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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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The top row shows a schematic implementation of the local subtraction scheme. A Schematic representation of a phase-vs.-log10 CF curve consisting of a shallow portion plus a steeper portion having a region where the slope is locally maximal. B Output of an array of second-order neurons, whereby the instantaneous output of cells at a given CF are subtracted from that of their neighbour. This subtraction is implemented by subtracting the phases of adjacent channels. The bottom row shows the knee point model. C Phase-vs.-CF function consisting of a shallow and a steep portion. D Output of an array of second-order neurons, whereby the instantaneous output of cells at a given CF are subtracted from that of their neighbour. E Output of an array of third-order neurons operating on the different between adjacent second-order neurons shown in (D).
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Fig3: The top row shows a schematic implementation of the local subtraction scheme. A Schematic representation of a phase-vs.-log10 CF curve consisting of a shallow portion plus a steeper portion having a region where the slope is locally maximal. B Output of an array of second-order neurons, whereby the instantaneous output of cells at a given CF are subtracted from that of their neighbour. This subtraction is implemented by subtracting the phases of adjacent channels. The bottom row shows the knee point model. C Phase-vs.-CF function consisting of a shallow and a steep portion. D Output of an array of second-order neurons, whereby the instantaneous output of cells at a given CF are subtracted from that of their neighbour. E Output of an array of third-order neurons operating on the different between adjacent second-order neurons shown in (D).

Mentions: Perhaps the simplest scheme for estimating the frequency (and therefore, the pitch) of pure tones, that takes advantage of the PT, is one in which an array of “second-order” neurons subtracts the response of each AN fibre from that of a fibre having a “neighbouring” CF. If the function relating phase to CF in the first-order (AN) neurons has a slope that is locally steeper over some region compared with surrounding regions, there will be a corresponding local maximum in the output of the second-order array. This concept is illustrated in Fig. 3, where part A) shows a schematic phase-vs-CF curve and where part B) shows the output of a hypothetical second-order array. It is also illustrated in Figure 2 which shows some schematic “neurograms” to 1,000- and 1,200-Hz pure tones, with the results of a local subtraction algorithm shown to the right of the plot. Note that the neurograms in Figure 2 include the effects of peripheral filtering on response phase, but not on its magnitude (firing rate). Note also that the phase-vs.-CF functions shown in Figure 3 and in all remaining plots in this article show CF increasing from low to high along the abscissa, opposite to the convention adopted by Kim et al. and shown in Figure 1.FIG. 3


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)

The top row shows a schematic implementation of the local subtraction scheme. A Schematic representation of a phase-vs.-log10 CF curve consisting of a shallow portion plus a steeper portion having a region where the slope is locally maximal. B Output of an array of second-order neurons, whereby the instantaneous output of cells at a given CF are subtracted from that of their neighbour. This subtraction is implemented by subtracting the phases of adjacent channels. The bottom row shows the knee point model. C Phase-vs.-CF function consisting of a shallow and a steep portion. D Output of an array of second-order neurons, whereby the instantaneous output of cells at a given CF are subtracted from that of their neighbour. E Output of an array of third-order neurons operating on the different between adjacent second-order neurons shown in (D).
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Related In: Results  -  Collection

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Fig3: The top row shows a schematic implementation of the local subtraction scheme. A Schematic representation of a phase-vs.-log10 CF curve consisting of a shallow portion plus a steeper portion having a region where the slope is locally maximal. B Output of an array of second-order neurons, whereby the instantaneous output of cells at a given CF are subtracted from that of their neighbour. This subtraction is implemented by subtracting the phases of adjacent channels. The bottom row shows the knee point model. C Phase-vs.-CF function consisting of a shallow and a steep portion. D Output of an array of second-order neurons, whereby the instantaneous output of cells at a given CF are subtracted from that of their neighbour. E Output of an array of third-order neurons operating on the different between adjacent second-order neurons shown in (D).
Mentions: Perhaps the simplest scheme for estimating the frequency (and therefore, the pitch) of pure tones, that takes advantage of the PT, is one in which an array of “second-order” neurons subtracts the response of each AN fibre from that of a fibre having a “neighbouring” CF. If the function relating phase to CF in the first-order (AN) neurons has a slope that is locally steeper over some region compared with surrounding regions, there will be a corresponding local maximum in the output of the second-order array. This concept is illustrated in Fig. 3, where part A) shows a schematic phase-vs-CF curve and where part B) shows the output of a hypothetical second-order array. It is also illustrated in Figure 2 which shows some schematic “neurograms” to 1,000- and 1,200-Hz pure tones, with the results of a local subtraction algorithm shown to the right of the plot. Note that the neurograms in Figure 2 include the effects of peripheral filtering on response phase, but not on its magnitude (firing rate). Note also that the phase-vs.-CF functions shown in Figure 3 and in all remaining plots in this article show CF increasing from low to high along the abscissa, opposite to the convention adopted by Kim et al. and shown in Figure 1.FIG. 3

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