Scalp field potentials of human pain: spatial effects and temporal relation in finger stimulation

In the present study, the spatial extent and temporo-spatial correlations of the human brain responses were investigated by electrically stimulating thumb (D1) and little finger (D5) under painful and non-painful intensity levels. High-density (124-ch) somatosensory evoked potentials (SEPs) were recorded (-50 to +450 ms) from 15 healthy male volunteers. Early (0-50 ms), and late phases (150-450 ms) of the responses were analyzed. Peak stages 33 ms, 42 ms, 210 ms, 328 ms for D1; and 33 ms, 44 ms, 240 ms, 350 ms for D5 were isolated from a compressed waveform; the relationships within stages and phases were investigated using topography, Focal maximum Amplitude (FA, single central site at 0.2 cm2) and Area Magnitude (AM, summated FA and amplitudes of neighboring four sites in a region of nearly 9.9 cm2 area). In the early phase, the response of FA at 33 ms was significantly higher during painful than non-painful stimulation for both D1 and D5. In the late phase D1 (210 ms and 328 ms), and D5 (240 ms and 350 ms) the spatial areas of activation were significantly enlarged from non-painful to painful stimulation. For temporo-spatial relationship, D1 at 33 ms of early phase showed a significant correlation between the negative maximal site (sink) and the positive maximal site (source) under both non-painful and painful stimulations. For the late phase, the AM potentials at N2 correlated with that of P2 for both D1 and D5 under painful stimulation. The focal effects in FA of contralateral early potential indicates a shallow dipole in the primary somatosensory area of SI, while the large spatial extent in AM indicates a deep dipole of the putative cingulum activation under painful stimulation. No correlation between early and late activities implied that both activations are operated independently at the early SI and late cingulate processing of evoked pain.