the content of stimulus images. The overlap in the physiological properties of the CM and BL neurons suggest distributed processing across the nuclear groups. The differences, therefore, appear to be a processing bias rather than a hallmark of mutually exclusive functions.

Individual differences in Scanpaths correspond with serotonin transporter genotype and behavioral phenotype in rhesus monkeys (Macaca mulatta).

Scanpaths (the succession of fixations and saccades during spontaneous viewing) contain information about the image but also about the viewer. To determine the viewer-dependent factors in the scanpaths of monkeys, we trained three adult males (Macaca mulatta) to look for 3 s at images of conspecific facial expressions with either direct or averted gaze. The subjects showed significant differences on four basic scanpath parameters (number of fixations, fixation duration, saccade length, and total scanpath length) when viewing the same facial expression/gaze direction combinations. Furthermore, we found differences between monkeys in feature preference and in the temporal order in which features were visited on different facial expressions. Overall, the between-subject variability was larger than the within- subject variability, suggesting that scanpaths reflect individual preferences in allocating visual attention to various features in aggressive, neutral, and appeasing facial expressions. Individual scanpath characteristics were brought into register with the genotype for the serotonin transporter regulatory gene (5-HTTLPR) and with behavioral characteristics such as expression of anticipatory anxiety and impulsiveness/hesitation in approaching food in the presence of a potentially dangerous object.