In the fi ve nontremulous pallidotomy patients, single GP neurons did not display oscillatory activity, and all of the cross- correlograms in the GPe (15 pairs) and GPi (73 pairs) were fl at. The percentage of spikes in bursts in the GPi was also similar to that reported in another patient group (Levy et al., 2001) and was signi fi cantly lower than the percentage of spikes in bursts found in the GPe. The mean fi ring rates of neurons in these nuclei were similar to those reported in other patient groups (Hutchison et al., 1994, 1998). The mean fi ring rate and percentage of spikes in bursts of single neurons in each nucleus are given in Figure 2 B. In the bottom pair, it can be observed that the bursts of spikes are periodic ( ϳ 5 Hz) and that the neurons display oscillatory syn- chronization. Examples of raw spike trains simultaneously re- corded from pairs of GPi neurons are shown in Figure 2 A. Three of eight pallidotomy patients and three of four STN DBS patients had limb tremor at the time of the recordings, and oscillatory neuronal activity was limited to these patients (Fig. 1 A, bottom row ) to show the location of the microelectrode trajectories and to provide information about the neuronal activity and the locations of the simultaneously recorded neuron pairs in each patient. Figure 1 A displays digitized parasagittal plates (Schaltenbrand and Wahren, 1977) of the GP (Fig. D, A non fl at cross-correlogram in a tremulous pallidotomy patient that is indicative of an inhibitory connection between a pair of GPe neurons (0.5 msec bins 7010 ϫ 9252 events horizontal dashed lines indicate the 99% con fi dence interval about the mean). C, Examples of coherence plots showing high-frequency oscillatory synchronization in the GPe ( left ) and GPi ( right ) of a tremulous pallidotomy patient (calculated from 30 sec of data, 0.98 Hz resolution the number beside the peak is the phase difference in degrees, and the dashed horizontal line is p Ͻ 0.01). Neurons in each pair were separated by 250 m, and pairs were located ϳ 200 m from one another. These plots demonstrate tremor-related synchronization and phase variability in the GPi of a tremulous patient. B, Example of power spectra ( insets ) and coherence of two pairs of GPi TCs from a single patient (0.49 Hz resolution, calculated using 30 sec of data for each pair the number beside the peak is the phase difference in degrees, and the dashed horizontal line is p Ͻ 0.01 for coherence). Right, Coherence during a subsequent 30 sec with limb tremor (0.98 Hz resolution the number beside the peak is the phase difference in degrees, and the dashed horizontal line is p Ͻ 0.01). Left, Coherence during 30 sec with no tremor. A, Example of coherence between contralateral hand tremor and GPi cell activity. Synchronized activities between pairs of neurons in tremulous patients.
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