Cognitive vision, its disorders and differential diagnosis in adults and children: knowing where and what things are.

As ophthalmologists we need a basic model of how the higher visual system works and its common disorders. This presentation aims to provide an outline of such a model. Our ability to survey a visual scene, locate and recognise an object of interest, move towards it and pick it up, recruits a number of complex cognitive higher visual pathways, all of which are susceptible to damage. The visual map in the mind needs to be co-located with reality and is primarily plotted by the posterior parietal lobes, which interact with the frontal lobes to choose the object of interest. Neck and extraocular muscle proprioceptors are probably responsible for maintaining this co-location when the head and eyes move with respect to the body, and synchronous input from both eyes is needed for correct localisation of moving targets. Recognition of what is being looked at is brought about by comparing the visual input with the "image libraries" in the temporal lobes. Once an object is recognised, its choice is mediated by parietal and frontal lobe tissue. The parietal lobes determine the visual coordinates and plan the visually guided movement of the limbs to pick it up, and the frontal lobes participate in making the choice. The connection between the occipital lobes and the parietal lobes is known as the dorsal stream, and the connection between the occipital lobes and the temporal lobes, comprises the ventral stream. Both disorders of neck and extraocular muscle proprioception, and disorders leading to asynchronous input along the two optic nerves are "peripheral" causes of impaired visually guided movement, while bilateral damage to the parietal lobes can result in central impairment of visually guided movement, or optic ataxia. Damage to the temporal lobes can result in impaired recognition, problems with route finding and poor visual memory. Spontaneous activity in the temporal lobes can result in formed visual hallucinations, in patients with impaired central visual function, particularly the elderly. Deficits in cognitive visual function can occur in different combinations in both children and adults depending on the nature and distribution of the underlying brain damage. In young children the potential for recovery can lead to significant improvement in parietal lobe function with time. Patients with these disorders need an understanding of their deficits and a structured positive approach to their rehabilitation.