Localization of the binaural mechanisms underlying masking level differences, using fMRI.

We present the first fMRI study of masking level differences (MLD). Whereas MLDs have been typically been studied at the single unit level of the superior olivary complex and inferior colliculus, I discovered regions in the thalamus and insulae involved in the neural processing of MLDs. Furthermore, I found corpus callosum activation, which can be interpreted as a strong indication that the MLD processing involves inter-hemispheric communication. The specific MLD condition NthS0 is shown to have higher activity relative to the condition N0Sth, in the pulvinar thalamus, the insulae, and the genu of the corpus callosum. Unexpectedly, primary and secondary auditory cortex were not consistently activated by comparisons of MLD and non-MLD conditions. These results are compared to somewhat similar studies available in the literature, and interpreted in terms of a popular binaural processing model.