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Noninvasive fMRI investigation of interaural level difference processing in the rat auditory subcortex.

Lau C, Zhang JW, Cheng JS, Zhou IY, Cheung MM, Wu EX - PLoS ONE (2013)

Bottom Line: Our current understanding of ILD encoding has come primarily from invasive studies of individual structures, which have implicated subcortical structures such as the cochlear nucleus (CN), superior olivary complex (SOC), lateral lemniscus (LL), and inferior colliculus (IC).For all three regions, the larger amplitude response is located in the hemisphere contralateral from the higher SPL stimulus.This study is the first step towards future studies examining subcortical binaural processing and sound localization in animal models of hearing.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Biomedical Imaging and Signal Processing, The University of Hong Kong, Pokfulam, Hong Kong, China.

ABSTRACT

Objective: Interaural level difference (ILD) is the difference in sound pressure level (SPL) between the two ears and is one of the key physical cues used by the auditory system in sound localization. Our current understanding of ILD encoding has come primarily from invasive studies of individual structures, which have implicated subcortical structures such as the cochlear nucleus (CN), superior olivary complex (SOC), lateral lemniscus (LL), and inferior colliculus (IC). Noninvasive brain imaging enables studying ILD processing in multiple structures simultaneously.

Methods: In this study, blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is used for the first time to measure changes in the hemodynamic responses in the adult Sprague-Dawley rat subcortex during binaural stimulation with different ILDs.

Results and significance: Consistent responses are observed in the CN, SOC, LL, and IC in both hemispheres. Voxel-by-voxel analysis of the change of the response amplitude with ILD indicates statistically significant ILD dependence in dorsal LL, IC, and a region containing parts of the SOC and LL. For all three regions, the larger amplitude response is located in the hemisphere contralateral from the higher SPL stimulus. These findings are supported by region of interest analysis. fMRI shows that ILD dependence occurs in both hemispheres and multiple subcortical levels of the auditory system. This study is the first step towards future studies examining subcortical binaural processing and sound localization in animal models of hearing.

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BOLD signal ratios for different structures and ILDs.Mean and standard error (across all animals) of BOLD signal ratios computed for each structure with the ROIs in Fig. 2. The ratios were computed at each ILD by dividing the BOLD signal in the left hemisphere ROI by that in the right hemisphere ROI. The units are percentage of right hemisphere signal. A ratio of 100% indicates equal signal in the two hemispheres. The SOC/LL, DLL, and IC ratios exhibit a trend of decreasing ratio from −18dB to +18dB ILD while the CN ratio does not exhibit significant changes with ILD.
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pone-0070706-g005: BOLD signal ratios for different structures and ILDs.Mean and standard error (across all animals) of BOLD signal ratios computed for each structure with the ROIs in Fig. 2. The ratios were computed at each ILD by dividing the BOLD signal in the left hemisphere ROI by that in the right hemisphere ROI. The units are percentage of right hemisphere signal. A ratio of 100% indicates equal signal in the two hemispheres. The SOC/LL, DLL, and IC ratios exhibit a trend of decreasing ratio from −18dB to +18dB ILD while the CN ratio does not exhibit significant changes with ILD.

Mentions: To complement the above voxel-by-voxel analysis, Fig. 5 plots the mean and standard error (across all animals) of BOLD signal ratios from the CN, SOC/LL, DLL, and IC ROIs defined in Fig. 2. At ILD  = 0dB, the ratios of all structures are not statistically significantly different from 100% (p>0.05). The SOC/LL, DLL, and IC ratios exhibit a significant downward trend from−18dB to +18dB ILD, indicating that the left hemisphere responses are greater than the right hemisphere responses at negative ILDs (right ear SPL greater than left ear SPL), and vice versa. The CN ratio does not exhibit significant changes with ILD. The p-values in Table 1 show that the SOC/LL BOLD signal ratios measured at −18dB and −12dB ILD are statistically significantly greater than those measured at +18dB ILD (p<0.05). For the DLL, Table 2 shows that in general, ratios measured at zero and negative ILDs are statistically significantly greater than those measured at +12dB and +18dB ILD (p<0.05). For the IC, Table 3 similarly shows a negative correlation. In general, the ratios measured at negative ILDs are statistically significantly greater than those measured at zero and positive ILDs (p<0.05).


