Tremor-correlated cortical activity detected by electroencephalography
Introduction
Resting tremor in Parkinson's disease is assumed to be generated by a central oscillatory neuronal network (Elble, 1996). As yet, the structures involved in this network are not precisely known. There is some evidence for a thalamic origin of Parkinsonian tremor (Paré et al., 1990, Lenz et al., 1994). Experiments with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys and results of stereotactic operations in man suggest that the subthalamic nucleus as well as the internal pallidum contribute to the generation of tremor in Parkinson's disease (Bergman et al., 1994, Wichmann et al., 1994, Benabid et al., 1998, Rodriguez et al., 1998, Lozano et al., 1998). Finally, the cerebral cortex seems to be necessary for the clinical expression of tremor. For instance, extirpation of the motor cortex suppresses Parkinsonian tremor (Putnam, 1940). Furthermore, rhythmic electrical activity at the tremor frequency can be recorded directly from the cortical surface of Parkinsonian patients (Alberts et al., 1969). Recently, Volkmann et al. (1996) have shown that magnetoencephalography is able to reveal cortical activity related to Parkinsonian tremor. Earlier attempts to detect tremor-correlated activity by electroencephalography have not been successful (Jasper and Andrews, 1938, Schwab and Cobb, 1939). In this paper it is shown for the first time that, using coherency analysis, cortical activity related to Parkinsonian resting tremor can indeed be revealed by electroencephalography. We investigate also whether time delays between cortical and muscular activity can be inferred from the phase spectrum.
Preliminary results of this study have been published in abstract form (Hellwig et al., 1998).
Section snippets
Patients
Seven patients (4 men, 3 women) with idiopathic Parkinson's disease participated in the study. The patients were on average 59.9 years old (range: 52–68 years). The severity of the disease ranged between Hoehn and Yahr stages I and III. Patients were selected for the study because they showed a moderate, strictly unilateral (3 on the left, 4 on the right) resting tremor. There was no evidence for neurological diseases other than Parkinson's disease. All patients gave informed consent to
Results
As an example, Fig. 1 displays for a patient with tremor of the left arm two EEG traces (electrodes C3 and C4), one EMG trace (left wrist extensor), the autospectra of the EEG and EMG traces and the corresponding coherencies. Scrutinizing EEG and EMG traces does not reveal any correlation between tremor and cortical activity (Fig. 1a–c). While the autospectrum of the EMG trace shows peaks at the tremor frequency (about 5 Hz) and its higher harmonic, the autospectra of the EEG traces do not
Discussion
Clinical and experimental data suggest that the generation of Parkinsonian resting tremor involves not only the basal ganglia and the thalamus but also the cerebral cortex (Putnam, 1940, Alberts et al., 1969). In particular, it has been shown that tremor-correlated cortical activity can be detected by magnetoencephalography (Volkmann et al., 1996). Electroencephalography, on the other hand, failed to reveal electric activity correlated with Parkinsonian tremor (Jasper and Andrews, 1938, Schwab
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