Correct tonotopic representation is necessary for complex pitch perception

Abstract

The ability to extract a pitch from complex harmonic sounds, such as human speech, animal vocalizations, and musical instruments, is a fundamental attribute of hearing. Some theories of pitch rely on the frequency-to-place mapping, or tonotopy, in the inner ear (cochlea), but most current models are based solely on the relative timing of spikes in the auditory nerve. So far, it has proved to be difficult to distinguish between these two possible representations, primarily because temporal and place information usually covary in the cochlea. In this study, "transposed stimuli" were used to dissociate temporal from place information. By presenting the temporal information of low-frequency sinusoids to locations in the cochlea tuned to high frequencies, we found that human subjects displayed poor pitch perception for single tones. More importantly, none of the subjects was able to extract the fundamental frequency from multiple low-frequency harmonics presented to high-frequency regions of the cochlea. The experiments demonstrate that tonotopic representation is crucial to complex pitch perception and provide a new tool in the search for the neural basis of pitch.

Fig. 1.

Schematic diagram of a pure tone (Upper) and a transposed tone (Lower). The transposed tone is generated by multiplying a high-frequency sinusoidal carrier with a half-wave rectified low-frequency sinusoidal modulator. As a first-order approximation, the peripheral auditory system acts as a half-wave rectifier (HWR) and low-pass filter (LPF), so that the temporal representations of both the low-frequency (LF) pure tone and the high-frequency (HF) transposed tone are similar.

Fig. 2.

Mean performance in frequency discrimination (A) and interaural time discrimination (B) as a function of frequency. Open symbols represent performance with transposed tones (TT) on carrier frequencies ranging from 4,000 to 10,000 Hz. •, performance with pure tones.

Fig. 3.

Behavioral results using harmonic complex tones consisting of either pure tones (filled bars) or transposed tones (open bars). F0 difference limens (A) using transposed tones were not measurable for three of the four subjects. All three subjects tested on an F0 matching task (B) showed good performance for pure tones but no indication of complex pitch perception for the transposed tones.

Fig. 4.

An SACF model (20) representation of harmonic complex tones consisting of either pure tones (solid curve) or transposed tones (dashed curve). Both representations in the model show a clear peak at a 10-ms lag, the inverse of the F0 of 100 Hz. In contrast, the F0 was perceived by human subjects only in the case of the pure tones.