Holmes and Horrax (1919) revisited: impaired binocular fusion as a cause of "flat vision" after right parietal brain damage - a case study.

The complete loss of binocular depth perception ("flat vision") was first thoroughly described by Holmes and Horrax (1919), and has been occasionally reported thereafter in patients with bilateral posterior-parietal lesions. Though partial spontaneous recovery occurred in some cases, the precise cause(s) of this condition remained obscure for almost a century. Here, we describe a unique patient (EH) with a large right-sided occipito-parietal hemorrhage showing a complete loss of visual depth perception for several months post-stroke. EH could well simultaneously describe multiple visual objects - hence did not show simultanagnosia - but at the same time was completely unable to estimate their distance from him. In every 3-D visual scene objects appeared equidistant to him, thus experiencing a total loss of depth perception ("flat vision"). Neurovisual assessments revealed normal functions of the eyes. EH showed bilateral lower field loss and a severely impaired binocular convergent fusion, but preserved stereopsis. Perceptual re-training of binocular fusion resulted in a progressive and finally complete recovery of objective binocular fusion values and subjective binocular depth perception in a far-to-near-space, gradient-like manner. In parallel, visual depth estimation of relative distances improved, whereas stereopsis remained unchanged. Our results show that a complete loss of 3-D depth perception can result from an isolated impairment in binocular fusion. On a neuroanatomical level, this connection could be explained by a selective lesion of area V6/V6A in the medial occipito-parietal cortex that has been associated with the integration of visual space coordinates and sustained eye-positions into a cyclopean visual 3-D percept.