6 7

35 The cortical potentials evoked by cutaneous application of a laser stimulus (laser evoked 36 potentials, LEP) often include potentials in the primary somatic sensory cortex (S1), which may 37 be located within the subdivisions of S1 including Brodmann areas 3A, 3B, 1 and 2. The precise 38 location of the LEP generator may clarify the pattern of nociceptive activation of human S1 by 39 painful stimuli. We now test the hypothesis that the generators of the LEP are located in human 40 Brodmann areas 1 or 3A within S1. 41 Local field potential (LFP) source analysis of the LEP was obtained from subdural grids 42 over sensorimotor cortex in two patients undergoing epilepsy surgery. The relationship of LEP 43 dipoles was compared to dipoles for somatic sensory potentials evoked by median nerve 44 stimulation (SEP), and recorded in area 3B (see the companion paper). Both patients had an 45 early radial dipole in S1. The LEP dipole was located medial, anterior and deep to the SEP 46 dipole, which suggests a nociceptive dipole in area 3A. One patient had a later tangential dipole 47 with positivity posterior, which is opposite to the orientation of the SEP dipole in area 3B. The 48 reversal of orientations between modalities is consistent with the cortical surface negative 49 orientation resulting from the superficial termination of thalamocortical neurons which receive 50 inputs from the spinothalamic tract. Therefore, the present results suggest that the LEP may 51 result in a radial dipole consistent with a generator in area 3A and a putative later putative 52 tangential generator in area 3B. 53