by David S. Wack, Jennifer L. Cox, Claudiu V. Schirda, Christopher R. Magnano, Joan E. Sussman, Donald Henderson, Robert F. Burkard
Masking level differences (MLDs) are differences in the hearing threshold for the detection of a signal presented in a noise background, where either the phase of the signal or noise is reversed between ears. We use N0/Np to denote noise presented in-phase/out-of-phase between ears and S0/Sp to denote a 500 Hz sine wave signal as in/out-of-phase. Signal detection level for the noise/signal combinations N0Sp and NpS0 is typically 10–20 dB better than for N0S0. All combinations have the same spectrum, level, and duration of both the signal and the noise. Methods
Ten participants (5 female), age: 22–43, with N0Sp-N0S0 MLDs greater than 10 dB, were imaged using a sparse BOLD fMRI sequence, with a 9 second gap (1 second quiet preceding stimuli). Band-pass (400–600 Hz) noise and an enveloped signal (.25 second tone burst, 50% duty-cycle) were used to create the stimuli. Brain maps of statistically significant regions were formed from a second-level analysis using SPM5. Results
The contrast NpS0- N0Sp had significant regions of activation in the right pulvinar, corpus callosum, and insula bilaterally. The left inferior frontal gyrus had significant activation for contrasts N0Sp-N0S0 and NpS0-N0S0. The contrast N0S0-N0Sp revealed a region in the right insula, and the contrast N0S0-NpS0 had a region of significance in the left insula. Conclusion
Our results extend the view that the thalamus acts as a gating mechanism to enable dichotic listening, and suggest that MLD processing is accomplished through thalamic communication with the insula, which communicate across the corpus callosum to either enhance or diminish the binaural signal (depending on the MLD condition). The audibility improvement of the signal with both MLD conditions is likely reflected by activation in the left inferior frontal gyrus, a late stage in the what/where model of auditory processing.