Functional topography of cortical thumb movement representations in human primary motor cortex.

Intracortical microstimulation and single cell recordings in non-human primates showed that both, muscles and movements are represented in primary motor cortex (M1). This was also suggested in humans using electrophysiological and neuroimaging techniques. Transcranial magnetic stimulation (TMS) thus far was used to study motor cortical muscle representations, but data on movement representations in man are scarce. Therefore, we used TMS of M1 to evoke directional (flexion, extension) thumb movements and generated topographic movement maps (TMS-mov-map) in fourteen healthy individuals. TMS-mov-maps were related to the individual voluntary motor output (Vol-mov, directional thumb acceleration). Conventional TMS-motor evoked potentials (TMS-MEP) were simultaneously recorded from the prime movers (flexor and extensor pollicis brevis) and also compared to the voluntary motor output. Remarkably stable topographic maps were generated with both, thumb flexion and extension being multiply represented, overlapped and interspersed. Thumb Flexion was usually more robust than extension. TMS-mov-maps rather than TMS-MEP-maps seemed to better reflect the individual voluntary motor output. The findings emphasize existing models of multiple, overlapping finger movement representations in human M1, and indicate that this model also adapts to antagonistic thumb movements. The results suggest that investigating topographic movement maps may be an interesting adjunct in studying human motor cortical topography and its relevance for motor control.

Improving hand function in chronic stroke.

BACKGROUND: Recovery of function following stroke plateaus in about 1 year, typically leaving upper arm function better than that in the hand. Since there is competition among body parts for territory in the sensorimotor cortex, even limited activity of the upper arm might prevent the hand from gaining more control, particularly when the territory is reduced in size because of the stroke. Deafferentation of a body part in a healthy brain enhances cortical representations of adjacent body parts, and this effect is markedly increased by voluntary activity of the adjacent part. OBJECTIVE: To explore whether deafferentation of the upper arm, produced by a new technique of regional anesthesia during hand motor practice, helps recovery of hand function in patients with long-term stable weakness of their hand following stroke. METHODS AND RESULTS: Deafferentation, produced by a new technique of regional anesthesia of the upper arm during hand motor practice, dramatically improved hand motor function including some activities of daily living. The improvement was associated with an increase in transcranial magnetic stimulation-evoked motor output to the practice hand muscles. CONCLUSION: This is a novel therapeutic strategy that may help improve hand function in patients with long-term weakness after stroke.

Early consolidation in human primary motor cortex.

Behavioural studies indicate that a newly acquired motor skill is rapidly consolidated from an initially unstable state to a more stable state, whereas neuroimaging studies demonstrate that the brain engages new regions for performance of the task as a result of this consolidation. However, it is not known where a new skill is retained and processed before it is firmly consolidated. Some early aspects of motor skill acquisition involve the primary motor cortex (M1), but the nature of that involvement is unclear. We tested the possibility that the human M1 is essential to early motor consolidation. We monitored changes in elementary motor behaviour while subjects practised fast finger movements that rapidly improved in movement acceleration and muscle force generation. Here we show that low-frequency, repetitive transcranial magnetic stimulation of M1 but not other brain areas specifically disrupted the retention of the behavioural improvement, but did not affect basal motor behaviour, task performance, motor learning by subsequent practice, or recall of the newly acquired motor skill. These findings indicate that the human M1 is specifically engaged during the early stage of motor consolidation.