Biology subjects

Snyder, L. H.

Publications and source records attributed to Snyder, L. H..

7 recordsLinked to original sources

bioRxiv2025

A causal role for the posterior corpus callosum in bimanual coordination

Inter-areal communication is crucial for brain function. Given the largely contralateral organization of the brain, bimanual coordination likely involves interactions across the two cerebral hemispheres for motor planning and execution. The parietal reach region (PRR) is an early node in the sensorimotor transformation stream. Here we examine the contributions of direct callosal connections between left and right PRR to bimanual coordination. Using manganese-enhanced magnetic resonance imaging, we traced callosal pathways crossing the midline and found that PRR-PRR connections are restricted to the splenium. We then temporarily blocked these fibers with lidocaine while measuring behavioral performance and interhemispheric coherence. Blockade reduced task-specific PRR-PRR coherence during bimanual movements. Behaviorally, blockade sped movement initiation across tasks, consistent with an inhibitory role of interhemispheric communication, reduced the temporal synchrony of bimanual movements to a common target and reduced errors for bimanual movements to separate targets. These findings provide causal evidence that posterior callosal communication supports spatial coordination of bimanual actions but may also constrain independent limb control. Significance StatementClassic split-brain studies revealed that severing the corpus callosum impairs bimanual coordination, but the specific pathways and mechanisms remain unclear. In macaques, we transiently disrupted the posterior corpus callosum connecting left and right parietal reach regions (PRR), which encode planned contralateral arm movements. This targeted blockade reduced task-specific neural synchrony between PRRs and selectively impaired coordination when both arms reached to a common target, while improving performance when the arms moved to separate targets. Movement initiation was also sped up across tasks, consistent with an inhibitory role of interhemispheric communication. These findings provide causal evidence that posterior callosal communication enables spatially coordinated bimanual movements, extending foundational split-brain insights to defined cortical circuits in a non-human primate model.

neuroscience↗

bioRxiv2025

Compositional architecture: Orthogonal neural codes for task context and spatial memory in prefrontal cortex

The prefrontal cortex (PFC) is crucial for maintaining working memory across diverse cognitive tasks, yet how it adapts to varying task demands remains unclear. Compositional theories propose that cognitive processes in neural network rely on shared components that can be reused to support different behaviors. However, previous studies have suggested that working memory components are task specific, challenging this framework. Here, we revisit this question using a population-based approach. We recorded neural activity in macaque monkeys performing two spatial working memory tasks with opposing goals: one requiring movement toward previously presented spatial locations (look task) and the other requiring avoidance of those locations (no-look task). Despite differences in task demands, we found that spatial memory representations were largely conserved at the population level, with a common low-dimensional neural subspace encoding memory across both tasks. In parallel, task identity was encoded in an orthogonal subspace, providing a stable and independent representation of contextual information. These results provide neural evidence for a compositional model of working memory, where representational geometry enables the efficient and flexible reuse of mnemonic codes across behavioral contexts while maintaining an independent representation of context.

neuroscience↗

bioRxiv2023

The most prominent electrophysiological effect of ketamine is not sufficient to cause anesthesia

Electrophysiological recordings during ketamine anesthesia have revealed a slow alternating pattern of high- and low- frequency activity (a "gamma-burst" pattern) that develops with the onset of general anesthesia. We examine the role of NMDA receptor antagonism in generating the gamma-burst pattern and the link between gamma-bursts and dissociative anesthesia. We compare the effects of ketamine with those of the highly selective NMDA receptor antagonist CGS 19755 on multi-site intracranial electrophysiology and behavior in rhesus macaques. Remarkably, we find that animals given a moderate dose of CGS 19755 are able to perform a difficult memory task, while at the same time showing electrophysiological activity similar to ketamine anesthesia, with one key difference: a lack of delta-band LFP modulation. This difference demonstrates that ketamines ability to drive strong delta-band oscillations relies on additional mechanisms beyond NMDA receptor antagonism alone, and points to a key role for the activity underlying delta-band oscillations in causing anesthesia.

