The incidence and response properties of nociresponsive neurons, their locations relative to spinothalamic terminals, and their relations to cytoarchitectonic borders were studied in the lateral thalamus of the squirrel monkey. Nociceptive neurons were found in ventral posterior inferior nucleus (VPI), in the lateral and medial nuclei (VPL and VPM) of the ventral posterior complex (VP = VPL + VPM), as well as the posterior complex (PO). The overall incidence of nociresponsive cells was 19% (50 of 270 cells). The proportion of nociresponsive neurons within VPI was 50% (23 of 46), 38% in PO (8 of 21), and 10% in VP (19 of 203). Most nociresponsive cells (90%) in VP were of wide-dynamic- range type, while within VPI 43% of nociresponsive cells were nociceptive-specific type. Most of these nociresponsive cells had thermal and mechanical responses, and a small number also responded to cooling. The receptive fields of nociresponsive cells in VPL were in continuity, in both size and body location, with surrounding low- threshold units. The receptive fields of VPI and PO nociresponsive cells were larger than those in VPL. The probability of encountering nociresponsive cells located within 100 microns of spinothalamic terminations was high in VPI (73%) and low in VPL (33%). On the other hand, the probability of encountering non-nociceptive cells located within 100 microns of spinothalamic terminals was low in both VPI (20%) and VPL (26%). The results indicate segregation of nociresponsive cell types across VP, VPI, and PO and suggest that VPI, and perhaps PO, is an important region for discriminative processing and perception of painful stimuli.

The termination sites of the dorsolateral (DSTT) and ventral (VSTT) spinothalamic pathways were determined by using anterograde transport of horseradish peroxidase from the lumbar spinal cord in primates. One animal had no spinal cord lesion, while of two other animals, one received a midthoracic dorsolateral funiculus lesion, and the other received a midthoracic ventral quadrant lesion contralateral to the injection. The thalamic label in the animal with no spinal cord lesion was much less than the label in the two animals with spinal lesions. Moreover, in the animals with spinal lesions, HRP-labeled cells were found within the thalamus. Therefore, the remaining six animals received ipsilateral hemisections and bilateral dorsal column lesions, irrespective of the contralateral lesions. The thalamic label in the animals without contralateral lesions were assumed to represent the total spinothalamic input to the diencephalon. In these animals, label was located mainly in suprageniculate and pulvinar oralis, caudal and oral divisions of ventral posterior lateral nucleus, the lateral half of ventral posterior inferior nucleus, and zona incerta, while in the medial thalamus label was primarily in two distinct bands in medial dorsal nucleus and in the posterior dorsal portion of central lateral nucleus. Scattered lighter labeling was found in other thalamic nuclei. The pattern of terminal labeling observed in the ventral posterior lateral region was arranged in patches, while elsewhere in the thalamus a more uniform labeling pattern was observed. The thalamic label in animals with contralateral ventral quadrant lesions represented the terminations of the DSTT, while the label in animals with contralateral dorsolateral funiculus lesions represented VSTT terminations. The labeling pattern was similar between these two groups. However, there were small differences between them. These results indicate that DSTT and VSTT terminations largely overlap and innervate the lateral and medial thalamamus.

In a recent functional magnetic resonance imaging study (fMRI), we reported the cortical areas activated in a thermal painful task and compared the extent of overlap between this cortical network and those activated during a vibrotactile task and a motor task. In the present study we examine the temporal properties of the cortical activations for all three tasks and use linear systems identification techniques to functionally differentiate the cortical regions identified in the painful thermal task. Cortical activity was examined in the contralateral middle third of the brain of 10 right-handed subjects, using echo-planar imaging and a surface coil. In another eight subjects the temporal properties of the thermal task were examined psychophysically. The fMRI impulse response function was estimated from the cortical activations in the vibrotactile and motor tasks and shown to correspond to earlier reports. Given the fMRI impulse response function and the time courses for the thermal stimulus and the associated pain ratings, predictor functions were generated. The correlation between these predictor functions and cortical activations in the painful thermal task indicated a gradual transition of information processing anteroposteriorly in the parietal cortex. Within this region, activity in the anterior areas more closely reflected thermal stimulus parameters, whereas activity more posteriorly was better related to the temporal properties of pain perception. Insular cortex at the level of the anterior commissure was the region best related to the thermal stimulus, and Brodmann's area 5/7 was the region best related to the pain perception. The functional implications of these observations are discussed.

The present study examines relationships between regional brain chemistry (as identified by localized in vivo three-dimensional single-voxel proton magnetic resonance spectroscopy (

The response properties of 106 visceroceptive lateral thalamic neurons were investigated in anesthetized squirrel monkeys. Most neurons were located in the ventral posterior lateral nucleus (VPL), and a smaller number of cells was also found in a variety of thalamic nuclei around VPL. Ninety (85%) of these cells responded to distension of the urinary bladder, the distal colon, and/or the lower esophagus. The majority of the visceral-responsive cells also had convergent somatic and multivisceral responses (71% of the 85%). A small population (6%) was visceral specific; that is, these neurons were not activated with somatic stimuli. Visceral responses were excitatory, inhibitory, or mixed, and most were either visceral nociceptive specific (65%) or visceral wide-dynamic-range type (34%). Very few visceral responses (1%) could be classified as low threshold. The incidence of these response types were highly dependent on the specific viscus stimulated. Most visceral responses were able to code stimulus duration and intensity. The majority (69%) of the visceroceptive neurons had somatic low-threshold convergent input mainly from the surface of the lower body. The somatic receptive field locations and the somatic response properties seem unrelated to the convergent visceral input and the visceral response properties, although there were some exceptions. No obvious viscerotopical organization was found in VPL. The results lead us to propose two different modes of representation for processing of and distinguishing between visceral and somatic inputs: a distributed population code for visceral inputs, and a local code for somatic inputs. Based on these codes, we discuss a new hypothesis for referred pain.