Journal of neurophysiology最新文献

Animals adjust their locomotor pattern to increased speed demands by decreasing stance/extensor phase duration while the swing/flexor phase remains relatively unchanged, which we refer to here as "stance/extensor dominance." The control of locomotor speed involves dynamic interactions between spinal circuits, supraspinal drive, and somatosensory feedback. Whereas complete spinal cord injuries abolish brain-spinal cord interactions, incomplete lesions, such as lateral hemisections, preserve some connectivity between brain and spinal circuits. In this study, we investigated adjustments in the locomotor pattern at different treadmill speeds before and after staggered lateral thoracic hemisections performed on opposite sides of the spinal cord (first at right T5-T6 and then at left T10-T11). We collected kinematic and electromyographic data during treadmill locomotion from 0.4 to 0.8 m/s before and 8 wk after each spinal lesion in eight adult cats. Our main results show left-right asymmetries in hindlimb phase durations after each lesion, with prolonged swing on the ipsilesional side and prolonged stance on the contralesional side across speeds. Hindlimb stance dominance was also weakened on the side of each lesion, first on the right and then on the left after the first and second hemisections, respectively. In contrast to phase durations, hindlimb stride lengths remained symmetrical after both injuries across speeds. Using our recent computational models and experimental data of the present study, we provide predictions of altered interactions between supraspinal drive and somatosensory feedback onto flexor and extensor half-centers to explain left-right changes in hindlimb phase durations across speeds after staggered lateral thoracic hemisections.NEW & NOTEWORTHY Staggered lateral thoracic hemisections reversibly altered the temporal structure of the hindlimb locomotor cycle by reducing stance/extensor phase dominance in the ipsilesional hindlimb in favor of the swing/flexor phase. The contralateral hindlimb compensated by prolonging stance and reducing swing. The forelimbs started taking more steps within a hindlimb cycle independently of speed or lesion side. These results can be explained by reorganized sensorimotor interactions based on network architecture from recently published computational models.

Humans adjust their motor actions to correct for errors both with and without being aware of doing so. Little is known, however, about what makes errors detectable for the actor, and how researchers can know when errors are detected. Here, we investigated pupillometry as an unobtrusive, no-report marker of perturbation detection. We also replicate and extend prior work showing that motor adjustments may mask the very errors they correct for. Participants (n = 48) grasped objects while a visuo-haptic size mismatch was applied either sinusoidally (with a continuously changing perturbation) or abruptly (with a constant perturbation that could more easily be adapted to). When mismatches started abruptly and thereafter stayed the same, participants adapted well but also showed decreasing discrimination performance and decreasing confidence in their responses. This was not the case for sinusoidally introduced perturbations. As hypothesized, parameters that characterize phasic and tonic pupil responses were predicted by stimulus parameters and differed depending on participants' grasping and behavioral responses. In particular, tonic responses were smaller for stronger perturbations but larger in trials with correct and confident responses, whereas phasic responses were correlated positively with perturbation magnitude and response correctness, but negatively with response confidence. However, predicting response characteristics from pupil-dilation features using support-vector machine classifiers was not successful. This shows that although pupillometry may yet prove to be a useful no-report marker of perturbation and error detection, there are some challenges for trial-by-trial prediction.NEW & NOTEWORTHY Actors detecting the need to adjust motor actions can make adjustments more efficiently. We show that pupil dilation during perturbed actions reflected perturbation properties and participants' responses, but trial-wise prediction of responses using pupil-dilation parameters was close to chance. Error signals, in addition to perturbation magnitude, play a specific central role in humans' detection of and meta-cognition about motor perturbations. This is a step toward potentially using pupillometry as a no-report marker of perturbation detection.

The use of a high-density microelectrode array (>4,000 microelectrodes) allows the recording of evoked and ongoing field activity of the whole hippocampal formation and other tissues with high spatiotemporal resolution. From these recordings, it is possible to obtain the current source density (CSD), which separates the current generators into sinks and sources; this differentiation provides the means to distinguish correlated, disjoint loci of activity and track them separately at near-to-cell resolution. By obtaining the vectorial average of an area regarded as a sink or source, we obtain a putative center of action or center of mass that can be traced in time. Thus, successive centers would reveal the immediately nearby correlated units that "inherit" or otherwise are affected by this activity, revealing a putative route of transfer of information that can be quantitated with high spatiotemporal definition. For structured tissues, this methodology provides a means to infer effective information transmission with clear parameters that can be further analyzed in several ways. Importantly, the CSD over time reveals patterns of activity obscured by the representation of activity in the voltage domain, which can aid in uncovering synaptic interactions in restricted microcircuits.NEW & NOTEWORTHY We developed a novel methodology to analyze electrophysiological recordings obtained from hippocampal slices with a high-density microelectrode array. Current generators are separated into sinks and sources, and disjoint components are identified within active loci in the hippocampal substructures. A center of mass is obtained from each component, which can be tracked in time at near-cell resolution, revealing a putative route of transfer of information that can be quantitated with high spatiotemporal definition.

