Disorders of perceived auditory lateralization after lesions of the right hemisphere

The disorders of perceived auditory lateralization after unilateral damage of the cerebral hemispheres were investigated by dichotic presentation of pure tones, lateralization being obtained by interaural intensity differences. Observations were made on 107 unilateral brain-damaged patients and 30 normal subjects who were required to localize the perceived position of fused sound images. The images were generated by dichotic stimuli delivered through headphones and the patients were asked to point the perceived position. A clear-cut hemispheric asymmetry was found. Whereas left brain-damaged patients and right brain-damaged patients without visual field defects were not impaired in this task in comparison with controls, the performance of right brain-damaged patients with visual field defects was defective, with a systematic directional error towards the right side, that is, the side ipsilateral to the damaged hemisphere. The defect of auditory lateralization involved both the half-space contralateral to the lesion and the ipsilateral one. Different hypotheses are discussed in order to explain the present results. It is argued that they cannot easily be interpreted in terms of defective processing of sensory information and the proposal is made that lesions of the right hemisphere may produce a distortion of the internal representation of egocentric space to the right, yielding a systematic error of auditory lateralization are discussed.