A method of detecting higher-order auditory laterality in normal humans

A prevalent method of determining functional differences between human cerebral hemispheres has been the medical examination and clinical psychiatric observation of brain-injured patients, which is believed to provide sufficient data for assuming the lateralization of specific cerebral functions. But this clinical approach is clearly limited in its capacity to measure the functions of normal brains, and it is important to note that even a small injury can affect the mechanism of the whole brain.

In recent years, several techniques of investigating laterality in normal human subjects free of brain injury have been developed, among which the sensory approach to brain laterality has opened up new possibilities previously unexplored by clinical researchers. The dichoticlistening test developed by D. Kimura (1961) is the best known and most widely employed method.

In this test, the subject is required to listen simultaneously to two different melodies delivered to the respective ears and then identify the melodies from a pool of sample melodies. It is generally found that the subject recalls the melody heard from the left ear more accurately. If verbal or arithmetic problems are delivered to both ears, information heard in the right ear more than that heard in the left ear helps the subject correctly solve these problems.

When inputs from each ear are placed in a mutually competing situation, the ipsilateral nerve routes are suppressed and the contralateral nerve routes are activated, causing the ear on the contralateral side of the verbal hemisphere to function dominantly when the subject solves verbal and arithmetic problems.