Auditory Laterality : Dichotic Listening and fMRI Studies

Auditory laterality and hemispheric asymmetry The differing perceptual and cognitive functions of the two cerebral hemispheres is one of the great discoveries and mysteries of the human brain (see Hugdahl & Westerhausen, 2009 for a recent review of the field). Ever since the groundbreaking work by the Nobel Laureate Roger Sperry (see Sperry 1974), mapping the different functions of the cerebral hemispheres has been a major research area. Most research on hemispheric asymmetry was however focused on the visual system, although the principles governing the workings of the hemispheres should be supra-modal. In 1984 (Hugdahl & Andersson, 1984) our group began research on auditory laterality, using a simple technique called dichotic listening (DL) which had been introduced to neuropsychology in 1961 by Doreen Kimura. We used a variant of the technique with dichotic presentations of simple speech sounds consisting of pairs of consonant-vowel (CV) syllables (see Figure 1) that was first used by Studdert-Kennedy and Shankweiler (1970). The DL technique means that two different auditory stimuli are presented at the same time, one in each ear, and without informing the subject or patient that there are two different sounds. The task of the subject is simply to report which sound she/he perceives on each trial, typically repeating the sequence for a 100 such trials. The result is normally a ‘right-ear-advantage’ (REA) with better performance for the right ear stimulus, also when controlling for interaural differences in hearing acuity, caused by left temporal lobe processing advantage for speech sounds. This simple technique has several advantages when studying perceptual and cognitive processes particularly in psychiatric and neurological patients, not the least its simple structure which means that patients with severe disorders and diseases can be compared on the same parameters across processes. Over the years our group has used the DL technique to study deficits in hemispheric asymmetry for speech sound processing in numerous psychiatric and neurological disorders, and the CV-syllables variant we developed in the 1980s is today in use in clinics and laboratories worldwide.