neuroscience↗

bioRxiv2023

Inferring functional organization of posterior parietal cortex circuitry based on information flow

Many studies infer the role of neurons by asking what information can be decoded from their activity or by observing the consequences of perturbing their activity. An alternative approach is to consider information flow between neurons. We applied this approach to the Parietal Reach Region (PRR) and the Lateral Intraparietal area (LIP) in posterior parietal cortex. Two complementary methods show that, across a range of reaching tasks, information flows primarily from PRR to LIP but not vice versa. This suggests that PRR determines the spatial goals of coordinated eye and arm movements and instructs LIP of those goals. Based on these findings, we conclude that PRR and LIP operate in a parallel rather than hierarchical manner to plan arm and eye movements, respectively. Similar methodology can be applied to other areas to infer their relative relationships.

neuroscience↗

bioRxiv2018

Comparison of stimulus-evoked BOLD responses in human and monkey visual cortex

We characterized the blood oxygenation level dependent (BOLD) signal in humans and macaque monkeys by comparing the response in visual cortex to a single checkerboard or two checkerboards, spaced 1.5, 3.0, or 4.5 s apart. We found that the magnitude and shape of the BOLD response to a single checkerboard was similar in the two species. In addition, we found that the BOLD responses summed similarly, and that at an inter-stimulus interval (ISI) of 4.5 sec BOLD summation was nearly linear in both species. When comparing the ratio of the amplitude of the response to the second checkerboard at the 4.5 sec ISI with that of the single checkerboard between subjects in both species, the results from both monkey subjects fell within one standard deviation of the mean human results (human mean (n=12): .95 +/- .31 second/single response amplitude; monkey 1: 1.16; monkey 2: .86). At the shorter ISIs, both species demonstrated increased suppression of the BOLD response to the second checkerboard. These findings indicate that the magnitude of the BOLD response to events separated by 4.5 seconds can be accurately measured in and compared between human and monkey visual cortex.

neuroscience↗

bioRxiv2018

Topographic Organization of Extraoccipital Visual Processing Areas in the Macaque

The macaque visual system has long been used as a model for investigating the processing of incoming visual stimuli results from the coordinated work of a distributed network of areas interacting at many different levels (Felleman and Van Essen, 1991). While much is known about the organization and layout of the occipital visual areas, there are still substantial gaps in our understanding of layout and organization of the higher-level areas. The goal of this study is to describe the whole brain functional anatomy using BOLD-fMRI in macaques performing a series of demanding visuospatial attention tasks. We wish to study the spatial specificity of visual responses in terms of contralateral preference, i.e. stronger responses to contralateral visual stimuli, as well as retinotopic organization both in terms of polar angle and eccentricity. We found that most visuospatial processing areas only respond to contralaterally presented stimuli; ipsilaterally presented stimuli evoked little or no activity in these areas. Additionally, we found that LIP, MT, and possibly PITd contained polar-angle maps of the contralateral hemifield. These same areas, plus FEF and area 46, appear to have separate representations of the fovea and periphery. When compared to previous human fMRI studies, these results indicate that there may be significant differences between macaque visual processing areas and their putative human homologues.

neuroscience↗

bioRxiv2018

Attentional Modulation of Macaque Visual Processing Areas

Introduction Introduction Materials and Methods Results Discussion References The visual expectation that an object will appear at a certain location or time may increase the accuracy and speed of its detection (Eriksen and Hoffman, 1974; Posner, 1980; Carrasco et al., 2000; Dosher and Z-L., 2000). The ability to use prior information to enhance the processing of visual stimuli at a specific location is critical for quickly sorting through the myriad of incoming stimuli and making decisions regarding future actions. This selection process, known as visual attention, appears to be fundamental to our ability to navigate and interact with the environment, and accordingly has been the focus of innumerable psychological and neurobiological studies (Pa ...

neuroscience↗