Magnitude of diotic summation in speech-in-noise tasks: performance region and appropriate baseline

The diotic summation effect is one of the three types of binaural advantage for perceiving speech-in-noise. Using a within-subject design in normally-hearing subjects having no significant noise exposure, we examined its magnitude. The average advantage was +5%, at an overall performance level of 70-80% rising to +9% when an appropriate control was used to exclude order effects introduced by the within-subject design. Diotic summation is an important component of the advantage to be derived from presentation to two ears, but the size of the binaural advantage obtained depends critically on the particular monotic baseline chosen for comparison. The methodological problems of demonstrating binaural advantages (as in binaural aiding studies) are discussed.