Subthalamic nucleus functional organization revealed by parkinsonian neuronal oscillations and synchrony.

The emergence of oscillations and synchrony among neurons of the basal ganglia is a well-known characteristic of Parkinson's disease. In this study we used intra-operative microelectrode recording to investigate this interrelationship between these two phenomena in the subthalamic nucleus (STN) neurons of 39 human Parkinson's disease patients undergoing deep brain stimulation surgery. From the recorded neuronal traces both neuronal spike trains and their background activity were extracted, and their spectral characteristics were evaluated. We have used the background oscillations as a marker for synchronized activity in the local population in the neuron vicinity and studied its relation to single neuron oscillations. Spike train background oscillations were evaluated using a procedure of background reconstruction that consisted of spikes removal from the original traces and full wave rectification followed by standard spectral analysis. Coherence and phase analysis between oscillatory spike trains and their oscillatory background were also conducted to study the phase relationship between the two. Of the 231 neuronal spike-trains which were sorted offline, 82 (35%) showed significant oscillatory activity. These neurons were found to oscillate mostly in two bands; 3-7 Hz, termed the Tremor Frequency Band (TFB), and 8-20 Hz, termed the High-Frequency Band (HFB). While HFB neurons oscillated for longer periods and always coherently with their background activity, TFB neurons oscillated more episodically and only a half were coherent with their background. These findings indicate that the two neuronal populations are the outcome of very different oscillatory drives deriving from different local functional neuronal organizations.