Mapping the basal ganglia: fMRI evidence for somatotopic representation of face, hand, and foot.

OBJECTIVE: To noninvasively investigate the somatotopy of the basal ganglia in humans. METHODS: Functional MRI, 1.5-T, was performed on six normal right-handed volunteers during simple acoustically paced motor tasks involving the right hand, foot, and face. RESULTS: In a single-subject analysis, statistical parametric maps showed overlapping activation extending along the anteroposterior extent of the left lentiform nucleus (LLN) for the hand, foot, and face representations. Within the LLN, the centers of gravity of each body part, reflecting both the extent and gradient of activation, were all located in the retrocommissural portion of the putamen. Their spatial relationship followed a similar pattern across subjects-face was medial to toes and fingers, toes were dorsal and rostral to fingers. CONCLUSIONS: The somatotopic organization of hand, face, and foot representation in the human lentiform nucleus suggests a triangular pattern, rather than the linear pattern seen in primate studies. The overlap observed between the distinct body parts differs from the cortical sensorimotor representation, indicating a different organizational concept of the basal ganglia.

Electroencephalographic measurement of motor cortex control of muscle activity in humans.

OBJECTIVE: To detect and measure correlation between cortical and muscle activities, coherence analysis was used. METHODS: The electroencephalogram (EEG) and electromyogram (EMG) were recorded in 9 normal volunteers during tonic contraction of upper and lower limb muscles on the right side. Coherence between EEG and EMG was computed to analyze their linear association. RESULTS: EEG over the contralateral sensorimotor area was coherent with EMG, with peak coherence at 11-36 Hz (mean, 22 Hz). For the abductor pollicis brevis (APB) muscle, peak coherence, as determined by functional brain mapping with focal transcranial magnetic stimulation (TMS), was over or slightly posterior to the hand area on the primary motor cortex determined by focal transcranial magnetic stimulation (TMS). Peak coherence over the scalp was somatotopically organized. The temporal relation between EEG and EMG was analyzed with a new model for interpreting the phase shift ('constant phase shift plus constant time lag' model). For the APB muscle, the phase relation between cortical and muscular oscillations differed in the frequency ranges of 3-13 Hz and 14-50 Hz, respectively, suggesting that different coupling mechanisms operate in different bands. Only the phase shift between cortical and motoneuronal firing at 14-50 Hz was reliably estimated by a linear model. At 14-50 Hz, motoneuronal firing was led by surface-negative cortical activity with a constant time lag that depended on the cortical-muscular distance. For the APB muscle, the time lag was slightly shorter than the cortical-muscular conduction time determined by TMS. Vibratory stimulation (100 Hz) of a muscle tendon during tonic contraction had no significant effect on cortical-muscular coherence, indicating that cortical oscillation reflected motor rather than sensory activity. CONCLUSIONS: The present findings suggest temporal coding of the oscillatory motor control system (3-13 Hz vs. 14-50 Hz), and confirm the functional importance of cortical beta and gamma rhythms in the motor efferent command. Cortical-muscular synchronization is most likely mediated by the direct corticospinal pathway within the frequency range of 14-50 Hz.

Functional coupling of human cortical sensorimotor areas during bimanual skill acquisition.

Bimanual co-ordination of skilled finger movements is a high-level capability of the human motor system and virtually always requires training. Little is known about the physiological processes underlying successful bimanual performance and skill acquisition. In the present study, we used task-related coherence (TRCoh) and task-related power (TRPow) analysis of multichannel surface EEG to investigate the functional coupling and regional activation of human sensorimotor regions during bimanual skill acquisition. We focused on changes in interhemispheric coupling associated with bimanual learning. TRCoh and TRPow were estimated during the fusion of two overlearned unimanual finger-tapping sequences into one novel bimanual sequence, before and after a 30-min training period in 18 normal volunteers. Control experiments included learning and repetition of complex and simple unimanual finger sequences. The main finding was a significant increase in interhemispheric TRCoh selectively in the early learning stage (P < 0.0001). Interhemispheric TRCoh was also present during the unimanual control tasks, but with lower magnitude, even if learning was involved. Training improved bimanual sequence performance (from 58.3+/-24.1 to 83.7+/-15.3% correct sequences). After training, interhemispheric (bimanual) TRCoh decreased again, thereby approaching levels similar to those in the unimanual controls. We propose that the initial increase in TRCoh reflects changes in interhemispheric communication that are specifically related to bimanual learning and may be relayed through the corpus callosum. The present data might also offer a neurophysiological explanation for the clinical observation that patients with lesions of the corpus callosum may show deficits in the acquisition of novel bimanual tasks but not necessarily in the execution of previously learned bimanual activities.

