Dichotic Listening Studies of Brain Asymmetry

Abstract

Dichotic listening (DL) is a noninvasive technique for the study of brain lateralization, or hemispheric asymmetry. DL is the most frequently used method to reveal a left-hemisphere dominance for language processing, particularly extraction of the phonetic code from the speech signal. By recording the pattern of verbal responses to dichotic presentations of simple speech sounds (e.g., syllables), it is possible to determine the hemisphere to which receptive language capabilities are most likely localized in an individual. DL is frequently used in both experimental and clinical studies of language asymmetry, or laterality, which can also be used to study lateralization of emotion and affect. DL involves simultaneously presenting two different auditory stimuli, one in each ear, and the task of the individual is to report, after each presentation, which sound is heard. The individual is not informed beforehand that there are two different syllables on each trial. We have used the DL method in our own research to study the pathology of left temporal lobe language processing in dyslexic children, auditory hallucinations in schizophrenia, and patients with left-hemisphere arachnoid cysts, to mention a few examples. DL can also be used as a complement to invasive sodium barbital techniques when investigating language asymmetry in epileptic patients undergoing surgical treatment. However, although mainly used as a technique for the study of language laterality, in a general sense DL is a behavioral technique to study a broad range of cognitive and emotional processes, related not only to brain laterality and hemispheric asymmetry, but also to attention, conditioning, and learning and memory psychopathology and psycholinguistics. Thus, DL is a measure of both temporal and frontal lobe function, attention and information processing, and stimulus processing speed, in addition to being a measure of hemispheric asymmetry.