In vivo optogenetics using a Utah Optrode Array with enhanced light output and spatial selectivity.

Optogenetics allows manipulation of neural circuits in vivo with high spatial and temporal precision. However, combining this precision with control over a significant portion of the brain is technologically challenging (especially in larger animal models). Here, we have developed, optimised, and tested in vivo, the Utah Optrode Array (UOA), an electrically addressable array of optical needles and interstitial sites illuminated by 181 µLEDs and used to optogenetically stimulate the brain. The device is specifically designed for non-human primate studies. Thinning the combined µLED and needle backplane of the device from 300 µm to 230 µm improved the efficiency of light delivery to tissue by 80%, allowing lower µLED drive currents, which improved power management and thermal performance. The spatial selectivity of each site was also improved by integrating an optical interposer to reduce stray light emission. These improvements were achieved using an innovative fabrication method to create an anodically bonded glass/silicon substrate with through-silicon vias etched, forming an optical interposer. Optical modelling was used to demonstrate that the tip structure of the device had a major influence on the illumination pattern. The thermal performance was evaluated through a combination of modelling and experiment, in order to ensure that cortical tissue temperatures did not rise by more than 1°C. The device was tested in vivo in the visual cortex of macaque expressing ChR2-tdTomato in cortical neurons. It was shown that the strongest optogenetic response occurred in the region surrounding the needle tips, and that the extent of the optogenetic response matched the predicted illumination profile based on optical modelling - demonstrating the improved spatial selectivity resulting from the optical interposer approach. Furthermore, different needle illumination sites generated different patterns of low-frequency potential (LFP) activity.

An optrode array for spatiotemporally-precise large-scale optogenetic stimulation of deep cortical layers in non-human primates.

Optogenetics has transformed studies of neural circuit function, but remains challenging to apply to non-human primates (NHPs). A major challenge is delivering intense, spatiotemporally-precise, patterned photostimulation across large volumes in deep tissue. Such stimulation is critical, for example, to modulate selectively deep-layer corticocortical feedback circuits. To address this need, we have developed the Utah Optrode Array (UOA), a 10×10 glass needle waveguide array fabricated atop a novel opaque optical interposer, and bonded to an electrically addressable µLED array. In vivo experiments with the UOA demonstrated large-scale, spatiotemporally precise, activation of deep circuits in NHP cortex. Specifically, the UOA permitted both focal (confined to single layers/columns), and widespread (multiple layers/columns) optogenetic activation of deep layer neurons, as assessed with multi-channel laminar electrode arrays, simply by varying the number of activated µLEDs and/or the irradiance. Thus, the UOA represents a powerful optoelectronic device for targeted manipulation of deep-layer circuits in NHP models.

A neural correlate of perceptual segmentation in macaque middle temporal cortical area.

High-resolution vision requires fine retinal sampling followed by integration to recover object properties. Importantly, accuracy is lost if local samples from different objects are intermixed. Thus, segmentation, grouping of image regions for separate processing, is crucial for perception. Previous work has used bi-stable plaid patterns, which can be perceived as either a single or multiple moving surfaces, to study this process. Here, we report a relationship between activity in a mid-level site in the primate visual pathways and segmentation judgments. Specifically, we find that direction selective middle temporal neurons are sensitive to texturing cues used to bias the perception of bi-stable plaids and exhibit a significant trial-by-trial correlation with subjective perception of a constant stimulus. This correlation is greater in units that signal global motion in patterns with multiple local orientations. Thus, we conclude the middle temporal area contains a signal for segmenting complex scenes into constituent objects and surfaces.

An Open Resource for Non-human Primate Optogenetics.

Optogenetics has revolutionized neuroscience in small laboratory animals, but its effect on animal models more closely related to humans, such as non-human primates (NHPs), has been mixed. To make evidence-based decisions in primate optogenetics, the scientific community would benefit from a centralized database listing all attempts, successful and unsuccessful, of using optogenetics in the primate brain. We contacted members of the community to ask for their contributions to an open science initiative. As of this writing, 45 laboratories around the world contributed more than 1,000 injection experiments, including precise details regarding their methods and outcomes. Of those entries, more than half had not been published. The resource is free for everyone to consult and contribute to on the Open Science Framework website. Here we review some of the insights from this initial release of the database and discuss methodological considerations to improve the success of optogenetic experiments in NHPs.

Distinct Laminar Processing of Local and Global Context in Primate Primary Visual Cortex.

