Our Research Lab
Spatial Hearing - Individual Differences (IACC)
There are many ways to measure binaural hearing - the effect of combining sound at the two ears. Many investigations over the last 70 years have been modeled successfully by assuming detection or discrimination is based on a change in the correlation of the sound at the two ears - known as interaural cross correlation (IACC). Diotic noise consists of separate noises with a correlation of 1.0 presented to the two ears. The perceived image is compact and lateralized centrally. As the two noises are progressively de-correlated, the perceptual image widens and becomes more diffuse. Previous investigators have reported small just-noticeable differences in IACC (IACC-JNDs) for most listeners but some listeners have abnormally large IACC-JNDs. The relatively large JNDs suggest very poor binaural processing in the latter group, despite other binaural measures that may contradict that conclusion. This is consistent with numerous published and anecdotal reports of a substantial number of listeners with normal hearing, by all accounts, having very poor performance on certain binaural tasks. The binaural masking level difference (BMLD) task is one such measure. Because BMLD is one of the tasks that can be modeled based on change in IACC, perhaps these and other variances among listeners is related to a fundamental difference in the ability to register IACC. Another possibility is that the binaural cues related to the various tasks may be subtle or not well conceptualized by the listener and thus apparent deficits are more related to task performance than underlying binaural hearing abilities.
The aim of this study is to determine whether the apparent dichotic deficit in some individuals is more task relevant or is associated with some underlying central processing mechanism. The IACC-JND was measured in listeners who show normal or abnormal IACC-JNDs. We compared these measures to correlation-change evoked potentials using electroencephalography (EEG) for various interaural correlations. Results indicate a clear difference in the morphology of the evoked responses between groups. Remarkably, the abnormal group showed a shift in their cortical detection of a correlation change similar to their behavioral data, indicating that these listeners are indeed binaurally disadvantaged despite otherwise normal audiometric profiles and lack of spatial hearing complaints. If this result holds for other binaural tasks, it may be the case that the apparent deficits are related to an underlying deficit in the representation of IACC. Additional studies are planned to investigate this possibility.