Masking Level Differences – A Diffusion Tensor Imaging and Functional MRI Study

David S Wack,Robert F Burkard,Paul Polak,Jon Furuyama,Wang Zhan

doi:10.1371/journal.pone.0088466

Abstract

In our previous study we investigated Masking Level Differences (MLD) using functional Magnetic Resonance Imaging (fMRI), but were unable to confirm neural correlations for the MLD within the auditory cortex and inferior colliculus. Here we have duplicated conditions from our previous study, but have included more participants and changed the study site to a new location with a newer scanner and presentation system. Additionally, Diffusion Tensor Imaging (DTI) is included to allow investigation of fiber tracts that may be involved with MLDs. Twenty participants were included and underwent audiometric testing and MRI scanning. The current study revealed regions of increased and decreased activity within the auditory cortex when comparing the combined noise and signal of the dichotic MLD stimuli (N0Sπ and NπS0) with N0S0. Furthermore, we found evidence of inferior colliculus involvement. Our DTI findings show strong correlations between DTI measures within the brainstem and signal detection threshold levels. Patterns of correlation when the signal was presented only to the right ear showed an extensive network in the left hemisphere; however, the opposite was not true for the signal presented only to the left ear. Our current study was able to confirm what we had previously hypothesized using fMRI, while extending our investigation of MLDs to include the characteristics of connecting neural pathways.

Highlights

Masking Level Differences (MLDs) are determined with dichotic and diotic listening conditions that typically use a combination of band-pass noise and a pure tone
This current protocol is nearly identical to our previous study with the minor changes including the following: 20 participants were used in the present study; auditory testing was performed at the MRI scanning center immediately before scanning, with the required hearing screening threshold relaxed to 30 dB SPL; there was no minimum MLD
The functional Magnetic Resonance Imaging (fMRI) contrast N0S0 – NoStim had activation in the left and right auditory cortices (Table 3, rows 34 and 35), whereas the contrast, NoStim – N0S0, had a spatially large (4745 voxels) activation extending from the inferior colliculus to the red nucleus (Table 3, row 36)

Summary

Introduction

Masking Level Differences (MLDs) are determined with dichotic and diotic listening conditions that typically use a combination of band-pass noise and a pure tone. There is a substantial signal-detection advantage for the dichotic condition when either the noise or the signal is presented out-ofphase compared to the diotic condition when both the noise and signal are presented in-phase (identically) between ears. We use the notation N0S0, N0Sp, NpS0, N0SL, and N0SR to denote whether the noise or signal are in-phase (N0 or S0), out-of-phase A person’s MLD can be found by subtracting the signal detection threshold for a dichotic condition (such as: N0Sp, NpS0, N0SL, N0SR) from the signal detection threshold obtained for N0S0. The. MLD for N0Sp is usually the greatest, NpS0 is slightly less, and. Licklider [2] and are still of interest for their ability to shed light on the binaural listening system

Methods

Results

Discussion

Conclusion