Blink reflex recovery in facial weakness: an electrophysiologic study of adaptive changes.

OBJECTIVE: To study the electrophysiologic effects of unilateral facial weakness on the excitability of the neuronal circuitry underlying blink reflex, and to localize the site of changes in blink reflex excitability that occur after facial weakness. BACKGROUND: Eyelid kinematic studies suggest that adaptive modification of the blink reflex occurs after facial weakness. Such adaptations generally optimize eye closure. A report of blepharospasm following Bell's palsy suggests that dysfunctional adaptive changes can also occur. METHODS: Blink reflex recovery was evaluated with paired stimulation of the supraorbital nerve at different interstimulus intervals. Comparisons were made between normal control subjects and patients with Bell's palsy who either recovered facial strength or who had persistent weakness. RESULTS: Blink reflex recovery was enhanced in patients with residual weakness but not in patients who recovered facial strength. Facial muscles on weak and unaffected sides showed enhancement. In patients with residual weakness, earlier blink reflex recovery occurred when stimulating the supraorbital nerve on the weak side. Sensory thresholds were symmetric. CONCLUSION: Enhancement of blink reflex recovery is dependent on ongoing facial weakness. Faster recovery when stimulating the supraorbital nerve on the paretic side suggests that sensitization may be lateralized, and suggests a role for abnormal afferent input in maintaining sensitization. Interneurons in the blink reflex pathway are the best candidates for the locus of this plasticity.

Functional coupling and regional activation of human cortical motor areas during simple, internally paced and externally paced finger movements.

We studied the activation and interaction of cortical motor regions during simple, internally paced and externally paced right-hand finger extensions in healthy volunteers. We recorded EEGs from 28 scalp electrodes and analysed task-related coherence, task-related power and movement-related cortical potentials. Task-related coherence reflects inter-regional functional coupling of oscillatory neuronal activity, task-related power reflects regional oscillatory activity of neuronal assemblies and movement-related cortical potentials reflect summated potentials of apical dendrites of pyramidal cells. A combination of these three analytical techniques allows comprehensive evaluation of different aspects of information processing in neuronal assemblies. For both externally and internally paced finger extensions, movement-related regional activation was predominant over the contralateral premotor and primary sensorimotor cortex, and functional coupling occurred between the primary sensorimotor cortex of both hemispheres and between the primary sensorimotor cortex and the mesial premotor areas, probably including the supplementary motor area. The main difference between the different types of movement pacing was enhanced functional coupling of central motor areas during internally paced finger extensions, particularly inter-hemispherically between the left and right primary sensorimotor cortexes and between the contralateral primary sensorimotor cortex and the mesial premotor areas. Internally paced finger extensions were also associated with additional regional (premovement) activation over the mesial premotor areas. The maximal task-related coherence differences between internally and externally paced finger extensions occurred in the frequency range of 20-22 Hz rather than in the range of maximal task-related power differences (9-11 Hz). This suggests that important aspects of information processing in the human motor system could be based on network-like oscillatory cortical activity and might be modulated on at least two levels, which to some extent can operate independently from each other: (i) regional activation (task-related power) and (ii) inter-regional functional coupling. We propose that internal pacing of movement poses higher demands on the motor system than external pacing, and that the motor system responds not only by increasing regional activation of the mesial premotor system, including the supplementary motor area, but also by enhancing information flow between lateral and mesial premotor and sensorimotor areas of both hemispheres, even if the movements are simple and unimanual.