Representation of

Although the role of somatosensory cortex (SI) in the mapping of body parts has been well documented,1–3 its involvement in the processing of thermal (hot and cold) sensations remains poorly defined. Clinical and physiological evidence suggests that SI, which is divided into Brodmann’s areas, 1,2, 3a, and 3b, may be involved in thermal and even pain processing.4–7 Cerebral evoked response studies on humans6,7 and studies of single cortical units in monkeys8 and rats9 support a role for SI in the perception of thermal stimuli. In addition, selective SI ablation studies in rats and clinical studies of humans sustaining damage to SI cortex have shown disturbances in the performance of thermal discrimination tasks.10 More recently, single unit recording studies in cats, rats, and monkeys have revealed that some neurons in SI receive a nociceptive input.11 Despite this progress in understanding mechanisms in thermatosensory processing, very little is known about the precise representation in SI for humans. Thus, it is not clear whether thermal information is transmitted within the cerebral cortex as a generalized message or converges on focal areas associated with somesthetic information (somatotopy). Neuro-imaging techniques have opened new avenues to investigate the central processing of nociceptive input in the awake human. PET studies have identified SI activation during application of painful heat stimuli applied to the contralateral limb12,13 and recently, SI activation during noxious heat stimulation of the contralateral digits has been shown by fMRI.14 In this study, we applied fMRI to investigate the role of SI in higher order processing of nociceptive thermal stimuli using stimulations of two body parts (hand and foot) and two thermal sensations (cool/cold and warm/hot). Our goal is to determine whether representations of thermal sensation are ordered in a somatotopic or a distributed fashion in SI.