title: Holmes and Horrax (1919) revisited: impaired binocular fusion as a cause of "flat vision" after right parietal brain damage - a case study.
abstract: 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.
authors.affiliation: Clinical Neuropsychology Unit & Neuropsychological Outpatient Service, Saarland University, Campus, Building A.1.3., 66123 Saarbruecken, Germany; International Research and Training Group "Adaptive Minds" (IRTG 1457), Saarland University, Saarbruecken, Germany. Electronic address: annakatharina.schaadt@uni-saarland.de.
authors.forename: Anna-Katharina
authors.initials: AK
authors.lastname: Schaadt
authors.affiliation: Campus Charité Mitte, CC 15: Neurology, Neurosurgery and Psychiatry, Vision and Motor System Group, Department of Neurology with Experimental Neurology, Charité Universitaetsmedizin Berlin, Charitéplatz 1, 10117 Berlin, Germany. Electronic address: stephan.brandt@charite.de.
authors.forename: Stephan A
authors.initials: SA
authors.lastname: Brandt
authors.affiliation: Campus Charité Mitte, CC 15: Neurology, Neurosurgery and Psychiatry, Vision and Motor System Group, Department of Neurology with Experimental Neurology, Charité Universitaetsmedizin Berlin, Charitéplatz 1, 10117 Berlin, Germany; Berlin Institute of Health, Charité Universitaetsmedizin Berlin, Berlin, Germany. Electronic address: antje.kraft@charite.de.
authors.forename: Antje
authors.initials: A
authors.lastname: Kraft
authors.affiliation: Clinical Neuropsychology Unit & Neuropsychological Outpatient Service, Saarland University, Campus, Building A.1.3., 66123 Saarbruecken, Germany; International Research and Training Group "Adaptive Minds" (IRTG 1457), Saarland University, Saarbruecken, Germany. Electronic address: kerkhoff@mx.uni-saarland.de.
authors.forename: Georg
authors.initials: G
authors.lastname: Kerkhoff
mesh_terms: D002543:Cerebral Hemorrhage / Q000150:complications / Q000473:pathology / Q000503:physiopathology / Q000534:rehabilitation; D003867:Depth Perception / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D009483:Neuropsychological Tests; D009778:Occipital Lobe / Q000473:pathology / Q000503:physiopathology; D010296:Parietal Lobe / Q000473:pathology / Q000503:physiopathology*; D010468:Perceptual Disorders / Q000209:etiology / Q000473:pathology / Q000503:physiopathology* / Q000534:rehabilitation; D020127:Recovery of Function; D020521:Stroke / Q000150:complications / Q000473:pathology / Q000503:physiopathology; D000071939:Stroke Rehabilitation; D014786:Vision Disorders / Q000209:etiology / Q000473:pathology / Q000503:physiopathology* / Q000534:rehabilitation; D014787:Vision Tests; D015348:Vision, Binocular / Q000502:physiology*
keywords: Binocular vision; Depth perception; Fusion; Parietal lobe; Stereopsis; V6/V6A