Source current density mapping of middle latency response and slow vertex response using a source derivation technique

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Source current density (SCD) waveforms were obtained from potential waveforms of the middle latency response (MLR) and slow vertex response (SVR) over the human scalp using a source derivation technique. The SCD waveforms were different from the potential waveforms in the peak latency and polarity. The SCD map of the Pa component in MLR might be composed of more than one generator spatially : the bilateral dipoles in the temporal region and small dipole in the frontal region contralateral to stimulation. The SCD maps of Ni and P2 in SVR suggested bilateral generators in the temporal region as previously reported. However, the two were not identical because of there being a difference in the spatial and temporal patterns between the SCD maps of N1 and P2.By subtracting the SCD waveform from the potential, the low spatial frequency component(LSFC) was obtained. The latencies of both N1 and P2 in the SCD waveform were decreased in comparison to the potential waveform on the scalp. In contrast, the latencies in the LSFC waveform were prolonged. The source region of N1 in the SCD maps appeared initially in the vicinity of the temporal region, contralateral to the stimulated ear, and with time, appeared as restricted source regions in both temporal regions. The maps of P2 in the SCD and LSFC showed a sink region located in both temporal areas, and source regions in the vicinity of the vertex. This P2 sink-source pattern did not change with time.It was concluded that the N1 might be generated from the bilateral temporal regions including contributions from moving deep dipoles. The P2 was thought to originate from an anatomically fixed location in the bilateral temporal sites.