Operculo-insular responses to nociceptive skin stimulation in humans (a review).

CO2 laser stimulation selectively activates the endings of small myelinated A delta fibers, involved with non-myelinated C fibers in the processing of nociceptive information. Thus, potentials evoked by CO2 laser stimulation reflect the activation of cortical areas receiving inputs from the spinothalamic tract. In this article we review data on the early pain-related CO2 laser evoked potentials recorded on the scalp, or by intracortical electrodes, during presurgical assessment of patients with drug-resistant epilepsy. A combination of surface and depth recordings allows the description of early cortical pain responses in terms of latency, polarity and scalp topography. Such a technique also allows the localization of the anatomical generators of these early responses using dipolar source modeling of scalp-recorded evoked potentials, or intracortical recordings, in stereotactical conditions. The earliest response recorded on the scalp to CO2 laser stimulation was an N1-P1 dipolar potential field at a latency of 140-200 ms. The N1 and P1 maximal voltages are recorded in the temporal region contralateral to stimulation cmd mid-frontal region, respectively Intracerebral electrodes record nn activation of a dipolar cortical source in the same latency range located in the upper bank of the sylvian fissure, corresponding to the second somatosensory (SII) area ipsi- and contralateral to the stimulation and insular cortex. The SII-insular responses ipsilateral to stimulation ore likely to be triggered via transcallosal fibers coming from the opposite SII area. The operculo-insular cortex contralateral to stimulation, activated through direct thalamocortical projections, is likely to represent the first step in the cortical processing of peripheral A delta fiber pain inputs. (C) 1999 Editions scientifiques et medicales Elsevier SAS.