Comparison of orientation encoding across layers within single columns of primate V1 revealed by high-density recordings.

Primary visual cortex (V1) has been the focus of extensive neurophysiological investigations, with its laminar organization serving as a crucial model for understanding the functional logic of neocortical microcircuits. Utilizing newly developed high-density, Neuropixels probes, we measured visual responses from large populations of simultaneously recorded neurons distributed across layers of macaque V1. Within single recordings, myriad differences in the functional properties of neuronal subpopulations could be observed. Notably, while standard measurements of orientation selectivity showed only minor differences between laminar compartments, decoding stimulus orientation from layer 4C responses outperformed both superficial and deep layers within the same cortical column. The superior orientation discrimination within layer 4C was associated with greater response reliability of individual neurons rather than lower correlated activity within neuronal populations. Our results underscore the efficacy of high-density electrophysiology in revealing the functional organization and network properties of neocortical microcircuits within single experiments.

Common and distinct neural mechanisms of attention.

Despite a constant deluge of sensory stimulation, only a fraction of it is used to guide behavior. This selective processing is generally referred to as attention, and much research has focused on the neural mechanisms controlling it. Recently, research has broadened to include more ways by which different species selectively process sensory information, whether due to the sensory input itself or to different behavioral and brain states. This work has produced a complex and disjointed body of evidence across different species and forms of attention. However, it has also provided opportunities to better understand the breadth of attentional mechanisms. Here, we summarize the evidence that suggests that different forms of selective processing are supported by mechanisms both common and distinct.

Large-scale high-density brain-wide neural recording in nonhuman primates.

High-density, integrated silicon electrodes have begun to transform systems neuroscience, by enabling large-scale neural population recordings with single cell resolution. Existing technologies, however, have provided limited functionality in nonhuman primate species such as macaques, which offer close models of human cognition and behavior. Here, we report the design, fabrication, and performance of Neuropixels 1.0-NHP, a high channel count linear electrode array designed to enable large-scale simultaneous recording in superficial and deep structures within the macaque or other large animal brain. These devices were fabricated in two versions: 4416 electrodes along a 45 mm shank, and 2496 along a 25 mm shank. For both versions, users can programmatically select 384 channels, enabling simultaneous multi-area recording with a single probe. We demonstrate recording from over 3000 single neurons within a session, and simultaneous recordings from over 1000 neurons using multiple probes. This technology represents a significant increase in recording access and scalability relative to existing technologies, and enables new classes of experiments involving fine-grained electrophysiological characterization of brain areas, functional connectivity between cells, and simultaneous brain-wide recording at scale.

Covert attention regulates saccadic reaction time by routing between different visual-oculomotor pathways.

Covert attention modulates saccadic performance, e.g., the abrupt onset of a task-irrelevant visual stimulus grabs attention as measured by a decrease in saccadic reaction time (SRT). The attentional advantage bestowed by the task-irrelevant stimulus is short-lived: SRT is actually longer ~200 ms after the onset of a stimulus than it is when no stimulus appears, known as inhibition of return. The mechanism by which attention modulates saccadic reaction is not well-understood. Here, we propose two possible mechanisms: by selective routing of the visuomotor signal through different pathways (routing hypothesis) or by general modulation of the speed of visuomotor transformation (shifting hypothesis). To test them, we designed a cue gap paradigm in which a 100-ms gap was introduced between the fixation point disappearance and the target appearance to the conventional cued visual reaction time paradigm. The cue manipulated the location of covert attention, and the gap interval resulted in a bimodal distribution of SRT, with an early mode (express saccade) and a late mode (regular saccade). The routing hypothesis predicts changes in the proportion of express saccades vs. regular saccades, whereas the shifting hypothesis predicts a shift of SRT distribution. The addition of the cue had no effect on mean reaction time of express and regular saccades, but it changed the relative proportion of two modes. These results demonstrate that the covert attention modification of the mean SRT is largely attributed to selective routing between visuomotor pathways rather than general modulation of the speed of visuomotor transformation.

Functional interactions among neurons within single columns of macaque V1.

Recent developments in high-density neurophysiological tools now make it possible to record from hundreds of single neurons within local, highly interconnected neural networks. Among the many advantages of such recordings is that they dramatically increase the quantity of identifiable, functional interactions between neurons thereby providing an unprecedented view of local circuits. Using high-density, Neuropixels recordings from single neocortical columns of primary visual cortex in nonhuman primates, we identified 1000s of functionally interacting neuronal pairs using established crosscorrelation approaches. Our results reveal clear and systematic variations in the synchrony and strength of functional interactions within single cortical columns. Despite neurons residing within the same column, both measures of interactions depended heavily on the vertical distance separating neuronal pairs, as well as on the similarity of stimulus tuning. In addition, we leveraged the statistical power afforded by the large numbers of functionally interacting pairs to categorize interactions between neurons based on their crosscorrelation functions. These analyses identified distinct, putative classes of functional interactions within the full population. These classes of functional interactions were corroborated by their unique distributions across defined laminar compartments and were consistent with known properties of V1 cortical circuitry, such as the lead-lag relationship between simple and complex cells. Our results provide a clear proof-of-principle for the use of high-density neurophysiological recordings to assess circuit-level interactions within local neuronal networks.

A multilayered story of memory retrieval.

The successful retrieval of learned visual associations requires coordination of multiple brain regions involved in the encoding and association of visual images. In this issue of Neuron, Takeda et al. (2015) use a combination of modern recording and analytical methods to eavesdrop on this process.

Long term retention of saccadic adaptation is induced by a dark environmental context.

Under many circumstances, motor memory needs to be retained for a long period of time to enable accurate behavior. Since the first introduction of the saccadic adaptation paradigm in 1960s, saccadic adaptation protocols have been widely used to study the mechanisms of motor learning and motor memory. However, previous studies reported that the effect of saccadic adaptation on the oculomotor system was rather short (minutes to hours) in human and non-human primates. Here we ask whether the fast decay of the effects of saccadic adaptation is due to the influence of environmental context. To test this hypothesis, we asked human subjects to perform a saccadic adaptation task in a very dark environment. Our data showed that saccade gain remained at the post-adaptation level 24-72h after exposure to the saccadic adaptation task without significant recovery, and that the effect of saccadic adaptation on saccade gain could still be found 2 months later, much longer than previously reported. Our results indicate a vital role for environmental context in the retention of saccadic adaptation.