Effect of the spectral envelope on the auditory evoked N100m

The spectral envelope is one of the factors that are employed for the analysis of sounds in the central auditory system. To clarify the effect of the spectral envelope on sound processing in humans, we recorded auditory evoked magnetic responses to binaurally presented 24 periodic complex sounds in a passive listening condition. Stimuli were finely matched in all acoustic parameters except two different fundamental frequencies (110 and 220 Hz) and spectral envelopes with three different categories (vocal, instrumental, and linear). The peak latency, amplitude, and localization of the N100m component were analyzed. The peak latencies were significantly longer for the fundamental frequency of 110 than 220 Hz. The source strength for the vocal and the instrumental sounds was significantly larger than that for the linear sounds, and the effect was more pronounced in the left hemisphere than in the right. Equivalent current dipoles (ECDs) were localized around the Heschl's gyrus in both hemispheres, and the dipole locations for the vocal and the instrumental sounds were significantly anterior to those for the linear sounds. These findings suggest that complexity in the spectral envelope may enhance sound processing reflected in the N100m, and especially in the left hemisphere.