Sequential mapping favours the hypothesis of distinct generators for Na and Pa middle latency auditory evoked potentials.

The temporo-spatial organization of Na and Pa middle latency auditory components evoked by monaural clicks delivered separately to right and left ears was assessed by sequential mapping of scalp potentials. The potential field distribution was found to be different for the two components and was assessed by calculating the maximal potential differences in the Na/Pa time period. These data are compatible with the hypothesis that distinct generators are responsible for the two components. Scalp potential field configuration observed for Na suggests a deep generator, which could be situated at the mesencephalic or diencephalic level. Bilateral cortical generators tangentially orientated satisfactorily account for the distribution of the Pa potential field, which could be related to simultaneous activation of both supratemporal auditory cortices in response to monaural stimulation.