Measuring the electrophysiological effects of direct electrical stimulation after awake brain surgery.

OBJECTIVE: Direct electrical stimulation (DES) at 60 Hz is used to perform real-time functional mapping of the brain, and guide tumour resection during awake neurosurgery. Nonetheless, the electrophysiological effects of DES remain largely unknown, both locally and remotely. APPROACH: In this study, we lowered the DES frequency to 1-10 Hz and we used a differential recording mode of electro-corticographic (ECoG) signals to improve the focality with a simple algorithm to remove the artefacts due to the response of the acquisition chain. MAIN RESULTS: Doing so, we were able to observe different components in the evoked potentials triggered by simulating the cortex or the subcortical white matter pathways near the recording electrodes and by stimulating the cortex remotely from the recording site. More particularly, P0 and N1 components were repeatedly observed on raw ECoG signals without the need to average the data. SIGNIFICANCE: This new methodology is important to probe the electrophysiological states and the connectivity of the brain in vivo and in real time, namely to perform electrophysiological brain mapping on human patients operated in the neurosurgical room and to better understand the electrophysiological spreading of DES.

Motor-prediction improvements after virtual rehabilitation in geriatrics: frail patients reveal different learning curves for movement and postural control.

BACKGROUND: Postural control associated with self-paced movement is critical for balance in frail older adults. The present work aimed to investigate the effects of a 2D virtual reality-based program on postural control associated with rapid arm movement in this population. METHODS: Participants in an upright standing position performed rapid arm-raising movements towards a target. Practice-related changes were assessed by pre- and post-test comparisons of hand kinematics and centre-of-pressure (CoP) displacement parameters measured in a training group and a control group. During these pre- and post-test sessions, patients have to reach towards yellow balls appearing on the screen, form a standardized upright position (with 15cm between the two malleoli). Training group patients took part in six sessions of virtual game. In this, patients were asked to reach their arm towards yellow balls appearing on the screen, from an upright position. RESULTS: After training, we observed improvements in arm movements and in the initial phase of CoP displacement, especially in the anticipatory postural adjustments. Learning curves for these two types of motor improvements showed different rates. These were continuous for the control of the arm movement, and discontinuous for the control of the CoP during the anticipatory postural adjustments. CONCLUSION: These results suggest that some level of motor (re)-learning is maintained in frail patients with low functional reserves. They also suggest that re-learning of anticipatory postural control (i.e. motor prediction) is less robust than explicit motor learning involved for the arm reaching. This last point should encourage clinicians to extend the training course duration, even if reaching movement improvements seems acquired, in order to automate these anticipatory postural activities. However, other studies should be done to measure the retention of these two types of learning on a longer-term period.

Pointing to double-step visual stimuli from a standing position: motor corrections when the speed-accuracy trade-off is unexpectedly modified in-flight. A breakdown of the perception-action coupling.

The time required to complete a fast and accurate movement is a function of its amplitude and the target size. This phenomenon refers to the well known speed-accuracy trade-off. Some interpretations have suggested that the speed-accuracy trade-off is already integrated into the movement planning phase. More specifically, pointing movements may be planned to minimize the variance of the final hand position. However, goal-directed movements can be altered at any time, if for instance, the target location is changed during execution. Thus, one possible limitation of these interpretations may be that they underestimate feedback processes. To further investigate this hypothesis we designed an experiment in which the speed-accuracy trade-off was unexpectedly varied at the hand movement onset by modifying separately the target distance or size, or by modifying both of them simultaneously. These pointing movements were executed from an upright standing position. Our main results showed that the movement time increased when there was a change to the size or location of the target. In addition, the terminal variability of finger position did not change. In other words, it showed that the movement velocity is modulated according to the target size and distance during motor programming or during the final approach, independently of the final variability of the hand position. It suggests that when the speed-accuracy trade-off is unexpectedly modified, terminal feedbacks based on intermediate representations of the endpoint velocity are used to monitor and control the hand displacement. There is clearly no obvious perception-action coupling in this case but rather intermediate processing that may be involved.

Catching falling objects: the role of the cerebellum in processing sensory-motor errors that may influence updating of feedforward commands. An fMRI study.

The human motor system continuously adapts to changes in the environment by comparing differences between the brain's predicted outcome of a certain behavior and the observed outcome. This discrepancy signal triggers a sensory-motor error and it is assumed that the cerebellum is a key structure in updating this error and associated feedforward commands. Using fMRI, the aim of the present study was to determine the main cerebellar structures that are involved in the processing of sensory-motor errors and in updating feedforward commands when simply catching a falling ball without displacement of the hand. Subjects only grasped the ball with their fingers when receiving it in their hand. By contrasting functional imaging signal obtained in conditions in which it was possible and impossible to predict the weight of the ball, we aimed to highlight sensory-motor error processing which we expected to be more marked in the conditions without prediction (less accurate feedforward process or more important feedback corrections) with respect to conditions with prediction (more accurate feedforward process or less important feedback corrections). When catching a falling ball and the possibility of prediction about the ball weight was manipulated, our results showed that both the right and left cerebellum is engaged in processing sensory-motor errors. It may also be involved in updating feedforward motor commands, perhaps on a trial by trial basis. In addition, when subjects were blindfolded, we observed a similar network but centered in a more anterior portion of the right cerebellum and we noted the presence of a cerebellar-thalamo-prefrontral network that may be involved in cognitive prediction (rather than sensory prediction) about ball weight.

Pointing to double-step visual stimuli from a standing position: very short latency (express) corrections are observed in upper and lower limbs and may not require cortical involvement.

How fast can we correct a planned movement following an unexpected target jump? Subjects, starting in an upright standing position, were required to point to a target that randomly and unexpectedly jumps forward to a constant spatial location. Rapid motor corrections in the upper and lower limbs, with latency responses of less than 100 ms, were revealed by contrasting electromyographic activities in perturbed and unperturbed trials. The earliest responses were observed primarily in the anterior section of the deltoïdus anterior (shoulder) and the tibialis anterior (leg) muscles. Our findings indicate that visual on-going movement corrections may be accomplished via fast loops at the level of the upper and lower limbs and may not require cortical involvement.