Journal of neurophysiology最新文献

Gustatory cortical (GC) and basolateral amygdalar (BLA) taste responses consist of an inter-regionally coherent 3-part state sequence. This coherence suggests that reciprocal BLA-GC connectivity is important for taste processing, but it remains unknown: 1) whether BLA-GC coherence actually reflects a reciprocal "conversation" (as opposed to one region simply driving the other); and 2) whether such a "conversation" has anything to do with the taste processing observed within GC response dynamics. Here, we address these questions using network and single-neuron analysis of simultaneously-recorded GC and BLA taste responses in awake rats. We find asymmetric, reciprocal µ-frequency influences that reflect taste processing dynamics: BLA→GC influence dominates between 300 and 1000msec (the epoch in which BLA codes palatability); afterward, when GC responses become palatability-related and GC has been shown to release a behavior-relevant signal, the direction of influence reverses, becoming GC→BLA. Follow-up analyses demonstrate that this "turn-taking" exists alongside effectively synchronous amygdala-cortical coupling-the two regions functioning as a unified structure. Finally, to assess the implications of these interactions for single-neuron responses, we tested the response properties of GC neurons categorized by their inferred connectivity with BLA: GC neurons influenced by BLA produce stronger taste-specific and palatability-related responses than other GC neurons, and the strongest taste encoding is specifically found in GC neurons that both influence and receive influence from BLA-those most deeply embedded in the reciprocal circuit. These results, consistent with findings in multiple systems, support the novel conclusion that taste processing and decision-making is a function of the amygdala-cortical loop.

A clear understanding of how visual information affects postural sway is crucial for assessing normal balance control and developing diagnostic and rehabilitation methods for balance disorders. However, a quantitative model of sway responses to visual perturbations with improved accuracy is still needed. We used virtual reality to apply rotational visual perturbations (0.04-1 Hz, 2.5°-15°) to fourteen healthy adults. Participants were splinted at the knee and hip to ensure the ankle strategy was used. Postural responses, including body angles and ankle torques, were recorded. Initial analysis demonstrated that right-eye dominant subjects showed more coherent body sway responses, possibly related to the higher magnitude of the optical flow in the right half-plane of the visual field. Detailed analysis was therefore focused on eight subjects with large, coherent responses. A detrending method was applied to angles and torques based on inverse Fourier transform to remove frequencies below the smallest stimuli frequency. Our methodology yielded a model with improved accuracy between the visual input and body angle output, i.e., coherence values close to 1. Frequency response analysis revealed a low-pass gain characteristic and a linear phase decrease showing a consistent delay in the system across all amplitudes. A parametric model fitted to the frequency response yielded a delayed, second-order, low-pass transfer function. The transfer function gain decreased with increasing stimulus amplitude, demonstrating a nonlinear response reflecting reduced responsiveness to larger visual amplitudes. In conclusion, this paper provides an experimental and analytical framework to accurately quantify the nonlinear dynamics of postural responses to visual stimuli.

Based on electromyography (EMG) recordings, EMG-EMG coherence provides a practical approach to investigate neural mechanisms involved in locomotion. Although some studies indicated the influence of age and walking conditions on EMG-EMG coherence, no clear consensus emerged from the existing literature. The aim of this scoping review was to map the literature on EMG-EMG coherence in healthy individuals across ages and walking tasks. Six databases (CINAHL, Cochrane Central, CDSR, MEDLINE, Embase, and Web of Science) were searched, resulting in 31 studies included (551 individuals). These studies analyzed EMG EMG coherence of muscles involved during different locomotor tasks. The results revealed a consensus regarding the decrease in EMG-EMG coherence during walking with aging, particularly in the Beta and Gamma bands, which could be attributed to natural alterations in the corticospinal tract with age. Furthermore, Beta and Gamma EMG-EMG coherence showed an increased tendency during proprioceptive (perturbation-based tasks) and proactive (obstacle negotiation) locomotor tasks, which is interpreted as an enhancement of cortical involvement in gait control. This review also highlights the necessity for future research to examine EMG-EMG coherence in frequency bands such as Alpha using standardized signal processing techniques and frequency classifications, and to investigate coherence in children across various locomotor tasks.

Aging commonly leads to balance problems, yet the neural processes driving this decline remain unclear. Recent structural, resting-state, and EEG combined with virtual reality (VR) studies suggest that age-related instability stems from reduced flexibility in combining visual, vestibular, and somatosensory cues rather than from losses in any single system. These findings indicate that diminished neural adaptability is a key contributor to balance impairment and point toward specific network-level mechanisms that future interventions may target.

