Dissociation of auditory and visual straight ahead in hemianopia

The perception of body orientation in space can be significantly affected by unilateral brain damage. One syndrome associated with such a disorder is homonymous hemianopia, resulting from lesions in postchiasmatic afferent visual pathways. Several studies in patients with hemianopia have shown that their subjective straight ahead is shifted to the anopic (contralesional) side. However, subjective straight ahead was mostly measured with reference to visual stimuli. As visual space perception is known to be distorted in hemianopia, it thus remained unclear whether the previous findings reflected this unisensory deficit or a genuine abnormality of subjective body orientation in multisensory space. In the present study, the auditory and visual straight ahead directions were assessed in twelve patients with persistent pure hemianopia (without neglect) by using related adjustment methods. As in previous studies, the visual straight ahead was found to be significantly shifted toward the anopic side. By contrast, the patients' auditory straight ahead was approximately veridical, as in healthy controls. These results indicate that in hemianopia the subjective straight ahead, in its genuine sense as the perceived orientation of the body median sagittal plane in frontal multisensory space, is unaffected. This suggests that visual brain areas, as are damaged in hemianopia, are not directly involved in relating body position to the external space. With respect to therapies for rehabilitation of hemianopia, these findings may support recent approaches, in which visual stimuli are combined with auditory stimuli in order to improve visual search behaviour.