Objective: The human middle latency auditory evoked magnetic fields were recorded with different interstimulus intervals (ISI) to investigate the differential natures of P30m and the P50m, including whether the P50m source was spatially different or not from the P30m source. Methods: Twenty right-handed healthy subjects participated in the experiment. Auditory magnetic responses were recorded in the 0.5 s ISI (ISI were between 0.4 and 0.6 s) and the 1.5 s ISI conditions (ISI were between 1 and 2 s). Tone bursts were presented to the right ears 880 times consecutively for each condition. The P30m and the P50m responses were investigated, and the dipole source localization was performed. Results: The P50m latency was significantly prolonged, while the P30m latency did not vary in the shorter ISI. Both P50m and P30m amplitudes were significantly reduced in the shorter ISI. The P50m was located significantly more anteriorly than P30m. Conclusions: These results suggest the existence of differential characteristic and spatially different magnetic responses in the middle latency range. Significance: This study has revealed one aspect of the different natures between P30m and P50m, and may provide a key for auditory perceptional processes in humans.
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