Abstract
It is widely accepted that the perception of pure tone is mostly determined in the early sites of the auditory pathway: e. g., at the external acoustic meatus pathway and the basilar membrane. We measured EEG signals for pure tones that have the same A-weighted sound pressure level (73 dB) at several frequencies. Correction of the amplitude with A-weighted sound pressure level allows us to isolate the effects that occur at the upper cognitive sites from those at earlier sites, since the A-weighted sound pressure level compensates for the gain lost at the earlier sites. Eight normal subjects participated in the experiment. We measured alpha wave fluctuation and alpha power spectra that responded to the pure tones. We used pure tones at 12 different frequencies as test stimuli and white noise as a control stimulus. Alpha wave fluctuation was calculated from the slope of the fluctuation spectrum, obtained from the Fp1 and Fp2 electrodes. The results showed that the slope varied depending on the frequency of the pure tones and formed a U-shaped curve as a function of the frequency of the pure tones. The trough of the curve was around 1-2kHz and became close to zero as the frequency increased/decreased, suggesting that the pure tones around 1-2kHz induce the largest comfort and lowest arousal. The alpha power spectrum peaked at around 0.4-5.0kHz. Since the alpha power spectrum increases when subjects feel relaxed, it appears that pure tones around 1-4kHz induce relaxation, consistent with the results of alpha wave fluctuation.
