Cognitive visual dysfunction.

The human visual system is made up of an exquisitely organised array of functional units which have been the subject of investigation by many disciplines, including those of optics, electrophysiology, neurophysiology, psychology, and in the realm of cognate vision, philosophy. The nature of image degradation, resulting from pathology of the eye or anterior visual pathways, is not difficult to imagine and comprehend. By contrast, the forms that visual disturbances take resulting from cerebral damage are less amenable to analysis by inductive reasoning and tax the imagination. Recent developments in neurological imaging have highlighted the structural correlates ofa diverse range of cognitive visual disorders.' The resultant increased understanding and awareness of the origin of these disorders provides the potential to use the remarkable plasticity and adaptability of intact unaffected cerebral function to attempt to circumvent and adapt to these complex visual problems. During the first years of life, play and exploration using all the senses creates a progressively enlarging cross referenced memory store for all aspects of visual performance, ranging from visual resolution to image recognition and understanding in all its forms. An internalised cross referenced multisensory encyclopaedia of imagery and its significance is progressively built up in the mind and provides the substrate for recognition, imagination, and dreams. A continuous process of matching of incoming visual information with the stored data takes place. A pattern match leads to recognition and reinforcement. A failure to match results in learning for the purpose of subsequent recognition. Repetition enhances recall, but lack ofreinforcement renders subsequent recognition more difficult. It is thus not surprising that disturbance of this highly complex system of information storage, retrieval, and matching can cause subjective disorders of visual function at an intellectual level. Children with multiple handicaps due to cerebral disorders may show evidence of a wide range of cognitive visual disorders, but these remain to be classified and understood. In adults, focal brain pathology can disrupt specific components of the cognitive system including those subserving vision. The resultant functional deficits provide insights into the nature of higher visual processing. Many such patients may present first to the ophthalmologist complaining of visual disturbance. The aim of this paper is to review recent literature concerning the more common disorders of cognitive visual impairment as they affect patients likely to present to the ophthalmologist and, where appropriate, to allude to strategies which may assist in diagnosis and management.