Parallel activation of primary and secondary somatosensory cortices in human pain processing

Cerebral processing of pain has been shown to involve primary (SI) and secondary (SII) somatosensory cortices. However, the temporal activation pattern of these cortices in nociceptive processing has not been demonstrated so far. We therefore used whole-head magnetoencephalography to record cortical responses to cutaneous laser stimuli in six healthy human subjects. By using selective nociceptive stimuli our results confirm involvement of contralateral SI and bilateral SII in human pain processing. Beyond they show for the first time simultaneous activation onset of contralateral SI and SII after ~130 ms, indicating parallel thalamocortical distribution of nociceptive information. This contrasts to the serial cortical organization of tactile processing in higher primates and instead corresponds to the parallel cortical organization in lower primates and nonprimates. Thus our finding suggests preservation of the basic mammalian parallel organizational scheme in human pain processing, whereas in the tactile modality parallel organization appears to be abandoned in favor of a serial processing scheme. Functionally, preservation of direct access to SII underscores the relevance of this area in human pain processing, probably reflecting an important role of SII in nociceptive learning and memory.