The classical model of how the human visual system works is that image data are transferred from the eyes to the occipital cortex where the picture is "seen". Damage to the parts of the brain serving vision causes visual field impairment and reduced visual acuities. However, additional impairment of higher visual processing is common and may go unrecognised.Two higher visual pathways have recently been described, the dorsal and ventral streams. The dorsal stream connects the occipital lobes and posterior parietal lobes. It serves to appraise the whole scene, and perceive elements within the scene. It facilitates visual guidance of movement, by interacting with area V5 of the middle temporal lobes, or motion perception centre. It is automatic, immediate and unconscious. It is 'on line' and is not memory based. Damage impairs visual guidance of movement (optic ataxia) and visual search. The ventral stream links the occipital and temporal lobes, which contain the "image libraries". Recognition of faces, shapes, objects and routes, is attained by matching incoming data with "library data".Dorsal stream dysfunction results from posterior parietal damage and is associated with cerebral palsy, periventricular white matter injury, premature birth, hydrocephalus and Williams' syndrome, and similar visual difficulties are becoming apparent in children with autistic spectrum disorder. Ventral stream dysfunction is less frequent and usually accompanies dorsal stream dysfunction. It is not uncommon in children with hydrocephalus.A specific disorder of dorsal stream dysfunction is emerging, comprising difficulty handling the complexity of a visual scene (of varying degree) with impaired visual guidance of the limb movement (optic ataxia). Commonly, but not always this is associated with reduced visual acuities and visual field impairment, and occasionally with impaired recognition of people (which could be called "dorsal stream dysfunction plus").

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