Interaural intensity difference and ear advantage in listening to dichotic consonant-vowel syllable pairs.

The right-ear advantage (REA) is typically observed in verbal dichotic listening, indicating a left hemisphere superiority for speech processing. The REA could be thought of as a bottom-up, stimulus-driven laterality effect, caused by the preponderance of the contralateral neural fibers from the right ear to the auditory/speech processing areas in the left temporal lobe. The REA can, however, be modified by explicitly requiring the listeners to focus their attention alternatively on the left or right-ear stimuli, thus either countering or enhancing the bottom-up processes through top-down attentional control. In the present study, we manipulated the strength of the bottom-up REA by inducing an intensity difference between the right-ear and left-ear speech inputs in order to make the REA either weaker (left-ear input>right-ear input) or stronger (left-ear input<right-ear input) and also examined how this manipulation affected the top-down attention modulation effects. Twenty healthy participants listened to dichotic presentations of consonant-vowel syllable pairs with different attention instructions. The results showed that the interaural intensity difference significantly affected the ear advantage in the predicted way. It also interacted with the top-down control effects, attentional control having a stronger effect when attending to the ear that had a weaker sound intensity, as compared to when the intensities were equal.