Gaze-dependent hemianopia without hemispatial neglect.

A hemispheric field is the space defined by the midsagittal plane of the body. Hemianopia, in the absence of hemispheric spatial neglect, has been thought to be purely retinotopic. Confrontation testing of visual fields in eccentric positions of gaze in hemianopic patients permits the discrimination of hemispheric from retinotopic deficits. In the primary position of gaze, a patient with an ischemic lesion of the right occipital and temporal lobes, who was without unilateral spatial neglect, was unable to detect finger movement, name objects, or identify shapes or colors in the left retinotopic hemifield, but when gazing 30 degrees to the right (so that the left retinotopic field was in the right hemispheric field), he performed nearly as well in the left retinotopic field as in the right. The mechanism of this gaze-dependent hemifield visual impairment is unknown, but we discuss four possibilities: (1) eccentric gaze enhances an alternative visual system; (2) the lesion led to modality-specific hemispheric inattention that was corrected by directing intention (plan to act) and gazing to the right; (3) the lesion led to modality specific inattention that was corrected by compensatory mechanisms that may normally enhance attentive vigilance in central space during eccentric gaze; and (4) the neuronal substrate for hemispheric vision was destroyed, whereas that for retinotopic vision was preserved.