Biology subjects

Luabeya, G. N.

Publications and source records attributed to Luabeya, G. N..

6 recordsLinked to original sources

bioRxiv2026

Reach plans for grasping vs. placing objects evoke different neural dynamics within a shared cortico-cerebellar mechanism.

Many reach studies treat grasping as the endpoint of a reach, but grasping often initiates a sequence of actions, ending with placing the object. Grasping and placing both require object-specific encoding, but placing has different computational demands, introduced by the intent, sensory feedback, and tolerance for spatial error when alignment is required. Recent neuroimaging work suggests that object placement shares similar cortico-cerebellar networks with grasp, with modest increases in network segregation and cerebellar activity. Here, we ask whether Grasp and Place are better distinguished by their spatiotemporal dynamics in both behaviour and scalp-recorded EEG activity. Using high-density EEG and motion tracking, 31 participants performed alternating Grasp/Place movements of a 3D object from/toward an oriented target/template. Place movements were initiated faster but executed more slowly and with greater endpoint-orientation precision than Grasp, indicating distinct control strategies. ERP and microstate analyses revealed that Grasp and Place shared a peri-movement polarity switch, from posterior negativity/frontal positivity during preparation to posterior positivity/frontal negativity during execution. This shared pattern was amplified during Place preparation and prolonged during Place execution, with stronger expression of preparatory medial frontal/cerebellar and execution-related posterior/cerebellar microstate dynamics. Finally, SVM decoding indicated that task-specific information was primarily encoded in low-frequency EEG activity (below 6 Hz). These results confirm that grasping and placing recruit similar cortico-cerebellar mechanisms, yet different spatiotemporal dynamics are employed to support the distinct computational demands of these behaviours.

neuroscience↗

bioRxiv2026

Transcranial Magnetic Stimulation Differentiates Egocentric vs. Allocentric Network Hubs for Memory-Guided Reach

Previous patient and imaging studies suggest that dorsal and ventral visual stream mechanisms contribute selectively to egocentric (eye-centred) and allocentric (landmark-centred) goal-directed behaviours, respectively. However, causal evidence for this ego-allocentric dissociation in healthy brains is lacking. Based on a recent neuroimaging study, we hypothesized that perturbations to dorsal stream network hubs would disrupt both egocentric and allocentric reaches, whereas perturbations to ventral hubs would affect only allocentric reaches. Sixteen participants viewed a briefly presented target in the presence of a landmark that later shifted after a memory interval. Participants were instructed to 1) ignore the landmark and reach toward / opposite the original target location (egocentric task) or 2) reach toward the targets location fixed relative to the shifted landmark (allocentric task). Transcranial magnetic stimulation (TMS) was applied during the memory period, spanning the landmark shift, at two occipital sites, based on the network hub coordinates from our previous study and individual brain anatomy. As predicted, superior occipital TMS impaired reaction time, accuracy, and precision in both tasks, whereas inferior occipital TMS only impaired allocentric performance. Further, spatial asymmetries in the resulting error patterns were influenced by TMS site and task: superior occipital TMS influenced asymmetries more strongly in the egocentric task, whereas inferior occipital TMS only influenced spatial asymmetries in the allocentric task. Finally, a functional network analysis revealed a widespread influence of TMS on network-behavior relations and another dissociation: greater influence of superior occipital TMS on the egocentric task and of inferior occipital TMS on the allocentric task. These results establish specific causal relationships between occipital hubs, widespread cortical networks, and reach behavior, confirming a default dorsal egocentric transformation for visually aimed reaches, and a role for the ventral stream for landmark-centred actions.

neuroscience↗

bioRxiv2026

Beyond Reach I: Neuroimaging Reveals Different Cortico-cerebellar Networks for Grasp vs. Object Placement.

Object manipulation usually involves both acquisition and placement, but neuroimaging studies tend to focus on its initial reach-to-grasp component. To understand how grasp and placement are represented in the brain, we designed an event-related fMRI study in which 20 participants alternated between grasping / placing a rectangular object from / toward templates presented at variable locations and orientations on an inclined plane. Despite the similarity in target information (i.e., location and orientation), we expected to see sensory and intentional task differences, as the grasp relied on matching the hand to the object, whereas placement relied on matching the object to the template. For our analysis, we examined BOLD activation across two epochs (planning phase before movement onset and execution phase during the movement), and applied graph-theoretical analysis (GTA) of whole-brain functional connectivity based on the joint time series from these two epochs. In both tasks, we found that common activation was more focused on prefrontal regions during planning and on sensorimotor regions during execution, with a specific increase in cerebellar activation during placement. Furthermore, the greater placement activation was also found in action-related regions of interest during the planning phase, and they could accurately decode between our two tasks. GTA categorized the two networks into three identical modules (prefrontal, cerebellar-occipito-parietal, and sensorimotor) and found task-based differences in modularity scores in the prefrontal and cerebellar-occipito-parietal modules. Overall, these data show that although reach-to-grasp and reach-to-place movements share extensive neural circuitry, they also exhibit task-specific differences, likely related to differences in intentionality during planning and in sensory feedback during movement execution.

neuroscience↗

bioRxiv2026

Modular Integration of Auditory Instructions and Visual Cues into the Cortical Reach Network

