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Across-channel timing differences as a potential code for the frequency of pure tones.

Carlyon RP, Long CJ, Micheyl C - J. Assoc. Res. Otolaryngol. (2011)

Bottom Line: When a pure tone or low-numbered harmonic is presented to a listener, the resulting travelling wave in the cochlea slows down at the portion of the basilar membrane (BM) tuned to the input frequency due to the filtering properties of the BM.It has been suggested that the auditory system exploits these across-channel timing differences to encode the pitch of both pure tones and resolved harmonics in complex tones.We conclude that although the use of across-channel timing cues provides an a priori attractive and plausible means of encoding pitch, many of the most obvious metrics for using that cue produce pitch estimates that are strongly influenced by the overall level and therefore are unlikely to provide a straightforward means for encoding the pitch of pure tones.

View Article: PubMed Central - PubMed

Affiliation: MRC Cognition & Brain Sciences Unit, 15 Chaucer Rd., Cambridge, CB2 7EF, UK. bob.carlyon@mrc-cbu.cam.ac.uk

ABSTRACT
When a pure tone or low-numbered harmonic is presented to a listener, the resulting travelling wave in the cochlea slows down at the portion of the basilar membrane (BM) tuned to the input frequency due to the filtering properties of the BM. This slowing is reflected in the phase of the response of neurons across the auditory nerve (AN) array. It has been suggested that the auditory system exploits these across-channel timing differences to encode the pitch of both pure tones and resolved harmonics in complex tones. Here, we report a quantitative analysis of previously published data on the response of guinea pig AN fibres, of a range of characteristic frequencies, to pure tones of different frequencies and levels. We conclude that although the use of across-channel timing cues provides an a priori attractive and plausible means of encoding pitch, many of the most obvious metrics for using that cue produce pitch estimates that are strongly influenced by the overall level and therefore are unlikely to provide a straightforward means for encoding the pitch of pure tones.

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Value of the “π shift point” as a function of level and input frequency. The curves shown are derived from the best-fitting model described in the text for input frequencies spaced 3 semitones apart, starting at 250 Hz, and with levels of 50, 70 and 90 dB SPL.
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Fig10: Value of the “π shift point” as a function of level and input frequency. The curves shown are derived from the best-fitting model described in the text for input frequencies spaced 3 semitones apart, starting at 250 Hz, and with levels of 50, 70 and 90 dB SPL.

Mentions: One potential decoding method is illustrated in Figure 9 which shows “broken-stick” fits to stimuli at two frequencies and levels (shown in Fig. 6), shifted so that the horizontal portions are aligned. The shift is performed because the animal has, of course, no idea about “absolute” phase and can only compare the relative phase across fibers. It can be seen that the CF at which the phase is π radians lower than that at the knee point (indicated by the black horizontal dotted lines) appears to be similar at the two levels for each frequency. Figure 10 shows this value, which we term the “π shift point”, as a function of fs for input levels of 50, 70 and 90 dB SPL. It increases monotonically with fs and has modest error bars. A generalized linear model revealed a significant effect of fs (F1,28 = 524, P < 0.0001), but not of level (F1,28 = 0.23, P = 0.633). Unlike the other measures described here, the effect of level, averaged across fs, was much smaller (0.23 octaves) than the effect of fs, which was 2.19 octaves; this, in turn, was close to the range of input frequencies (250–1,189 Hz) which was 2.24 octaves.FIG. 9


Across-channel timing differences as a potential code for the frequency of pure tones.

Carlyon RP, Long CJ, Micheyl C - J. Assoc. Res. Otolaryngol. (2011)

Value of the “π shift point” as a function of level and input frequency. The curves shown are derived from the best-fitting model described in the text for input frequencies spaced 3 semitones apart, starting at 250 Hz, and with levels of 50, 70 and 90 dB SPL.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3298616&req=5

Fig10: Value of the “π shift point” as a function of level and input frequency. The curves shown are derived from the best-fitting model described in the text for input frequencies spaced 3 semitones apart, starting at 250 Hz, and with levels of 50, 70 and 90 dB SPL.
Mentions: One potential decoding method is illustrated in Figure 9 which shows “broken-stick” fits to stimuli at two frequencies and levels (shown in Fig. 6), shifted so that the horizontal portions are aligned. The shift is performed because the animal has, of course, no idea about “absolute” phase and can only compare the relative phase across fibers. It can be seen that the CF at which the phase is π radians lower than that at the knee point (indicated by the black horizontal dotted lines) appears to be similar at the two levels for each frequency. Figure 10 shows this value, which we term the “π shift point”, as a function of fs for input levels of 50, 70 and 90 dB SPL. It increases monotonically with fs and has modest error bars. A generalized linear model revealed a significant effect of fs (F1,28 = 524, P < 0.0001), but not of level (F1,28 = 0.23, P = 0.633). Unlike the other measures described here, the effect of level, averaged across fs, was much smaller (0.23 octaves) than the effect of fs, which was 2.19 octaves; this, in turn, was close to the range of input frequencies (250–1,189 Hz) which was 2.24 octaves.FIG. 9

Bottom Line: When a pure tone or low-numbered harmonic is presented to a listener, the resulting travelling wave in the cochlea slows down at the portion of the basilar membrane (BM) tuned to the input frequency due to the filtering properties of the BM.It has been suggested that the auditory system exploits these across-channel timing differences to encode the pitch of both pure tones and resolved harmonics in complex tones.We conclude that although the use of across-channel timing cues provides an a priori attractive and plausible means of encoding pitch, many of the most obvious metrics for using that cue produce pitch estimates that are strongly influenced by the overall level and therefore are unlikely to provide a straightforward means for encoding the pitch of pure tones.

View Article: PubMed Central - PubMed

Affiliation: MRC Cognition & Brain Sciences Unit, 15 Chaucer Rd., Cambridge, CB2 7EF, UK. bob.carlyon@mrc-cbu.cam.ac.uk

ABSTRACT
When a pure tone or low-numbered harmonic is presented to a listener, the resulting travelling wave in the cochlea slows down at the portion of the basilar membrane (BM) tuned to the input frequency due to the filtering properties of the BM. This slowing is reflected in the phase of the response of neurons across the auditory nerve (AN) array. It has been suggested that the auditory system exploits these across-channel timing differences to encode the pitch of both pure tones and resolved harmonics in complex tones. Here, we report a quantitative analysis of previously published data on the response of guinea pig AN fibres, of a range of characteristic frequencies, to pure tones of different frequencies and levels. We conclude that although the use of across-channel timing cues provides an a priori attractive and plausible means of encoding pitch, many of the most obvious metrics for using that cue produce pitch estimates that are strongly influenced by the overall level and therefore are unlikely to provide a straightforward means for encoding the pitch of pure tones.

Show MeSH