Many patients with small fiber neuropathy experience elevated pain sensation for cold stimuli, and the pathophysiology is highly unknown. Therefore, the aim of this study was to evaluate a new method for probing the peripheral cold-sending fibers by combining electrical and thermal stimulation. The cold and warm detection thresholds (CDT and WDT) were measured in 17 healthy participants under the conditions with and without electrical conditioning stimulation. The electrical stimulation was tested using both 4 and 250 Hz at 50% of the electrical perception threshold. A small area cathode electrode was used for the electrical stimulation, and a thermode was placed on top of the electrode to estimate the thermal thresholds. Two-way RMANOVA was performed to analyze the results. The CDT decreased from 26.8°C (SD -4.1, +2.5, log-transformed) to 25.8°C (SD -4.9, +2.9, log-transformed) by the conditioning electrical stimulation (P = 0.006). The mean WDT was 41.6°C (SD -3.0, +4.1, log-transformed) without and 40.7°C (SD -3.1, +4.6, log₁₀-transformed) with conditioning electrical stimulation (P = 0.12). No significant main effect was found for the frequency of electrical stimulation for the two thermal thresholds. Conditioning electrical stimulation significantly altered the CDT but not the WDT, which can be explained by the small cathode's preferential activation of Aδ fibers to a greater extent than C fibers since the cold, innoxious sensation is mainly mediated by Aδ fibers and the warm sensation by C fibers. Combining thermal and electrical stimulation may, in the future, be used for probing cold-sensing fiber excitability, but further studies are necessary to validate the results.NEW & NOTEWORTHY A novel approach is evaluated for probing peripheral cold-sensing fiber by using electrical stimulation's large variety of protocols in combination with activation of the fiber by thermal stimulation to ensure selective activation of the cold-sensing fibers. The results showed that the cold detection threshold could be altered by electrical stimulation, but no significant differences were found for warm temperatures or different frequencies of conditioning electrical stimulation. The result was promising, but further studies are needed.

Acoustic stimuli where the spectrum is time-varying are ubiquitous in natural sounds, including animal vocalizations, human speech, and music. Early studies of such stimuli involving frequency-modulated sweeps reveal that neurons in the primary auditory cortex of a variety of mammals show differences in firing rates depending on either the direction of the sweep and/or the sweep velocity. Psychophysical studies have also shown that the perception of such time-varying stimulus parameters is quite acute, underscoring the importance of such signals in normal acoustic perception. The responses of auditory neurons in alert primates have been little studied, and there is limited information relating neural activity to the perception of these signals. In this study, we investigated the neural discriminability of sweep direction and velocity for frequency-modulated sweeps presented to alert rhesus macaque monkeys in both core and belt auditory cortical areas. We quantified how well these information-bearing parameters were encoded using spike train pattern discriminators, and compared decoder performance when neural responses were restricted to temporal patterns or firing rates. Decoding accuracy for firing rate alone exceeded chance, and rate-normalized, spike-timing information was essentially equivalent to the complete firing pattern. Although most belt areas showed small decreases in decoding accuracy relative to the primary field, all fields encoded and represented sweeps similarly. Thus, there was little evidence of hierarchical processing between core and belt fields for these stimuli, indicating that frequency modulation sweep direction and velocity are not specifically extracted in the early auditory cortical hierarchy.NEW & NOTEWORTHY We recorded responses of single neurons in core and belt auditory cortical fields in alert macaque monkeys to frequency-modulated sounds, key feature of many natural sounds, including speech and vocalizations. We found that the timing of neural activity, rather than its magnitude, explained decoding accuracy in all cortical areas, and we did not observe compelling evidence of improved discriminability in core or belt fields beyond that seen in the primary auditory cortex.

Motor adaptation is a learning process that enables humans to regain proficiency when sensorimotor conditions are sustainably altered. Many studies have documented the properties of motor adaptation, yet the underlying mechanisms of motor adaptation remain imperfectly understood. In this study, we propose a computational analysis of adaptation to a visuomotor rotation task and examine it through an experiment. Our analysis suggests that two distinct processes contribute to produce adaptation: one which straightens trajectories, and another which redirects trajectories. We designed a visuomotor rotation task in a three-dimensional (3-D) virtual environment where human participants performed a pointing task using a head-mounted display controller represented by a cursor that was visually rotated by an angular deviation relative to its actual position. We observed that: 1) the trajectories were initially curved and misdirected, and became straighter and better directed with learning; 2) the straightening process occurred faster than the redirection process. These findings are consistent with our computational analysis and disclose a new and different perspective on motor adaptation.NEW & NOTEWORTHY This study investigates the visuomotor rotation protocol using a pointing task to compare the adaptation of 1) the movement initial angle, 2) the trajectory length. The experiment shows that these two outcomes adapt on different time scales. To account for this observation, we propose a computational analysis based on the control-estimation framework.

