Dichotic listening refers to listening to two different signals presented simultaneously through earphones, one signal to the left ear (LE) and a different signal to the right ear (RE). Although the results have been expressed in different ways, the most common approach is to calculate %_RE, %_LE, and the difference score (%_RE − %_LE). The difference score describes the percentage ear advantage and may be a right ear advantage (REA), left ear advantage (LEA), or no ear advantage (NoEA).

Dichotic listening is a psychophysical process, the testing of which is used to assess certain aspects of central auditory function. The outcomes of experiments have led to the development of ear–brain hypotheses; an REA is accepted as evidence of left hemispheric dominance for processing and an LEA as evidence of right hemispheric dominance. A NoEA is sometimes interpreted to mean that brain dominance has not been well established (Gerber and Goldman, 1971).

When the signals are speech (usually consonant-vowel [CV] nonsense syllables), the commonly reported outcome is an REA, and interpretation of the REA in relation to left hemispheric dominance has been based on four assumptions: (1) ipsilateral auditory pathways are suppressed during dichotic stimulation (Milner, Taylor, and Sperry, 1968); (2) information from each ear arrives at the contralateral hemisphere in equivalent states (Studdert-Kennedy and Shankweiler, 1970); (3) the left hemisphere, which is language dominant for at least 95% of the right-handed population and about 70% of the left-handed population (Penfield and Roberts, 1959; Annett, 1975; Rasmussen and Milner, 1977), is principally responsible for extracting phonetic information from the different signals presented to the RE and LE (Studdert-Kennedy and Shankweiler, 1970; Studdert-Kennedy, Shankweiler, and Pisoni, 1972); and (4) the lower LE score implies “loss of information” during interhemispheric transmission from the right hemisphere to the left hemisphere via the corpus callosum (Studdert-Kennedy and Shankweiler, 1970; Studdert-Kennedy, Shankweiler, and Pisoni, 1972; Berlin et al., 1973; Brady-Wood and Shankweiler, 1973; Cullen et al., 1974; Repp, 1976). Each assumption is critical to the validity of an ear–brain hypothesis.