Differential coding of pain intensity in the human primary and secondary somatosensory cortex

The primary (SI) and secondary (SH) somatosensory cortices have been shown to participate in human pain processing. However, in humans it is unclear h ow SI and SH contribute to the encoding of nociceptive stimulus intensity. Using magnetoencephalography (MEG) we recorded responses in Sl and SII in e ight healthy humans to four different intensities of selectively nociceptiv e laser stimuli delivered to the dorsum of the right hand. Subjects' pain r atings correlated highly with the applied stimulus intensity. Activation of contralateral SI and bilateral SH showed a significant positive correlatio n with stimulus intensity. However, the type of dependence on stimulus inte nsity was different for SI and SH. The relation between SI activity and sti mulus intensity resembled an exponential function and matched closely the s ubjects' pain ratings. In contrast, SH activity showed an S-shaped function with a sharp increase in amplitude only at a stimulus intensity well above pain threshold. The activation pattern of SI suggests participation of SI in the discriminative perception of pain intensity. In contrast, the all-or -none-like activation pattern of SII points against a significant contribut ion of SH to the sensory-discriminative aspects of pain perception. Instead , SH may subserve recognition of the noxious nature and attention toward pa inful stimuli.