Functional differences between auditory cortices of the two hemispheres revealed by whole‐head neuromagnetic recordings

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Abstract
We recorded auditory evoked magnetic fields from nine healthy subjects with a 122‐channel whole‐head SQUID magnetometer. This type of measurement ensures that responses from both hemispheres are collected in the same stage of vigilance. The stimuli were 50‐ms 1‐kHz tones, delivered alternately to the two ears at interstimulus intervals (ISIs) of 1, 2, 4, 8, and 16 s per ear. In both hemispheres, the prominent 100‐ms response (N100m) was, on average, 22% larger and 9 ms earlier for contralateral than ipsilateral tones. The N100m amplitude increased as a function of ISI and saturated at ISIs of 8–16 s. The average ISI dependence of N100m was similar over both hemispheres and for both contralateral and ipsilateral stimuli, implying that tones leave neural traces of similar duration in both hemispheres. Differences were observed as well: the response waveforms were clearly asymmetric in one subject, and four subjects displayed an additional right‐hemisphere‐dominant 250‐ms deflection (N250m).

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