Modified toolbox for optogenetics in the nonhuman primate.

Attracted by the appealing advantages of optogenetics, many nonhuman primate labs are attempting to incorporate this technique in their experiments. Despite some reported successes by a few groups, many still find it difficult to develop a reliable way to transduce cells in the monkey brain and subsequently monitor light-induced neuronal activity. Here, we describe a methodology that we have developed and successfully deployed on a regular basis with multiple monkeys. All devices and accessories are easy to obtain and results using these have been proven to be highly replicable. We developed the "in-chair" viral injection system and used tapered and thinner fibers for optical stimulation, which significantly improved the efficacy and reduced tissue damage. With these methods, we have successfully transduced cells in multiple monkeys in both deep and shallow cortical areas. We could reliably obtain neural modulation for months after injection, and no light-induced artifacts were observed during recordings. Further experiments using these methods have shown that optogenetic stimulation can be used to bias spatial attention in a visual choice discrimination task in a way comparable to electrical microstimulation, which demonstrates the potential use of our methods in both fundamental research and clinical applications.

Optogenetically induced spatiotemporal gamma oscillations and neuronal spiking activity in primate motor cortex.

Transient gamma-band (40-80 Hz) spatiotemporal patterns are hypothesized to play important roles in cortical function. Here we report the direct observation of gamma oscillations as spatiotemporal waves induced by targeted optogenetic stimulation, recorded by intracortical multichannel extracellular techniques in macaque monkeys during their awake resting states. Microelectrode arrays integrating an optical fiber at their center were chronically implanted in primary motor (M1) and ventral premotor (PMv) cortices of two subjects. Targeted brain tissue was transduced with the red-shifted opsin C1V1(T/T). Constant (1-s square pulses) and ramp stimulation induced narrowband gamma oscillations during awake resting states. Recordings across 95 microelectrodes (4 × 4-mm array) enabled us to track the transient gamma spatiotemporal patterns manifested, e.g., as concentric expanding and spiral waves. Gamma oscillations were induced well beyond the light stimulation volume, via network interactions at distal electrode sites, depending on optical power. Despite stimulation-related modulation in spiking rates, neuronal spiking remained highly asynchronous during induced gamma oscillations. In one subject we examined stimulation effects during preparation and execution of a motor task and observed that movement execution largely attenuated optically induced gamma oscillations. Our findings demonstrate that, beyond previously reported induced gamma activity under periodic drive, a prolonged constant stimulus above a certain threshold may carry primate motor cortex network dynamics into gamma oscillations, likely via a Hopf bifurcation. More broadly, the experimental capability in combining microelectrode array recordings and optogenetic stimulation provides an important approach for probing spatiotemporal dynamics in primate cortical networks during various physiological and behavioral conditions.