Propriospinal interneurons in the spinal cord integrate multiple modalities of supraspinal and sensory inputs to modulate motor activity and facilitate complex motor behaviors, such as locomotion, skilled reaching, or grasping. The important ability of modulating motor activity in response to changes in the environment is partly mediated by a population of spinal interneurons marked by the expression Isl1, called dI3 neurons. These dI3 neurons are located throughout the cervical and lumbar spinal cord, receive cutaneous and proprioceptive feedback, and project to motoneurons. Previous work has demonstrated that dI3s are implicated in cutaneous-evoked reflexes and play a role in behaviors such as locomotion and grip strength, as well as motor recovery after spinal cord injury; however, it is unclear how different dI3 populations are connected to motor networks across the spinal cord to facilitate these diverse and complex functions. Through optogenetic activation of individual dI3 subpopulations located in different segments of the spinal cord, we mapped the functional connectivity of dI3 premotor circuits across the lumbar and cervical enlargements. We demonstrate that individual dI3 subpopulations have unique connectivity patterns and together form short and long propriospinal circuits that are either ipsilateral or commissural. Our findings suggest that dI3 subpopulations modulate the activity of distinct motor pools to differentially modulate complex motor functions such as grasping or locomotion.NEW & NOTEWORTHY We used an isolated mouse spinal cord preparation to optogenetically stimulate individual subpopulations of dI3 neurons. We investigated for the first time the specific connectivity patterns of dI3 neurons and demonstrated that they activate premotor circuits beyond their local segment, including short propriospinal, long propriospinal, and commissural circuits.

Adaptation to consistently occurring sensorimotor errors is considered obligatory in nature. We probed the robustness of this finding by asking if humans can selectively attenuate adaptation based on the task-relevance of error signals. Subjects made planar reaches to three different targets: an arc (experiment 1), a bar (experiment 2), and a point (experiment 3). During the reach, perturbations in extent (visuomotor gain), direction (visuomotor rotation), or both simultaneously were employed. In experiment 1, subjects showed robust adaptation to the rotation when reaching to the arc, even though the presence of this perturbation was irrelevant for the achievement of the task goal. Interestingly, however, rotation adaptation was strongly attenuated when it was presented simultaneously with a task-relevant gain perturbation. In experiment 2, which involved reaches to the bar, again, subjects successfully adapted to the task-irrelevant gain perturbation when it occurred in isolation. However, adaptation was attenuated when the gain co-occurred with a task-relevant rotation. Experiment 3 revealed that the attenuation observed in the first two experiments was not due to an inability to adapt to co-occurring rotation and gain perturbations. Collectively, our results suggest that the sensorimotor system selectively tunes learning in the presence of multiple error signals, a finding that can potentially be explained by a biased competition mechanism. That is, given limited processing capacity, a salient attribute-the relevance of the error to the task goal in this case-is prioritized for processing and drives subsequent adaptive changes in motor output.NEW & NOTEWORTHY The motor system continuously uses error feedback to recalibrate movements in response to changes in body and environmental conditions. Such error-based adaptation is thought to be obligatory, occurring whenever error signals are present, and even if the learning interferes with the achievement of the task goal. Contrary to this classical view, we demonstrate selective modulation of motor adaptation in the presence of multiple error signals based on their task-relevance.

Intracortical microstimulation (ICMS) is a technique to provide tactile sensations for a somatosensory brain-machine interface (BMI). A viable BMI must function within the rich, multisensory environment of the real world, but how ICMS is integrated with other sensory modalities is poorly understood. To investigate how ICMS percepts are integrated with visual information, ICMS and visual stimuli were delivered at varying times relative to one another. Both visual context and ICMS current amplitude were found to bias the qualitative experience of ICMS. In two tetraplegic participants, ICMS and visual stimuli were more likely to be experienced as occurring simultaneously in a realistic visual condition compared with an abstract one, demonstrating an effect of visual context on the temporal binding window. The peak of the temporal binding window varied but was consistently offset from zero, suggesting that multisensory integration with ICMS can suffer from temporal misalignment. Recordings from primary somatosensory cortex (S1) during catch trials where visual stimuli were delivered without ICMS demonstrated that S1 represents visual information related to ICMS across visual contexts. This study was a part of a clinical trial (NCT01964261).NEW & NOTEWORTHY Little is known about how the brain integrates tactile sensations elicited through intracortical microstimulation (ICMS) with visual information. This work investigates how visual cues affect the perception of tactile sensations from ICMS in two human participants. The results suggest that visual context can influence the perceived timing and the qualitative nature of artificial sensations, which is directly relevant to the implementation of a viable brain-machine interface (BMI) for individuals with tactile impairments.