Functional magnetic resonance imaging of human auditory cortical tuning to interaural level and time differences
McLaughlin, Susan A
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Interaural level and time differences (ILD and ITD) are important cues for localizing sound and segregating signal from noise. Evidence indicates that auditory cortex (AC) plays a critical role in mammalian sound localization, but models of AC binaural processing remain underspecified. A series of experiments was conducted to systematically characterize fMRI tuning to binaural stimulation in human AC. To investigate processing of ILD versus monaural sound, Experiment 1 collected responses to narrowband-filtered Gabor click impulses varying in ILD, average binaural level, monotic level, and presentation rate. Response to monotic sound was greater contralateral to ear of presentation and binaural responses grew with increasingly contralateral ILD values. Contralateral bias was more modest for ILD than for monotic stimulation, and the relationship in bias across the stimulus types was weak, suggesting that ILD tuning is partially independent of monaural level sensitivity. Binaural suppression was observed for diotic and dichotic ILD sound and may actively shape ILD sensitivity. Experiments 2 and 3 presented parametrically modulated ILD and ITD cues carried by Gabor click trains (ILD<sub>GCT</sub> and ITD<sub>GCT</sub>). Consistent with Exp. 1, results showed contralateral tuning to ILD<sub>GCT</sub>, but little tuning to ITD<sub>GCT</sub>. Additional experiments manipulated ITD in broadband noise-burst trains (ITD<sub>NBT</sub>; Experiment 4) and in low-frequency noise (ITD<sub>LPN</sub>; Experiment 5). Modest contralateral tuning to ITD<sub>NBT</sub> was observed, but only in left hemisphere, consistent with previous neuroimaging and clinical lesion data. Further analysis revealed a similar asymmetry in response to ILD<sub>GCT</sub>. Analyses of stimulus history effects showed that contralateral tuning to ILD<sub>GCT</sub> and ITD<sub>NBT</sub> was enhanced by exposure to prior ipsilateral sound, particularly in right hemisphere responses to ITD, potentially serving to minimize the left hemisphere asymmetry observed. Stimulus history analyses also showed greater sensitivity to "outward" (probe sound more lateralized than adaptor) versus "inward" (adaptor more lateralized than probe) sound location change for both ILD and ITD, consistent with opponent-channel models of binaural processing. Differences in the degree of contralateral bias and hemispheric asymmetry across cue types may be due to reduced BOLD sensitivity to ITD, rather than reflecting distinct AC processing mechanisms for ILD and ITD.
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