The posterior insular-opercular region and the search of a primary cortex for pain.

To be considered specific for nociception, a cortical region should: (a) have plausible connections with ascending nociceptive pathways; (b) be activated by noxious stimuli; (c) trigger nociceptive sensations if directly stimulated; and (d) tone down nociception when injured. In addition, lesions in this area should have a potential to develop neuropathic pain, as is the case of all lesions in nociceptive pathways. The single cortical region approaching these requirements in humans encompasses the suprasylvian posterior insula and its adjoining medial operculum (referred to as "PIMO" in this review). This region does not contain, however, solely nociceptive networks, but represents in primates the main sensory receiving area of the spinothalamic system, and as such contributes to the processing of thermo-sensory, nociceptive, C-fibre tactile, and visceral input. Nociception (and, a fortiori, pain) should therefore not be considered as a separate sensory modality, like vision or audition, but rather as one component of a global system subtending the most primitive forms of somatosensation. Although a clear functional segregation of PIMO sub-areas has not yet been achieved, some preferential distribution has been described in humans: pain-related networks appear preferentially distributed within the posterior insula, and non-noxious thermal processing in the adjacent medial operculum. Thus, spinothalamic sub-modalities may be partially segregated in the PIMO, in analogy with the separate representation of dorsal column input from joint, muscle spindle and tactile afferents in S1. Specificity, however, may not wholly depend on ascending 'labelled lines' but also on cortical network properties driven by intrinsic and extrinsic circuitry. Given its particular anatomo-functional properties, thalamic connections, and tight relations with limbic and multisensory cortices, the PIMO region deserves to be considered as a third somatosensory region (S3) devoted to the processing of spinothalamic inputs.