FUNCTIONAL MRI STUDY OF THALAMIC AND CORTICAL ACTIVATIONS EVOKED BY CUTANEOUS HEAT, COLD, AND TACTILE STIMULI

Positron emission tomography studies have provided evidence for the in volvement of the thalamus and cortex in pain and temperature perceptio n. However, the involvement of these structures in pain and temperatur e perception of individual subjects has not been studied in detail wit h high spatial resolution imaging. As a first step toward this goal, w e have used functional magnetic resonance imaging (fMRI) to locate dis crete regions of the thalamus, insula, and second somatosensory cortex (S2) modulated during innocuous and noxious thermal stimulation. Resu lts were compared with those obtained during tactile stimulation of th e palm. High resolution functional images were acquired on a 1.5 T ech ospeed GE MR system with an in-plane resolution of 1.7 mm. A modified peltier-type thermal stimulator was used to deliver innocuous cool and warm and noxious cold and hot stimuli for 40-60 s to the thenar emine nce of normal male and female volunteers. Experimental paradigms consi sted of four repetitions of interleaved control and task stimuli. A pi xel by pixel statistical analysis of images obtained during each task versus control (e.g., noxious heat vs, warm, warm vs. neutral temperat ure, etc.) was used to determine task-related activations. Painful the rmal stimuli activated discrete regions within the lateral and medial thalamus, and insula, predominantly in the anterior insula in most sub jects, and the contralateral S2 in 50% of subjects. The innocuous ther mal stimuli did not activate the S2 in any of the subjects but activat ed the thalamus and posterior insula in 50% of subjects. By comparison , innocuous tactile stimulation consistently activated S2 bilaterally and the contralateral lateral thalamus. These data also demonstrate th at noxious thermal and innocuous tactile-related activations overlap i n S2. The data also suggest that innocuous and noxious-related activat ions may overlap within the thalamus but may be located in different r egions of the insula. Therefore, we provide support for a role of the anterior insula, S2, and thalamus in the perception of pain; whereas t he posterior insula appears to be involved in tactile and innocuous te mperature perception. These data demonstrate the feasibility of using fMRI for studies of pain, temperature, and mechanical stimuli in indiv idual subjects, even in small regions such as thalamic nuclei. However , the intersubject variability should be considered in future single s ubject imaging studies and studies that rely on averaged group respons es.