Psychophysical tuning curves for combination tones 2f1-f2 and f2-f1.

A simultaneous masking paradigm was employed to demonstrate that combination tones are "stimulus-like" in nature. Three musically sophisticated listeners adjusted the level of pure tones at various frequencies to just mask a pulsing signal corresponding to a tone at 1200 Hz and 20 dB SPL. The signal was produced by four stimulus conditions: (1) a pulsing tone at 1200 Hz; (2) a combination tone at 2f1-f2 = 1200 Hz produced by a continuous f1 at 1650 Hz and a pulsing f2 at 2100 Hz; (3) a control for condition 2 with a pulsing tone at 1200 Hz and a continuous f1 at 1650 Hz; and (4) a combination tone at f2-f1 = 1200 Hz produced by a continuous f1 at 1900 Hz and a pulsing f2 at 3100 Hz. Across subjects and stimulus conditions the shapes of the tuning curves were similar; whether for an externally presented single tone or a combination tone, minimum masker level (which corresponds to the tuning-curve "tip," the frequency at which masking is most effective) was always at frequencies near 1200 Hz. These findings are consistent with previous psychophysical as well as recent physiological evidence which indicates that, regardless of how generated, each of the combination tones 2f1-f2 and f2-f1 is present in the motion of the basilar membrane in the cochlear region around the characteristic place of the combination-tone frequency.