Noninvasive fMRI investigation of interaural level difference processing in the rat auditory subcortex.

Lau C, Zhang JW, Cheng JS, Zhou IY, Cheung MM, Wu EX - PLoS ONE (2013)

BOLD signal ratios for different structures and ILDs.Mean and standard error (across all animals) of BOLD signal ratios computed for each structure with the ROIs in Fig. 2. The ratios were computed at each ILD by dividing the BOLD signal in the left hemisphere ROI by that in the right hemisphere ROI. The units are percentage of right hemisphere signal. A ratio of 100% indicates equal signal in the two hemispheres. The SOC/LL, DLL, and IC ratios exhibit a trend of decreasing ratio from −18dB to +18dB ILD while the CN ratio does not exhibit significant changes with ILD.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3733930&req=5

pone-0070706-g005: BOLD signal ratios for different structures and ILDs.Mean and standard error (across all animals) of BOLD signal ratios computed for each structure with the ROIs in Fig. 2. The ratios were computed at each ILD by dividing the BOLD signal in the left hemisphere ROI by that in the right hemisphere ROI. The units are percentage of right hemisphere signal. A ratio of 100% indicates equal signal in the two hemispheres. The SOC/LL, DLL, and IC ratios exhibit a trend of decreasing ratio from −18dB to +18dB ILD while the CN ratio does not exhibit significant changes with ILD.
Mentions: To complement the above voxel-by-voxel analysis, Fig. 5 plots the mean and standard error (across all animals) of BOLD signal ratios from the CN, SOC/LL, DLL, and IC ROIs defined in Fig. 2. At ILD  = 0dB, the ratios of all structures are not statistically significantly different from 100% (p>0.05). The SOC/LL, DLL, and IC ratios exhibit a significant downward trend from−18dB to +18dB ILD, indicating that the left hemisphere responses are greater than the right hemisphere responses at negative ILDs (right ear SPL greater than left ear SPL), and vice versa. The CN ratio does not exhibit significant changes with ILD. The p-values in Table 1 show that the SOC/LL BOLD signal ratios measured at −18dB and −12dB ILD are statistically significantly greater than those measured at +18dB ILD (p<0.05). For the DLL, Table 2 shows that in general, ratios measured at zero and negative ILDs are statistically significantly greater than those measured at +12dB and +18dB ILD (p<0.05). For the IC, Table 3 similarly shows a negative correlation. In general, the ratios measured at negative ILDs are statistically significantly greater than those measured at zero and positive ILDs (p<0.05).

Bottom Line: Our current understanding of ILD encoding has come primarily from invasive studies of individual structures, which have implicated subcortical structures such as the cochlear nucleus (CN), superior olivary complex (SOC), lateral lemniscus (LL), and inferior colliculus (IC).For all three regions, the larger amplitude response is located in the hemisphere contralateral from the higher SPL stimulus.This study is the first step towards future studies examining subcortical binaural processing and sound localization in animal models of hearing.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Biomedical Imaging and Signal Processing, The University of Hong Kong, Pokfulam, Hong Kong, China.

ABSTRACT

Objective: Interaural level difference (ILD) is the difference in sound pressure level (SPL) between the two ears and is one of the key physical cues used by the auditory system in sound localization. Our current understanding of ILD encoding has come primarily from invasive studies of individual structures, which have implicated subcortical structures such as the cochlear nucleus (CN), superior olivary complex (SOC), lateral lemniscus (LL), and inferior colliculus (IC). Noninvasive brain imaging enables studying ILD processing in multiple structures simultaneously.

Methods: In this study, blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is used for the first time to measure changes in the hemodynamic responses in the adult Sprague-Dawley rat subcortex during binaural stimulation with different ILDs.

Results and significance: Consistent responses are observed in the CN, SOC, LL, and IC in both hemispheres. Voxel-by-voxel analysis of the change of the response amplitude with ILD indicates statistically significant ILD dependence in dorsal LL, IC, and a region containing parts of the SOC and LL. For all three regions, the larger amplitude response is located in the hemisphere contralateral from the higher SPL stimulus. These findings are supported by region of interest analysis. fMRI shows that ILD dependence occurs in both hemispheres and multiple subcortical levels of the auditory system. This study is the first step towards future studies examining subcortical binaural processing and sound localization in animal models of hearing.

Show MeSH
Related in: MedlinePlus