Sources of cortical responses to painful CO2 laser skin stimulation of the hand and foot in the human brain

Objectives: To investigate whether the same dipolar model could explain the scalp CO2 laser evoked potential (LEP) distribution after either hand or foot skin stimulation. Methods: LEPs were recorded in 14 healthy subjects after hand and foot skin stimulation and brain electrical source analysis of responses obtained in each individual was performed. Results: A 5 dipolar sources model explained the scalp LEP topography after both hand and foot stimulation. In particular, we showed that the co-ordinates of the two earliest activated dipoles were compatible with source locations in the upper bank of the Sylvian fissure on both sides. These sources did not change their location when the stimulation site was moved from the upper to the lower limb. The other 3 dipoles of our model were activated in the late LEP latency range with a biphasic profile and a location compatible with activation of the cingulate gyrus and deep temporo-insular structures. Conclusions: The dipolar model previously proposed for the hand stimulation LEPs can also satisfactorily explain the LEP distribution obtained after foot stimulation. The earliest activated Sylvian dipolar sources did not change their location when the upper or lower limb was stimulated, as expected from the close projections of hand and foot in the second somatosensory area. No source in the primary somatosensory area was necessary to model the scalp topography of LEPs to hand and foot stimulation.