Neuronal activity of the zona incerta in Parkinson's disease patients

The objective of this study was to describe the firing characteristics of the zona incerta (ZI) in Parkinson's disease patients. The ZI constitutes a band of gray matter lying dorsal to the subthalamic nucleus, whose firing properties have not been well defined in humans yet. ZI proved to become hyperactive in 6-OHDA-lesioned rats as compared to normal rats, and regarding these noticeable changes in the discharge patterns it was suggested that ZI could be a putative target for the surgical treatment of Parkinson's disease. Twelve patients who underwent microrecording-guided subthalamic surgery consented to the study. Neurons from different tracts were classified as belonging to the ZI according to their firing features, background extracellular activity, anatomical mapping of trajectories, and atlas confirmation. Fifty-nine neurons were classified as belonging to ZI. The mean firing rate proved to be 29.5 Hz, with a broad dispersion band, even covering subthalamic nucleus (STN) frequency ranges. Pattern analysis showed heterogeneous neuronal signals ranging from tonic to burst and paused neurons. A decrease in extracellular background activity in the defined ZI was also observed. Five of the recorded neurons showed rhythmical spike trains with oscillations of 8 to 14 Hz, and two units were found to discharge trains at 4 Hz. None of the recorded ZI neurons responded to proprioceptive maneuvers. ZI presented firing activities with a broad spectrum in terms of frequency and tonicity. It is differentiated from STN recordings in Parkinson's disease patients mainly because of absent proprioceptive-related units and diminished extracellular background activity.