The development of upper limb movements has been primarily described through reaching movements, which may not have the complex motor planning and execution demands of many daily tasks. In this study, we introduced a complex task in which individuals had to navigate their hand from a start target through two openings in a simple maze to reach an end target. In half the trials, participants received visual feedback of their hand position, and in half of the trials they did not. Thirty-one participants ages 8 to 17 years completed the study. We found that with visual feedback, reaction time, number of speed peaks, movement time, and hand path length all decreased with age. Number of speed peaks, movement time, and hand path length were all increased without visual feedback. Our results demonstrate that complex upper limb movements are refined across childhood and adolescence, potentially reflecting more anticipatory planning and feedforward control. This task can be applied to clinical populations such as cerebral palsy to assess impairments in motor planning and execution as well as determine how proprioceptive impairments contribute to complex movements.

Implicit sensorimotor adaptation-the automatic correction of movement errors-is driven by a perceptual prediction error, the mismatch between the perceived movement outcome and its intended goal. Although perceptual uncertainty is known to attenuate adaptation, the impact of goal uncertainty on adaptation remains unknown. We used a visuomotor adaptation task that isolates implicit adaptation (n = 180), manipulating goal uncertainty by varying how precisely the goal's midpoint could be identified. Display format was varied independently to control for the objective size and luminance of visual features, and targets were hidden at movement onset, ensuring that identical visual input at the moment the error was experienced. We found that goal uncertainty significantly attenuated implicit adaptation, independent of low-level visual and kinematic features. Together, these results demonstrate that a precise internal representation of the goal is critical for sensorimotor adaptation, motivating a fundamental revision of motor learning models to explicitly incorporate goal uncertainty as a key determinant of performance.NEW & NOTEWORTHY Implicit adaptation depends not only on feedback uncertainty but also on goal uncertainty. In a large-scale online visuomotor rotation study (n = 180), increasing spatial ambiguity in the target midpoint-while keeping visual input constant-systematically attenuated implicit adaptation. These results call for a revision of standard models of motor adaptation, positioning the precision of the motor goal as a fundamental component in modulating motor learning.

Beta band (13-30 Hz) oscillations have been closely associated with motor control, yet their precise functional significance remains a subject of debate. Recent research suggests that beta activity occurs in transient bursts, which may better capture its role in movement regulation than sustained oscillations. Although cortical and subcortical beta bursts have been extensively studied, their transmission to muscles-particularly in the upper limb-remains poorly understood and has been limited by traditional bipolar EMG techniques. In this study, we used high-density surface electromyography (HDsEMG) and electroencephalography (EEG) to investigate the cortico-peripheral dynamics of beta bursts in forearm extensor muscles during isometric contractions at the motor unit (MU) level. We show that MU activity in the upper limb exhibits discrete beta bursts that are temporally aligned with cortical beta activity. Notably, beta bursts in the periphery were time-locked to cortical bursts, suggesting strong coordination and synchronization of bursting across the corticospinal tract. We also found stronger beta synchronization in the extensor's ulnar regions compared with the radial regions, indicating muscle-specific differences in beta projections to the motor neuron pools. These findings provide the first demonstration of beta burst propagation from cortex to upper-limb MUs and show that HDsEMG can reliability detect such events in the upper limb. This work supports the cortical origin and structure of peripheral beta activity and demonstrates its potential as a neurophysiological biomarker for targeting corticospinal dynamics in motor disorders such as Parkinson's disease.NEW & NOTEWORTHY We provide the first evidence of cortical beta bursting in motor unit (MU) activity of forearm extensors across force levels. MU beta bursts are time-locked to cortical beta bursts, supporting direct corticospinal transmission in the upper limb. Our findings indicate that burst timing is robust to changes in contraction levels and demonstrate a robust cortical-peripheral beta relationship during isometric contractions. They also reinforce the utility of HDsEMG for investigating burst dynamics and refining neurophysiological biomarkers.

