Subthalamic Somatosensory Evoked Potentials in Parkinson’s Disease Alessandra Pesenti, MD, 1,2 Alberto Priori, MD, PhD, 1,2 * Marco Locatelli, MD, 1 Marcello Egidi, MD, 1 Paolo Rampini, MD, 1 Filippo Tamma, MD, 3 Elena Caputo, MD, 3 Valentina Chiesa, MD, 3 and Sergio Barbieri, MD, PhD 1 1 Dipartimento di Neuroscienze e Organi di Senso, IRCCS- Ospedale Maggiore Milano, Milano, Italy; 2 Universita ` di Milano, Milano, Italy; 3 Divisione di Neurologia, Ospedale S. Paolo Milano, Milano, Italy Abstract: Deep brain stimulation (DBS) of subthalamic nu- cleus (STN) is an effective treatment for advanced Parkinson’s disease. It also provides an opportunity to record neural ac- tivity from the human basal ganglia. In this study, to investi- gate the involvement of the human STN in sensory functions, we recorded somatosensory evoked potentials (SEPs) elicited by contralateral median-nerve stimulation, from STN elec- trodes implanted for DBS in patients with Parkinson’s disease. We suggest that the STN N18 component of SEPs in Parkin- son’s disease is a mainly local field potential elicited by muscle afferent input to the nucleus. © 2003 Movement Disorder Society Key words: subthalamic nucleus (STN); deep brain stimu- lation (DBS); somatosensory processing; Parkinson’s disease; evoked potentials Deep brain stimulation (DBS) of the subthalamic nu- cleus (STN) is an effective treatment for advanced Par- kinson’s disease (PD). 1–4 The electrodes implanted for DBS also offer a unique opportunity to record neural activity from the human basal ganglia. 5–7 Although the role of the STN in the basal ganglia circuit for motor functions is relatively clear, 8,9 several lines of evidence show that the human STN is also involved in a variety of nonmotor functions. 5,10 Some areas of the STN display associative and limbic func- tions. 10 Recent data also suggest a role of the STN in sensory processing in man. 5 Experiments in animals pro- vide extensive evidence that the basal ganglia have a function as a “sensory analyzer” for the motor system, converting sensory data into a form that is relevant for guiding movement. Hence, these nuclei affect movement by regulating the access of sensory inputs to motor areas. 11 In animals, there are three main portions in the STN: “motor,” “associative,” and “limbic.” 12 Studies in humans, as in monkeys, have described a somatotopic organization and the sensorimotor region has been local- ized in the dorsolateral two thirds of the STN. 13 Postop- erative autonomic imbalance in humans after coagulation of the subthalamus 14 and the acute changes in sympa- thetic skin responses and plasma renin activity after chronic DBS, 15 suggest that the human STN might also have an autonomic function. The psychotropic effects (loss of motivation, depression) induced by high-fre- quency stimulation of the STN could depend, at least in part, on the overactivity of the “limbic” area of the STN. 10,16 In addition, the prelemniscal radiations (a fun- nel of fibers in the posterior STN) represent a circuit related to the mesencephalic reticular formation, a struc- ture involved in selective attention. 9 Finally, observa- tions in patients with Parkinson’s disease undergoing DBS of the STN indirectly suggest that the STN has a nonmotor role in the processing of somatosensory and visual input. 15 In this study, we investigated the possible involvement of the human STN in sensory function by recording STN potentials evoked by mixed nerve and cutaneous stimulation from DBS electrodes implanted in patients with Parkinson’s disease. SUBJECTS AND METHODS Subjects Eight patients with Parkinson’s disease, 4 women and 4 men, age 58.9  5.0 years (range, 52– 69 years), were studied 2 or 3 days after the implantation of the DBS electrodes for DBS in both STN. All patients were stud- ied off-medication (levodopa withdrawal 24 hours before the surgery, dopamine agonists withdrawal from 7 days before the DBS; none of the patients assumed anticho- linergic drugs), after informed consent and local ethical approval. The mean levodopa daily dose of the patients before surgery was 1,290.3  495.1 mg (2,175– 600 mg). Before surgery, the Unified Parkinson Disease Rating Scale (UPDRS) motor score off-medication was 41.7  10.1 (range, 31.6 –51.8; n = 8 subjects). Two healthy subjects (a 30-year-old man and a 46-year-old woman) took part in additional control experiments testing the effect of cutaneous input. *Correspondence to: Dr. Alberto Priori, Dipartimento di Neuro- scienze, Padiglione Ponti, Associazione Amici del Centro Dino Ferrari, Ospedale Maggiore di Milano, Via F. Sforza 35, Milano, 20122 Italy. E-mail: alberto.priori@unimi.it Received 26 November 2002; Revised 24 March 2003; Accepted 22 April 2003 SUBTHALAMIC SEPs 1341 Movement Disorders, Vol. 18, No. 11, 2003