Visual perception is affected by spatial context. In visual cortex, neuronal responses to stimuli inside the receptive field (RF) are suppressed by stimuli in the RF surround. To understand the circuits and cortical layers processing spatial context, we simultaneously recorded across all layers of macaque primary visual cortex while presenting stimuli at increasing distances from the recorded cells' RF. We find that near versus far-surround stimuli activate distinct layers, thus revealing unique laminar contributions to the processing of local and global spatial context. Stimuli in the near-surround evoke the earliest subthreshold responses in superficial and upper-deep layers, and earliest suppression of spiking responses in superficial layers. Conversely, far-surround stimuli evoke the earliest subthreshold responses in feedback-recipient layer 1 and lower-deep layers, and earliest suppression of spiking responses almost simultaneously in all layers, except 4C, where suppression emerges last. Our results suggest distinct circuits for local and global signal integration.

Interaction between orbital prefrontal and rhinal cortex is required for normal estimates of expected value.

Predicting and valuing potential rewards requires integrating sensory, associative, and contextual information with subjective reward preferences. Previous work has identified regions in the prefrontal cortex and medial temporal lobe believed to be important for each of these functions. For example, activity in the orbital prefrontal cortex (PFo) encodes the specific sensory properties of and preferences for rewards, while activity in the rhinal cortex (Rh) encodes stimulus-stimulus and stimulus-reward associations. Lesions of either structure impair the ability to use visual cues or the history of previous reinforcement to value expected rewards. These areas are linked via reciprocal connections, suggesting it might be their interaction that is critical for estimating expected value. To test this hypothesis, we interrupted direct, intra-hemispheric PFo-Rh interaction in monkeys by performing crossed unilateral ablations of these regions (functional disconnection). We asked whether this circuit is crucial primarily for cue-reward association or for estimating expected value per se, by testing these monkeys, as well as intact controls, on tasks in which expected value was either visually cued or had to be inferred from block-wise changes in reward size in uncued trials. Functional disconnection significantly affected performance in both tasks. Specifically, monkeys with functional disconnection showed less of a difference in error rates and reaction times across reward sizes, in some cases behaving as if they expected rewards to be of equal magnitude. These results support a model whereby information about rewards signaled in PFo is combined with associative and contextual information signaled within Rh to estimate expected value.

Reward processing: a global brain phenomenon?

Rewards and punishments (reinforcement) powerfully shape behavior. Accordingly, their neuronal representation is of significant interest, both for understanding normal brain-behavior relationships and the pathophysiology of disorders such as depression and addiction. A recent article by Vickery and colleagues (Neuron 72: 166-177, 2011) provides evidence that the neural response to rewards and punishments is surprisingly widespread, suggesting the need for examination of the specific roles of areas not commonly included in the canonical reward circuitry in processing reinforcement.

Intersection of reward and memory in monkey rhinal cortex.

In humans and other animals, the vigor with which a reward is pursued depends on its desirability, that is, on the reward's predicted value. Predicted value is generally context-dependent, varying according to the value of rewards obtained in the recent and distant past. Signals related to reward prediction and valuation are believed to be encoded in a circuit centered around midbrain dopamine neurons and their targets in the prefrontal cortex and basal ganglia. Notably absent from this hypothesized reward pathway are dopaminergic targets in the medial temporal lobe. Here we show that a key part of the medial temporal lobe memory system previously reported to be important for sensory mnemonic and perceptual processing, the rhinal cortex (Rh), is required for using memories of previous reward values to predict the value of forthcoming rewards. We tested monkeys with bilateral Rh lesions on a task in which reward size varied across blocks of uncued trials. In this experiment, the only cues for predicting current reward value are the sizes of rewards delivered in previous blocks. Unexpectedly, monkeys with Rh ablations, but not intact controls, were insensitive to differences in predicted reward, responding as if they expected all rewards to be of equal magnitude. Thus, it appears that Rh is critical for using memory of previous rewards to predict the value of forthcoming rewards. These results are in agreement with accumulating evidence that Rh is critical for establishing the relationships between temporally interleaved events, which is a key element of episodic memory.

Stimulus onset quenches neural variability: a widespread cortical phenomenon.

Neural responses are typically characterized by computing the mean firing rate, but response variability can exist across trials. Many studies have examined the effect of a stimulus on the mean response, but few have examined the effect on response variability. We measured neural variability in 13 extracellularly recorded datasets and one intracellularly recorded dataset from seven areas spanning the four cortical lobes in monkeys and cats. In every case, stimulus onset caused a decline in neural variability. This occurred even when the stimulus produced little change in mean firing rate. The variability decline was observed in membrane potential recordings, in the spiking of individual neurons and in correlated spiking variability measured with implanted 96-electrode arrays. The variability decline was observed for all stimuli tested, regardless of whether the animal was awake, behaving or anaesthetized. This widespread variability decline suggests a rather general property of cortex, that its state is stabilized by an input.