Error-based motor adaptation is currently understood as a dual-rate process involving a fast adaptive process that learns quickly but also decays rapidly and a slow process that learns slowly but has good retention. Although the fast process is typically categorized as procedural learning, recent evidence suggests that it relies on the declarative memory system. To test this hypothesis, we investigated in what manner a declarative memory task interferes with two processes that supposedly underlie force field adaptation in reaching. This declarative memory task, which involved learning a list of words, was assessed through either recognition or recall and was compared with a nondeclarative, vowel-counting task, using a within-subject design (n = 32). We used a Bayesian hierarchical dual-rate process model to capture the observed force compensation across trials, expecting that the parameters of the fast process would be affected by the declarative memory task. We examined the 95% highest density interval of the posterior distribution of the difference between the experimental and control conditions for each parameter. Although most parameters remained unaffected by the declarative memory task, the retention rate of the fast process showed a hint of reduction, suggesting a complex interplay between declarative memory and ongoing motor adaptation processes.NEW & NOTEWORTHY Motor adaptation involves fast and slow adaptive processes. Recent research suggests declarative memory interacts with the fast process. We studied how a declarative task affects trial-to-trial force field adaptation in reaching. Results showed no significant impact on the learning and retention of the slow process or on the learning of the fast process, but hint at a subtle role of declarative processes on the retention of the fast process.

Evidence that adaptive motor learning coincides with a realignment of somatosensory perception has led to hypotheses that a shared mechanism underlies both processes. This implies that these two phenomena should exhibit similar properties. However, studies of somatosensory realignment with visuomotor adaptation have shown mixed support, possibly due to a confounding coactivation of sensory prediction errors and multisensory integration. Although the former is thought to drive adaptation, both processes may contribute to somatosensory realignment. Here, we examined somatosensory realignment following force-field adaptation, which is not confounded by multisensory integration. Across two experiments, we tested whether somatosensory realignment mimics three canonical properties of adaptation in this paradigm. Our first experiment examined whether sensory realignment (for the perception of movement or static position) correlated with adaptation across individuals and generalized beyond the trained reach direction. The results showed that force-field adaptation coincided with a selective realignment of somatosensory perception of movement in the direction of the perturbing force, but this realignment did not correlate with the magnitude of adaptation or generalize beyond the reach direction of the adaptation task. In a second experiment, we tested whether context-dependent dual adaptation to opposing force-field perturbations coincides with a context-dependent dual realignment of somatosensory perception. The results showed no evidence of context-dependent somatosensory realignment after dual adaptation. Overall, our results indicate that somatosensory realignment and adaptation exhibit different properties and are therefore unlikely to rely on the same underlying mechanism, although realignment does display some coherence with the nature of the perturbation.NEW & NOTEWORTHY This study is the first to demonstrate a dissociation in the realignment of somatosensory perceptions of static position and movement following adaptation to novel forces in the upper limb. By assessing whether perceptual realignment following force-field adaptation exhibits two canonical properties of this type of motor learning-i.e., generalization to nearby movement directions and context dependence-this study constitutes a key test of theories positing a shared mechanism underlying the motor and perceptual processes.

Posterior root reflexes elicited by transcutaneous spinal stimulation (TSS) are useful for assessing spinal excitability and guiding neuromodulation interventions. Although various stimulation parameters have been extensively studied, the effect of pulse duration on reflex characteristics has not been thoroughly examined. This study systematically characterized posterior root reflexes across eight pulse durations ranging from 50 to 2,000 µs in 12 healthy participants using unipolar lumbosacral TSS (cathode over T11-12 processes, bilateral anode paraumbilically). In addition, in a subgroup of six participants, the repeatability of reflex characteristics over 2-3 mo was evaluated, and differences between the unipolar and bipolar configurations were examined. Recruitment curves in the major leg muscles reached similar plateau amplitudes across the pulse durations but shifted toward higher stimulation intensities with shorter pulses. The strength-duration curves for the motor threshold intensity were similar across muscles, with an average rheobase of 44.4 mA and a chronaxie of 362.9 µs. The strength-duration curves corresponding to 90% of the recruitment plateau revealed a 24.8 mA higher rheobase and only a 46.9 µs shorter chronaxie. Onset latencies of amplitude-matched reflexes increased 0.81 ms from 50 to 2,000 µs. Paired-pulse suppression demonstrated minimal dependency on pulse duration, although some muscle-specific variations were observed. The two ancillary experiments demonstrated good test-retest repeatability of the unipolar configuration and higher rheobase without significant differences in chronaxie with the bipolar configuration. We conclude that a wide range of pulse durations can produce posterior root reflexes when the stimulation intensity is properly adjusted. These findings offer a framework for selecting stimulation parameters for electrical neuromodulation.NEW & NOTEWORTHY The study demonstrates the impact of pulse duration on posterior root reflex characteristics across the dynamic range of the recruitment curve. It shows that strength-duration parameters-rheobase and chronaxie-are specific to the transcutaneous spinal stimulation settings and cannot be generalized from peripheral nerve H-reflex studies. The adequate reliability of the unipolar configuration is relevant for longitudinal studies, whereas the similarity between the unipolar and bipolar configurations indicates that they are complementary.