Sound Localization and Visual Hemispatial Agnosia

Abstract

Pathophysiology of sound localization in stroke patients is reviewed. The mechanism of sound localization in humans has been eagerly studied and was explained at first as internaural time difference (ITD) or internaural intensity (pressure) difference (IID). Then the concept of a sound transfer function was introduced and a three dimensional system for testing using a headphone became available. Now, three test methods are utilized: speaker method, headphone method with ITD or IID, or headphone method with transfer function. The majority of the studies found disturbance of sound localization in stroke patients. Stroke patients having either a right or left hemispheric lesion less accurately identified the direction of sound than healthy subjects. In some studies, patients showed a systematic (relative) error in discrimination of direction. The term “systematic error” is used when the answered direction consistently deviated to one direction, clockwise or counterclockwise. Clockwise systematic deviation was frequently encountered in patients with a right hemispheric lesion. This phenomenon is closely related to hemispatial visual neglect. Disturbance of sound localization in stroke patients seems to be composed of two elements. The first cause is decreased accuracy of discrimination of direction. This phenomenon can be brought about by damage in either the right or left side of the brain. The second element is a systematic error that may be one expression of symptoms of hemispatial neglect. In clinical situations, a patient may manifest both elements, or possibly only one of the two elements. Higher cortical function concerning sound localization is not fully understood now. Further research may build a comprehensive theory for hemispatial neglect, including sound localization.