Distinct neural modes carry information about attempted grasp timing and force in the sensorimotor cortex.

Neural Evidence for Action-Related Somatosensory Predictions

The tactile consequences of self-initiated movements are thought to be predicted by a forward model (FM), yet the precise neural implementation of these predictions remains unclear. In nonmotor contexts, expectations are thought to activate sensory neurons tuned toward the expected stimulus. This acts as a predictive template against which afferent sensory input is compared. It is unclear whether FM predictions have a similar neural instantiation. Here we employed time-resolved multivariate decoding on human electroencephalography during self-generated movements to examine the content of predictive neural activity. Human participants (males and females) performed index finger movements which were predictably paired with a vibration to either the index or ring finger of the opposite, passive hand. On some trials the tactile stimulus was unexpectedly omitted. Results revealed above-chance finger decoding in the premovement period supporting a predictive representation of expected stimulation location. As the movement approached, this predictive activity became similar to late-stage processing of a physical tactile stimulus. On omission trials, we found that despite the absence of afferent input, finger location could be decoded ∼120 ms after expected stimulus onset. This shows a stimulus-specific omission response. Together these findings indicate that self-generated movement preactivates neurons tuned toward expected tactile consequences.

Dissociable Roles of Primary Motor and Supplementary Motor Cortex in Shaping the Neural Drive to Muscle.

The primary motor cortex (M1) and supplementary motor area (SMA) are critical for motor execution and planning, yet their distinct causal contributions to modulating the neural drive to muscles remain incompletely understood. To dissociate their roles, we applied low-frequency (1-Hz) transcranial magnetic stimulation (TMS) over M1, SMA, or a sham condition in 75 healthy participants and characterized the activity of single motor units from high-density EMG recorded during a sustained isometric contraction. Our results revealed a clear functional divergence. Applying TMS over M1 produced a direct failure of motor output, causing a rapid force decline compared to sham, which was strongly correlated with a reduction in motor unit firing rates. Conversely, TMS over SMA did not impair net force. Instead, it altered the fundamental structure of the motor command, compelling a compensatory strategy characterized by a reliance on smaller-amplitude motor units with lower firing rates and a marked degradation of the low-frequency (delta-band) coherence that organizes stable output. These results provide direct causal evidence that M1 directly dictates the magnitude of motor output via population firing rates, while SMA orchestrates the composition and temporal structure of the active motor unit pool to generate an efficient and stable command.

Neural Changes in Processing Visuo-Tactile Looming Stimuli Following Hand-to-Foot Sensorimotor Remapping

The functional and neural mechanisms underlying the processing of different action sequences in grasping.

Goal-directed grasping actions are often planned with respect to their intended final states, as reflected in the end-state comfort effect (ESC) where individuals adopt initially uncomfortable grasp postures to ensure a comfortable final posture. The present study investigates the neurophysiological mechanisms underlying the processing of action sequences in grasping using event-related potentials (ERPs) within a priming paradigm. Participants were presented with prime images showing initial grasp postures (overhand or underhand grip) followed by target images representing final grasp postures (thumb-up or thumb-down) that together depicted either possible or impossible action sequences. Participants responded to the color of the upper part of the bar in the target image, while grasp postures were task-irrelevant. The results reveal that responses to target images representing an uncomfortable final grasp (thumb-down) following an underhand grip in the prime were slower and elicited a stronger P300 amplitude compared to responses to comfortable final grasp targets (thumb-up). This effect was observed only when the prime-target sequence represented a physically possible action. These results suggest that perceiving an initial underhand grip leads to the anticipation of a functional final action state. Deviations from this expected outcome result in greater cognitive processing effort, as reflected by increased P300 amplitudes. This suggests that action representations in grasping are anticipatory in nature and incorporate information about likely future action outcomes. The findings are consistent with ideo-motor theories, highlighting the role of anticipated action effects in action processing, and provide novel insights into the cognitive and neural underpinnings of action representation.

Localized Tuning Fields for 3D Hand Position in the Primary Motor Cortex and Premotor Cortex of Macaques.

A central question in motor neuroscience is how the brain represents the state of the limbs to guide volitional movements. While the primate motor cortex is known to encode movement kinematics, such as velocity and direction, whether it also maintains a direct and explicit representation of hand position in 3D space remains debated. To address this, we recorded the activity of single neurons in the primary motor cortex (M1) and dorsal premotor cortex (PMd) of two male rhesus macaques performing a naturalistic, self-paced 3D reach-and-grasp task. We found significant populations of neurons in both M1 (36.2%) and PMd (21.3%) that are robustly tuned to the instantaneous 3D position of the hand. In these neurons, the tuning for hand position-characterized by localized, elongated fields-coexists with tunings for other kinematic variables, reflecting the principle of mixed selectivity. Critically, the spatial organization of these representations differs between the two areas: M1 fields are systematically oriented along cardinal axes and exhibit multi-scale spatial clustering, whereas PMd fields are more randomly organized. Furthermore, a small subset of these hand position-tuned cells is sufficient to decode the hand's 3D trajectory with high fidelity. Our findings demonstrate that an explicit and functionally organized representation of 3D hand position is a fundamental component of primate motor cortex, complementing dynamic motor signals to support high-fidelity motor control.Significance Statement To guide skilled actions, the brain must track the hand's location. While the motor cortex is known for controlling movement commands, we reveal it also creates an explicit, highly organized 3D map of hand position. This neural representation is systematically structured, differing between primary and premotor areas. This discovery reshapes our understanding of motor control, showing the brain merges spatial information ("where") with motor commands ("how") of the hand in the same areas. These insights are crucial for creating more effective brain-computer interfaces for individuals with paralysis.

Event-related Potential Dynamics of Unilateral Lower Limb Movement with Functional Connectivity Analysis.

BACKGROUND Locomotor lateralization represents a general evolutionary trait in primates and is particularly well documented in human upper limb movements. Whether a corresponding lateralization pattern exists in lower limb movements, however, has remained largely unexplored. NEW METHOD To address this question, electroencephalographic (EEG) signals were recorded from 15 healthy volunteers while they performed instructed foot‑lift tasks under both motor execution (ME) and motor imagery (MI) conditions. The study characterized the frequency‑domain network representation of lower limb movement by analyzing event‑related potential (ERP) components in conjunction with functional connectivity measures. RESULTS The results revealed that unilateral motor imagery of the lower limb elicits functionally opposing patterns, manifested as distinct alterations in the spatial distribution of lateralized neural activities across hemispheres. COMPARISON WITH EXISTING METHODS By systematically comparing motor imagery and motor execution within the same paradigm, the study provides empirical evidence that the laterality hypothesis previously established for upper limb tasks extends universally to unilateral lower limb movements, thereby advancing beyond descriptive lateralization accounts. CONCLUSIONS These findings confirm that lower limb movements exhibit robust lateralization patterns, and the observed similarity between actual and imagined movements suggests a common neural substrate, providing insights into the neural network organization that may inform future brain computer interface research.