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Low-frequency envelope sensitivity produces asymmetric binaural tuning curves.

Agapiou JP, McAlpine D - J. Neurophysiol. (2008)

Bottom Line: This suggests a stereotyped pattern of input to the IC.In the course of this analysis, we were also able to determine the contribution of time and phase components to neurons' internal delays.These findings have important consequences for the neural representation of interaural timing differences and interaural correlation-cues critical to the perception of acoustic space.

View Article: PubMed Central - PubMed

Affiliation: Ear Institute, Department of Neuroscience, Physiology and Pharmacology, University College London, London, UK. john.agapiou@mail.rockefeller.edu

ABSTRACT
Neurons in the auditory midbrain are sensitive to differences in the timing of sounds at the two ears--an important sound localization cue. We used broadband noise stimuli to investigate the interaural-delay sensitivity of low-frequency neurons in two midbrain nuclei: the inferior colliculus (IC) and the dorsal nucleus of the lateral lemniscus. Noise-delay functions showed asymmetries not predicted from a linear dependence on interaural correlation: a stretching along the firing-rate dimension (rate asymmetry), and a skewing along the interaural-delay dimension (delay asymmetry). These asymmetries were produced by an envelope-sensitive component to the response that could not entirely be accounted for by monaural or binaural nonlinearities, instead indicating an enhancement of envelope sensitivity at or after the level of the superior olivary complex. In IC, the skew-like asymmetry was consistent with intermediate-type responses produced by the convergence of ipsilateral peak-type inputs and contralateral trough-type inputs. This suggests a stereotyped pattern of input to the IC. In the course of this analysis, we were also able to determine the contribution of time and phase components to neurons' internal delays. These findings have important consequences for the neural representation of interaural timing differences and interaural correlation-cues critical to the perception of acoustic space.

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Time and phase contributions to the internal delay. A and C: the characteristic delay as a function of CF for both peak-type responses (upward triangles) and trough-type responses (downward triangles) in DNLL (A) and IC (C). The dotted lines show the characteristic delays (CDs) corresponding to ±1/8 cyc re CF and the shaded area shows the physiological range for the guinea pig (±300 μs). B and D: the characteristic phase (CP) as a function of CF for DNLL (B) and IC (D). CDs resemble the plots for best ITD (Fig. 6, A and C), whereas CPs are distributed around zero and have no systematic relationship with CF.
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f7: Time and phase contributions to the internal delay. A and C: the characteristic delay as a function of CF for both peak-type responses (upward triangles) and trough-type responses (downward triangles) in DNLL (A) and IC (C). The dotted lines show the characteristic delays (CDs) corresponding to ±1/8 cyc re CF and the shaded area shows the physiological range for the guinea pig (±300 μs). B and D: the characteristic phase (CP) as a function of CF for DNLL (B) and IC (D). CDs resemble the plots for best ITD (Fig. 6, A and C), whereas CPs are distributed around zero and have no systematic relationship with CF.

Mentions: Similar to the best ITD, the CD of peak-type neurons was negatively correlated with the CF in both DNLL (Fig. 7A; r = −0.56, P = 0.032, Spearman's rank correlation coefficient) and IC (Fig. 7C; r = −0.74, P = 0.002). No significant difference was seen between CD and the best/worst ITD (DNLL: P = 1.00, IC: P = 0.82, sign test; see Fig. 6, A and C). Since the CP determines the deviation of the CD from the best ITD, the similarity between CD and best ITD reflected the lack of a systematic contribution from the CP. The CP showed a broad range of values with no dependence on the CF (Fig. 7, B and D; DNLL: P = 0.65, IC: P = 0.20). As expected from the lack of a significant difference between the CD and the best ITD, the CP of peak-type neurons showed no significant bias away from 0 cyc in either the DNLL (P = 1.00, sign test; median 0.00 cyc) or the IC (P = 0.45, median 0.08 cyc). Thus the inverse relationship we observed between best ITD and CF was chiefly determined by internal time delays (i.e., CD) that varied with the CF of the neuron, with no significant contribution from internal phase delays.


