Articles in PresS . J Neurophysiol ( November 5 , 2003 ) . 10 . 1152 / jn . 00912 . 2003

Negative and positive laser evoked potential (LEP) peaks (N2*, P2**) were simultaneously recorded from the primary somatosensory (SI), parasylvian, and medial frontal (MF – anterior cingulate and supplementary motor area) cortical surfaces through subdural electrodes implanted for the surgical treatment of intractable epilepsy. Distribution of the LEP N2* and P2** peaks was estimated to be in cortical areas (SI, parasylvian and MF) identified by anatomic criteria, by their response to innocuous vibratory stimulation of a finger (v-SEP), and to electrical stimulation of the median nerve (e-SEP). The maximum of the LEP N2* peak was located on the CS, medial (dorsal) to the finger motor area, as determined by cortical stimulation, and to the finger somatosensory area, as determined from the e-SEP and v-SEP. This finding suggests that the generator source of the LEP N2* peak in SI was different from that of e-SEP or v-SEP in Brodmann’s areas 3b or 1. In parasylvian and MF, polarity reversal was often observed, indicating tangential current sources in these regions. In contrast to e-SEP and v-SEP, the LEP N2* latency over SI was not shorter than that over the parasylvian region. The amplitude of N2* was larger over SI than over MF and the latencies of the LEP peaks in those two regions were different. These findings provide evidence for a significant LEP generator in the post-central gyrus, perhaps SI cortex, that is situated outside the tactile homunculus in SI and that receives its input arising from nociceptors simultaneously with parasylvian and MF cortex.