Humans often need to integrate bottom-up stimulus attributes (e.g., object shape) with top-down instructions (e.g., how to pick it up) for adaptive behavior. To investigate how such cues are integrated into the cortical reach network, we employed an fMRI paradigm where participants viewed a cube to their left or right (visual cue) and were verbally instructed to use a horizontal or vertical grip (auditory cue), in varying order. Univariate voxel-wise and region-of-interest analysis confirmed the expected order-dependent activation of sensory regions, followed by robust parietofrontal activation. Graph-theory analysis revealed two significant subnetworks (modules): an Occipital-Parietal module (likely integrating the visual cue into the reach plan) and a Temporal-Frontal module (likely integrating the auditory cue into the grasp plan), both converging in somatomotor cortex. Cue-order influenced modularity, i.e., the module corresponding to the first cue tended to dominate contralateral motor cortex, and conversely a classification model decoded cue-order based on modularity. Local network hubs were order-independent and occupied traditional reach areas, whereas global (between module) hubs were order-dependent and more distributed. Overall, these results suggest that multisensory integration of top-down and bottom-up cues for action is a network phenomenon that relies on both lateral and serial communication between parallel sensorimotor modules.

neuroscience↗

bioRxiv2023

Influence of Gaze, Vision, and Memory on Hand Kinematics in a Placement Task

People usually reach for objects to place them in some position and orientation, but the placement component of this sequence is often ignored. For example, reaches are influenced by gaze position, visual feedback, and memory delays, but their influence on object placement is unclear. Here, we tested these factors in a task where participants placed and oriented a trapezoidal block against 2D visual templates displayed on a frontally located computer screen. In Experiment 1, participants matched the block to three possible orientations: 0{degrees} (horizontal), +45{degrees} and -45{degrees}, with gaze fixated 10{degrees} to the left/right. The hand and template either remained illuminated (closed-loop), or visual feedback was removed (open-loop). In Experiment 2, a memory delay was added, and participants sometimes performed saccades (centripetal, centrifugal, or opposite side). In Experiment 1, the hand consistently overshot the template relative to gaze (similar to reaching), especially in the open-loop task. After a memory delay, location errors were influenced by both template orientation and gaze position. Based on previous reach experiments, we expected these errors to be independent of the previous eye position, but placement overshoot also depended on previous saccade metrics. Hand orientation over-rotated relative to template orientation (all conditions). Orientation was influenced by gaze position in Experiment 1, but this vanished in the presence of a memory delay. These results demonstrate interactions between gaze, location, and orientation signals in the planning and execution of hand placement and suggest different neural mechanisms for closed-loop, open-loop, and memory delay placement. NEW & NOTEWORTHYEye-hand coordination studies usually focus on object acquisition, but placement is equally important. Here, we investigated how gaze position influences object placement toward a 2D template, with different levels of visual feedback. Like reach, placement overestimated goal location relative to gaze, but was also influenced by previous saccade metrics. Gaze also modulated hand orientation, which generally overestimated template orientation. Gaze influence was feedback-dependent, with location errors increasing but orientation errors decreasing after a memory delay.

neuroscience↗

bioRxiv2019

Parietal Cortex Integrates Visual and Oculomotor Signals to Update Grasp Plans

Coordinated reach-to-grasp movements are often accompanied by rapid eye movements (saccades) that displace the desired object image relative to the retina. Parietal cortex compensates for this by updating reach goals relative to current gaze direction, but its role in the integration of oculomotor and visual orientation signals for updating grasp plans is unknown. Based on a recent perceptual experiment, we hypothesized that inferior parietal cortex (specifically supramarginal gyrus; SMG) integrates saccade and visual signals to update grasp plans in more superior parietal areas. To test this hypothesis, we employed a functional magnetic resonance adaptation paradigm, where saccades sometimes interrupted grasp preparation toward a briefly presented object that later reappeared (with the same/different orientation) just before movement. Right SMG and several parietal grasp areas, namely left anterior intraparietal sulcus (aIPS) and bilateral superior parietal lobe (SPL), met our criteria for transsaccadic orientation integration: during movement preparation, they showed task-dependent saccade modulations and, during grasp execution, they were specifically sensitive to changes in object orientation that followed saccades. Finally, SMG showed enhanced functional connectivity with both prefrontal saccade areas (consistent with oculomotor input) and aIPS / SPL (consistent with sensorimotor output). These results support the general role of parietal cortex for the integration of visuospatial perturbations, and provide specific cortical modules for the integration of oculomotor and visual signals for grasp updating. Significance StatementThe cortical mechanisms that update reach goals during eye movements are well documented, but it is not known how object features are linked to oculomotor signals when updating grasp plans. Here, we employed functional magnetic resonance imaging adaptation (fMRIa) and functional connectivity analysis to identify a cluster of inferior parietal (supramarginal gyrus) and superior parietal (intraparietal and superior parietal) regions that show functional connectivity with frontal cortex saccade centers, and also show saccade-specific modulations during unexpected changes in object / grasp orientation. This provides a network - complementary to the goal updater network - that integrates visuospatial updating into grasp plans, and may help explain some of the more complex symptoms associated with parietal damage such as constructional ataxia.

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