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Correct tonotopic representation is necessary for complex pitch perception Andrew J. Oxenham, Joshua G. W. Bernstein, and Hector Penagos Proceedings of the National Academy of Siences, USA, published online
before print Jan 12, 2004 (pnas.0306958101) Speech and Hearing Bioscience and Technology Program, Harvard-MIT Division
of Health Sciences and Technology, and Research Laboratory of Electronics,
Massachusetts Institute of Technology, Cambridge, MA 02139, USA 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. (Bold text emphasis by Martin Braun) Comment: Experimental subjects could determine the pitch of a complex sound, if three partials of 300, 400, and 500 Hz were presented, but not, if three high-frequency (HF) carriers of 4, 6.35, and 10.08 kHz were amplitude modulated by 300, 400, and 500 Hz, respectively. The results show that temporal information alone is not always sufficient to elicit a perception of pitch. More importantly, these findings add considerably to the constraints on a physiological model of pitch. Currently, there are two such models. Both have in common that the pitch period is detected by period-tuned neurons in the central nucleus of the inferior colliculus (ICC). Langner (1997) suggested that the sound's envelop period is encoded in delay responses of the cochlear nuclei, then auto-correlated in the ICC, and finally detected by the period-tuned neurons there. Braun (1999) suggested that lamina-based, tonotopic cross-correlation of phase-locked partial periods in the ICC produces supra-threshold action potentials in period-tuned neurons. According to the first model, the experimental subjects in the study of Oxenham et al. should have heard a pitch from a complex of modulated HF carriers, because the sound signal contained a clear envelop period. According to the second model, however, the subjects should not have heard a pitch, because the signal could not elicit a lamina-based tonotopic cross-correlation between the three low-frequency components. In other words, only the second model is compatible with the new results. (Comment Martin Braun) |