inaudible high-frequency sounds with a nonstationary structure may cause non-negligible effects on the human brain when coexisting with audible low-frequency sounds. We term this phenomenon the “hypersonic effect” and the sounds introducing this effect the “hypersonic sound.” We do not think that the hypersonic effect is specific to the sound material used in the present study because we previously confirmed, by EEG analysis, that the same effect can be introduced by different sound sources containing a significant amount of nonstationary HFCs (e.g.,
Oohashi et al. 1994).
In contrast to the fact that the primary auditory cortex in the bilateral temporal lobes was similarly activated by FRS and HCS, it is noteworthy that the brain stem and thlamic foci activated by the presentation of FRS showed a decrease in rCBF when HCS was presented, as shown in Fig.
5. This finding suggests that
these areas may not belong to the conventional auditory perception system. Moreover, it is the combined presentation of HFCs and LFCs, not HFCs alone, that specifically induces the enhancement of alpha-EEG and activation in the deep-lying structures. We interpret these findings to mean that the hypersonic effect does not simply result from a neurophysiological response to isolated frequencies above an audible range, but from a more complex interaction to which HFCs and LFCs both contribute.