THERMAL AND PAIN SENSATIONS EVOKED BY MICROSTIMULATION IN THE AREA OFHUMAN VENTROCAUDAL NUCLEUS

1. We have studied the sensations evoked by threshold microstimulation (TMS) in the area of the human principal sensory nucleus of the thala mus [ventralis caudalis (Vc)] in patients (n = 11) undergoing stereota ctic surgery for the treatment of movement disorders and pain. Preoper atively, patients were trained to describe somatic sensory stimuli usi ng a standard list of descriptors. This same list was used to describe sensations evoked intraoperatively by thalamic microstimulation. Stim ulation sites (n = 216) were defined by location within the area where the majority of cells had a reproducible response to innocuous cutane ous stimulation (core region) or in the cellular area posterior and in ferior to the core region (posteroinferior region). 2. TMS-evoked sens ations were categorized as paresthetic if the descriptors ''tingle,'' ''vibration,'' or ''electric current'' were chosen by the patient to d escribe the sensation and as thermal/pain if the descriptors ''cool,'' ''warm,'' ''warm and cool,'' or ''pain'' were chosen. Thermal/pain se nsations were evoked by stimulation in 82% (9/11) of patients and at 1 9% of sites studied. These results suggest that thalamic microstimulat ion can evoke thermal/pain sensations reproducibly across patients. 3. Thermal/pain sensations were evoked more frequently by stimulation at sites in the posteroinferior region (30%) than by stimulation at site s in the core region (5%). Nonpainful thermal sensations composed the majority of thermal/pain sensations evoked by stimulation in both the core (80%) and posteroinferior regions (86%). Sites where stimulation evoked pain and nonpainful cool sensations were found anterior to the area where nonpainful warm sensations were evoked. Thermal/pain sensat ions were evoked at sites located medially near the border between the core and posteroinferior regions. 4. Radiologic techniques were used to determine the presumed nuclear location of stimulation sites. Therm al/pain sensations were evoked less frequently by stimulation in the p art of Vc included in the core region than by stimulation in any of th e following: the part of Vc included in the posteroinferior region, ve ntralis caudalis portae nucleus, ventralis caudalis parvocellularis nu cleus, or the white matter underlying the ventral nuclear group. 5. Th e location of the sensation evoked by stimulation [projected field (PF )] varied widely in size. PFs were categorized as large if they involv ed more than one part of the body (e.g., face and arm) or if they cros sed at least one joint proximal to the metacarpophalangeal joint or to the metatarsophalangeal joint. PFs were more frequently large at site s where thermal/pain sensations were evoked by TMS (33%) than at those where paresthesia were evoked (6%). 6. PFs were referred to subcutane ous structures more frequently for stimulation sites located in the po steroinferior region (50%) than in the core region (28%), where PFs we re often referred to cutaneous structures. 7. These results demonstrat e that the sensations evoked by stimulation in the posteroinferior reg ion are significantly different from those evoked by stimulation in th e cutaneous core of Vc. TMS in the posteroinferior region evokes therm al sensations or pain often referred to large receptive fields and sub cutaneous structures. These results suggest that neural elements in th e posteroinferior region and in the posterior inferior aspect of the c ore region are involved in the perception of nonpainful thermal sensat ions as well as pain.