Some systematic observations on monaural diplacusis.
Anomalies of monaural pitch perception, including pitch changes with level, roughness and beats, were mapped in the intensity-frequency plane by a listener using continuous-tone stimulation between approximately 1 900 and 2 750 Hz. A narrow region characterized by rapid threshold adaptation was also mapped between approximately 2 590 and 2 690 Hz. In the procedure used to localize the internally generated tones, which were interacting with the externally presented single tones to produce beats, the listener adjusted the frequency of a signal presented to this right ear so that the beat rate heard there was the same as a criterion beat rate produced by two external tones in his left ear. A plot in the intensity-frequency plane of adjustments for a constant-beat rate revealed a remarkable correspondence to the region of adaptation (between 2 590 and 2 690 Hz) in that the former curve was simply shifted in frequency away from the latter by an amount equal to the two-tone interval producing the beat rate. Thus, the locations of 'internal tones' correspond predictably to both the lower and upper-frequency boundaries of the region of adaptation. Pitch changes, as a function of signal level, at frequencies below the region of adaptation were also investigated and appear to reflect a change from interaction to cessation of interaction between the signal and the region of adaptation. The locus of these pitch-related abnormalities, over a broad range of frequencies, points to interaction between that range and a smaller-frequency region where abrupt transitions in sensitivity occur.