A case study of monaural diplacusis

A detailed examination of a case of monaural diplacusis is reported. Low-intensity pure tones presented within a certain frequency range do not sound "pure"; instead, the percept is that of "roughness", "multiple tones" or "beats". In addition, an aftertone is heard upon the cessation of certain tones. Psychophysical experiments (e.g., simultaneous masking, best beats and pitch matching) suggest that the monaural diplacusis results from an interaction between the external tone and an internal tone. The internal tone, however, does not appear to be manifest as a spontaneous oto-acoustic emission.