Beta oscillatory neurons in the motor thalamus of movement disorder and pain patients

Excessive beta oscillations (15-25 Hz) in the basal ganglia have been linked to the akineto-rigid symptoms of Parkinson's disease (PD) although it remains unclear whether the underlying mechanism is causative or associative. While a number of studies have reported beta activity in the subthalamic nucleus and globus pallidus internus, relatively little is known about the beta rhythm of the motor thalamus and its relation to movement disorders. To test whether thalamic beta oscillations are related to parkinsonian symptoms, we examined the spectral properties of neuronal activity in the ventral thalamic nuclei of five Parkinson's disease patients (two female, age range 50-72 years) and compared them to five essential tremor (three female, aged 41-75) and four central pain patients (one female, aged 38-60). Spike and local field potential recordings were obtained during microelectrode-guided localization of thalamic nuclei prior to the implantation of deep brain stimulating electrodes. A total of 118 movement-related neurons in the region of the ventral intermediate nucleus (Vim) were analyzed across all patient groups. Eighty of these neurons (68%) displayed significant oscillatory firing in the beta range with the limbs at rest. In contrast, only 5.7% of the ventral oral posterior (Vop) (χ2 test, p < 0.05) and only 7.2% of the ventral caudal (Vc) neurons fired rhythmically at beta frequency (χ2 test, p < 0.05). Beta power was significantly decreased during limb movements (ANOVA, p < 0.05) and was inversely related to tremor-frequency power during tremor epochs in ET and PD (r2 = 0.44). Comparison between patient groups showed that Vim beta power was significantly higher in ET patients versus pain and PD groups (ANOVA, p < 0.05). The findings suggest that beta oscillations are found predominantly in Vim and are involved in movement but are not enhanced in tremor-dominant Parkinson's patients.