Low-frequency envelope sensitivity produces asymmetric binaural tuning curves.

Agapiou JP, McAlpine D - J. Neurophysiol. (2008)

Time and phase contributions to the internal delay. A and C: the characteristic delay as a function of CF for both peak-type responses (upward triangles) and trough-type responses (downward triangles) in DNLL (A) and IC (C). The dotted lines show the characteristic delays (CDs) corresponding to ±1/8 cyc re CF and the shaded area shows the physiological range for the guinea pig (±300 μs). B and D: the characteristic phase (CP) as a function of CF for DNLL (B) and IC (D). CDs resemble the plots for best ITD (Fig. 6, A and C), whereas CPs are distributed around zero and have no systematic relationship with CF.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2576218&req=5

f7: Time and phase contributions to the internal delay. A and C: the characteristic delay as a function of CF for both peak-type responses (upward triangles) and trough-type responses (downward triangles) in DNLL (A) and IC (C). The dotted lines show the characteristic delays (CDs) corresponding to ±1/8 cyc re CF and the shaded area shows the physiological range for the guinea pig (±300 μs). B and D: the characteristic phase (CP) as a function of CF for DNLL (B) and IC (D). CDs resemble the plots for best ITD (Fig. 6, A and C), whereas CPs are distributed around zero and have no systematic relationship with CF.
Mentions: Similar to the best ITD, the CD of peak-type neurons was negatively correlated with the CF in both DNLL (Fig. 7A; r = −0.56, P = 0.032, Spearman's rank correlation coefficient) and IC (Fig. 7C; r = −0.74, P = 0.002). No significant difference was seen between CD and the best/worst ITD (DNLL: P = 1.00, IC: P = 0.82, sign test; see Fig. 6, A and C). Since the CP determines the deviation of the CD from the best ITD, the similarity between CD and best ITD reflected the lack of a systematic contribution from the CP. The CP showed a broad range of values with no dependence on the CF (Fig. 7, B and D; DNLL: P = 0.65, IC: P = 0.20). As expected from the lack of a significant difference between the CD and the best ITD, the CP of peak-type neurons showed no significant bias away from 0 cyc in either the DNLL (P = 1.00, sign test; median 0.00 cyc) or the IC (P = 0.45, median 0.08 cyc). Thus the inverse relationship we observed between best ITD and CF was chiefly determined by internal time delays (i.e., CD) that varied with the CF of the neuron, with no significant contribution from internal phase delays.

Bottom Line: This suggests a stereotyped pattern of input to the IC.In the course of this analysis, we were also able to determine the contribution of time and phase components to neurons' internal delays.These findings have important consequences for the neural representation of interaural timing differences and interaural correlation-cues critical to the perception of acoustic space.

View Article: PubMed Central - PubMed

Affiliation: Ear Institute, Department of Neuroscience, Physiology and Pharmacology, University College London, London, UK. john.agapiou@mail.rockefeller.edu

ABSTRACT
Neurons in the auditory midbrain are sensitive to differences in the timing of sounds at the two ears--an important sound localization cue. We used broadband noise stimuli to investigate the interaural-delay sensitivity of low-frequency neurons in two midbrain nuclei: the inferior colliculus (IC) and the dorsal nucleus of the lateral lemniscus. Noise-delay functions showed asymmetries not predicted from a linear dependence on interaural correlation: a stretching along the firing-rate dimension (rate asymmetry), and a skewing along the interaural-delay dimension (delay asymmetry). These asymmetries were produced by an envelope-sensitive component to the response that could not entirely be accounted for by monaural or binaural nonlinearities, instead indicating an enhancement of envelope sensitivity at or after the level of the superior olivary complex. In IC, the skew-like asymmetry was consistent with intermediate-type responses produced by the convergence of ipsilateral peak-type inputs and contralateral trough-type inputs. This suggests a stereotyped pattern of input to the IC. In the course of this analysis, we were also able to determine the contribution of time and phase components to neurons' internal delays. These findings have important consequences for the neural representation of interaural timing differences and interaural correlation-cues critical to the perception of acoustic space.

Show MeSH
Related in: MedlinePlus