While functional imaging studies in humans have consistently reported activation of primary somatosensory cortex (SI) with painful stimuli, the specific roles of subdivisions of areas 3a, 3b, and 1 within SI during pain perception are largely unknown, particularly in the representation of mechanical evoked pain. In this study, we investigated how modality, location, and intensity of nociceptive stimuli are represented within SI by using high-spatial resolution optical imaging of intrinsic signals in Pentothal-anesthetized squirrel monkeys. Perceptually comparable mechanical nociceptive and innocuous tactile stimuli were delivered by indenting the glabrous skin of the distal finger pads with 0.2 and 2 mm diameter probes, respectively. Within each of areas 3a, 3b, and 1, activations to mechanical nociceptive stimulation of individual distal finger pads were spatially distinct and somatotopically organized. We observed differential cortical activation patterns. Areas 3a, 3b, and 1 were all activated during mechanical nociceptive stimulation and were modulated by nociceptive stimulus intensity. However, with innocuous tactile stimulation, mainly areas 3b and 1 exhibited response modulation with different levels of stimulation. In summary, mechanical nociceptive inputs are area-specific and topographically represented within SI. We propose that all areas of SI are implicated in encoding the features of mechanical nociception, where areas 3a and 3b are distinctively involved in coding nociceptive and pressure sensation components of stimulation.
aDepartment of Radiology and Radiological Science, Vanderbilt University, Nashville, TN 37232, USA
bInstitute of Imaging Science, Vanderbilt University, Nashville, TN 37232, USA
cDepartment of Psychology, Vanderbilt University, Nashville, TN 37203, USA
*Corresponding author. Address: Department of Radiology and Radiological Science, Institute of Imaging Science, Vanderbilt University, 1161 21st Ave. S, Nashville, TN 37232-2310, USA. Tel.: +1 615 936 7069; fax: +1 615 322 0734.
Received May 28, 2008
Received in revised form October 21, 2008
Accepted November 21, 2008.