Missing kinaesthesia challenges precise naturalistic cortical prosthetic control

A major assumption of brain-machine interface (BMI) research is that patients with disconnected neural pathways can still volitionally recall precise motor commands that could be decoded for naturalistic prosthetic control. However, the disconnected condition of these patients also blocks kinaesthetic feedback from the periphery, which has been shown to regulate centrally generated output responsible for accurate motor control. Here we tested how well motor commands are generated in the absence of kinaesthetic feedback by decoding hand movements from human scalp electroencephalography (EEG) in three conditions: unimpaired movement, imagined movement, and movement attempted during temporary disconnection of peripheral afferent and efferent nerves by ischemic nerve block. Our results suggest that the recall of cortical motor commands is impoverished in absence of kinaesthetic feedback, challenging the possibility of precise naturalistic cortical prosthetic control.

Changes in TMS Measures induced by repetitive TMS

This article reviews several protocols of repetitive transcranial magnetic stimulation (rTMS)-induced plasticity. rTMS, when applied to the motor cortex or other cortical regions of the brain, may induce effects that outlast the stimulation period. The neural plasticity, which emerges as a result of such interventions, has been studied to gain insight into plasticity mechanisms of the brain. In two protocols the structure of rTMS trains is modified, informed by the knowledge of the physiological properties of the corticospinal system. Pulse configuration, stimulus frequency, stimulus intensity, the duration of the application period, and the total number of stimuli are some variables that have to be taken into account when reviewing the physiological effects of rTMS. This article also introduces the concept of patterned rTMS pulses and rTMS with ischemic nerve block. In addition, rTMS has raised considerable interest because of its therapeutic potential; however, much needs to be done in this field.

Increase in flexor but not extensor corticospinal motor outputs following ischemic nerve block

Human motor cortex is capable of rapid and long-lasting reorganization, evident globally, as shifts in body part representations, and at the level of individual muscles as changes in corticospinal excitability. Representational shifts provide an overview of how various body parts reorganize relative to each other but do not tell us whether all muscles in a given body part reorganize in the same manner and to the same extent. Transcranial magnetic stimulation (TMS) provides information about individual muscles and can therefore inform us about the uniformity of plastic changes within a body part. We used TMS to investigate changes in corticospinal excitability of forearm flexors and extensors after inflation of a tourniquet around the wrist. Motor evoked potential (MEP) amplitudes and input/output (I/O) curves were obtained from wrist flexors and extensors simultaneously before and during block. TMS was delivered to the optimal site for eliciting MEPs in flexors in experiment 1, extensors in experiment 2, and both flexors and extensors in experiment 3. In all experiments flexor MEP amplitude increased during block while extensor MEP amplitude showed no systematic change, and the slope of flexor but not extensor I/O curves increased. Flexor H-reflex amplitude normalized to maximal M wave showed negligible changes during block, suggesting that the increase in corticospinal excitability in the flexors cannot be completely explained by increased excitability at the spinal cord level. These findings show that forearm flexors and extensors differ in their potential for plastic changes, highlight the importance of investigating how experimentally induced plasticity affects anatomically close, but functionally distinct, muscle groups, and suggest that rehabilitation interventions aiming to alter cortical organization should consider the differential sensitivity of various muscle groups to plasticity processes.