Subcortical structures involved in pain processing: evidence from single-trial fMRI.

Pain is processed in multiple cortical and subcortical brain areas. Subcortical structures are substantially involved in different processes that are closely linked to pain processing, e.g. motor preparation, autonomic responses, affective components and learning. However, it is unclear to which extent nociceptive information is relayed to and processed in subcortical structures. We used single-trial functional magnetic resonance imaging (fMRI) to identify subcortical regions displaying hemodynamic responses to painful stimulation. Thulium-YAG (yttrium-aluminum-granate) laser evoked pain stimuli, which have no concomitant tactile component, were applied to either hand of healthy volunteers in a randomized order. This procedure allowed identification of areas displaying differential fMRI responses to right- and left-sided stimuli. Hippocampal complex, amygdala, red nucleus, brainstem and cerebellum were activated in response to painful stimuli. Structures related to the affective processing of pain showed bilateral activation, whereas structures involved in the generation of withdrawal behavior, namely red nucleus, putamen and cerebellum displayed differential (i.e. asymmetric) responses according to the side of stimulation. This suggests that spatial information about the nociceptive stimulus is made available in these structures for the guidance of defensive and withdrawal behavior.