Development of an optogenetic toolkit for neural circuit dissection in squirrel monkeys.

Optogenetic tools have opened a rich experimental landscape for understanding neural function and disease. Here, we present the first validation of eight optogenetic constructs driven by recombinant adeno-associated virus (AAV) vectors and a WGA-Cre based dual injection strategy for projection targeting in a widely-used New World primate model, the common squirrel monkey Saimiri sciureus. We observed opsin expression around the local injection site and in axonal projections to downstream regions, as well as transduction to thalamic neurons, resembling expression patterns observed in macaques. Optical stimulation drove strong, reliable excitatory responses in local neural populations for two depolarizing opsins in anesthetized monkeys. Finally, we observed continued, healthy opsin expression for at least one year. These data suggest that optogenetic tools can be readily applied in squirrel monkeys, an important first step in enabling precise, targeted manipulation of neural circuits in these highly trainable, cognitively sophisticated animals. In conjunction with similar approaches in macaques and marmosets, optogenetic manipulation of neural circuits in squirrel monkeys will provide functional, comparative insights into neural circuits which subserve dextrous motor control as well as other adaptive behaviors across the primate lineage. Additionally, development of these tools in squirrel monkeys, a well-established model system for several human neurological diseases, can aid in identifying novel treatment strategies.

The need for calcium imaging in nonhuman primates: New motor neuroscience and brain-machine interfaces.

A central goal of neuroscience is to understand how populations of neurons coordinate and cooperate in order to give rise to perception, cognition, and action. Nonhuman primates (NHPs) are an attractive model with which to understand these mechanisms in humans, primarily due to the strong homology of their brains and the cognitively sophisticated behaviors they can be trained to perform. Using electrode recordings, the activity of one to a few hundred individual neurons may be measured electrically, which has enabled many scientific findings and the development of brain-machine interfaces. Despite these successes, electrophysiology samples sparsely from neural populations and provides little information about the genetic identity and spatial micro-organization of recorded neurons. These limitations have spurred the development of all-optical methods for neural circuit interrogation. Fluorescent calcium signals serve as a reporter of neuronal responses, and when combined with post-mortem optical clearing techniques such as CLARITY, provide dense recordings of neuronal populations, spatially organized and annotated with genetic and anatomical information. Here, we advocate that this methodology, which has been of tremendous utility in smaller animal models, can and should be developed for use with NHPs. We review here several of the key opportunities and challenges for calcium-based optical imaging in NHPs. We focus on motor neuroscience and brain-machine interface design as representative domains of opportunity within the larger field of NHP neuroscience.

Dynamic, rhythmic facial expressions and the superior temporal sulcus of macaque monkeys: implications for the evolution of audiovisual speech.

Audiovisual speech has a stereotypical rhythm that is between 2 and 7 Hz, and deviations from this frequency range in either modality reduce intelligibility. Understanding how audiovisual speech evolved requires investigating the origins of this rhythmic structure. One hypothesis is that the rhythm of speech evolved through the modification of some pre-existing cyclical jaw movements in a primate ancestor. We tested this hypothesis by investigating the temporal structure of lipsmacks and teeth-grinds of macaque monkeys and the neural responses to these facial gestures in the superior temporal sulcus (STS), a region implicated in the processing of audiovisual communication signals in both humans and monkeys. We found that both lipsmacks and teeth-grinds have consistent but distinct peak frequencies and that both fall well within the 2-7 Hz range of mouth movements associated with audiovisual speech. Single neurons and local field potentials of the STS of monkeys readily responded to such facial rhythms, but also responded just as robustly to yawns, a nonrhythmic but dynamic facial expression. All expressions elicited enhanced power in the delta (0-3Hz), theta (3-8Hz), alpha (8-14Hz) and gamma (> 60 Hz) frequency ranges, and suppressed power in the beta (20-40Hz) range. Thus, STS is sensitive to, but not selective for, rhythmic facial gestures. Taken together, these data provide support for the idea that that audiovisual speech evolved (at least in part) from the rhythmic facial gestures of an ancestral primate and that the STS was sensitive to and thus 'prepared' for the advent of rhythmic audiovisual communication.

Different neural frequency bands integrate faces and voices differently in the superior temporal sulcus.

The integration of auditory and visual information is required for the default mode of speech-face-to-face communication. As revealed by functional magnetic resonance imaging and electrophysiological studies, the regions in and around the superior temporal sulcus (STS) are implicated in this process. To provide greater insights into the network-level dynamics of the STS during audiovisual integration, we used a macaque model system to analyze the different frequency bands of local field potential (LFP) responses to the auditory and visual components of vocalizations. These vocalizations (like human speech) have a natural time delay between the onset of visible mouth movements and the onset of the voice (the "time-to-voice" or TTV). We show that the LFP responses to faces and voices elicit distinct bands of activity in the theta (4-8 Hz), alpha (8-14 Hz), and gamma (>40 Hz) frequency ranges. Along with single neuron responses, the gamma band activity was greater for face stimuli than voice stimuli. Surprisingly, the opposite was true for the low-frequency bands-auditory responses were of a greater magnitude. Furthermore, gamma band responses in STS were sustained for dynamic faces but not so for voices (the opposite is true for auditory cortex). These data suggest that visual and auditory stimuli are processed in fundamentally different ways in the STS. Finally, we show that the three bands integrate faces and voices differently: theta band activity showed weak multisensory behavior regardless of TTV, the alpha band activity was enhanced for calls with short TTVs but showed little integration for longer TTVs, and finally, the gamma band activity was consistently enhanced for all TTVs. These data demonstrate that LFP activity from the STS can be segregated into distinct frequency bands which integrate audiovisual communication signals in an independent manner. These different bands may reflect different spatial scales of network processing during face-to-face communication.