Reaction-Time Measures of Ear Advantage in Dichotic Listening

Experiments employing a dichotic listening test to study hemispheric specialization for speech have typically used probability of recall as the response measure to assess ear asymmetry. Reaction-time (RT) measures can be shown to provide additional information about the underlying processes and to reveal asymmetries not found with the traditional paradigm. In the first experiment, subjects monitored a dichotic consonant-vowel (CV) tape for each occurrence of a designated syllable. Whenever that item occurred, subjects manually depressed a button. For right-ear responses, percent correct was higher and reaction times were shorter, with no significant interaction of ear and hand-used-to-respond for either response measure. A second study showed that manual and verbal RT responses to targets produced identical ear differences, suggesting that at least the terminal portion of the processing of each speech input takes place in the left hemisphere. That RT may be a more sensitive tool to study ear asymmetries is suggested by a third study in which manual RT responses were obtained to CV syllables in the presence of contralateral noise. Right-ear RT responses averaged 14 msec shorter than left-ear responses, although percent correct scores did not reveal any asymmetry.