1. We have studied the sensations evoked by threshold microstimulation (TMS) in the area of the human principal sensory nucleus of the thalamus [ventralis caudalis (Vc)] in patients (n = 11) undergoing stereotactic surgery for the treatment of movement disorders and pain. Preoperatively, patients were trained to describe somatic sensory stimuli using a standard list of descriptors. This same list was used to describe sensations evoked intraoperatively by thalamic microstimulation. Stimulation sites (n = 216) were defined by location within the area where the majority of cells had a reproducible response to innocuous cutaneous stimulation (core region) or in the cellular area posterior and inferior to the core region (posteroinferior region). 2. TMS-evoked sensations were categorized as paresthetic if the descriptors 'tingle,' 'vibration,' or 'electric current' were chosen by the patient to describe the sensation and as thermal/pain if the descriptors 'cool,' 'warm,' 'warm and cool,' or 'pain' were chosen. Thermal/pain sensations were evoked by stimulation in 82% (9/11) of patients and at 19% of sites studied. These results suggest that thalamic microstimulation 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 sites 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 sensations 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. Thermal/pain sensations were evoked less frequently by stimulation in the part of Vc included in the core region than by stimulation in any of the following: the part of Vc included in the posteroinferior region, ventralis caudalis portae nucleus, ventralis caudalis parvocellularis nucleus, or the white matter underlying the ventral nuclear group. 5. The location of the sensation evoked by stimulation [projected field (PF)] varied widely in size. PFs were categorized as large if they involved more than one part of the body (e.g., face and arm) or if they crossed at least one joint proximal to the metacarpophalangeal joint or to the metatarsophalangeal joint. PFs were more frequently large at sites where thermal/pain sensations were evoked by TMS (33%) than at those where paresthesia were evoked (6%). 6. PFs were referred to subcutaneous structures more frequently for stimulation sites located in the posteroinferior region (50%) than in the core region (28%), where PFs were often referred to cutaneous structures. 7. These results demonstrate that the sensations evoked by stimulation in the posteroinferior region are significantly different from those evoked by stimulation in the cutaneous core of Vc. TMS in the posteroinferior region evokes thermal sensations or pain often referred to large receptive fields and subcutaneous structures. These results suggest that neural elements in the posteroinferior region and in the posterior inferior aspect of the core region are involved in the perception of nonpainful thermal sensations as well as pain.
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