Neuromagnetic auditory steady state response to chords: effect of frequency ratio

Perceptual degree of consonance or dissonance of a chord is known to be varied as a function of frequency ratio between tones composing the chord. It has been indicated that generation of a sense of dissonance is associated with the auditory steady-state response (ASSR) phase-locked to difference frequencies which are salient in the chords with complex frequency ratios. This study further investigated how the neuromagnetic ASSR would be modulated as a function of the frequency ratio when the acoustic properties of the difference frequency, to which the ASSR was synchronized, was identical in terms of its number, energy and frequency. Neuronal frequency characteristics intrinsic to the ASSR were compensated by utilizing responses to a SAM (Sinusoidally Amplitude Modulated) chirp tone sweeping through the corresponding frequency range. The results showed that ASSR was significantly smaller for the chords with simple frequency ratios than for those with complex frequency ratios. It indicates that the basic neuronal correlates underlying the sensation of consonance/dissonance might be associated with the attenuation rate applied to encode the input information through the afferent auditory pathway. Attentional gating of the thalamo-cortical function might also be one of the factors.