Visual Perception & Visuospatial Functions
Visual perception and recognition depend on the functions of the eye, the muscles that move the eye, the optic pathway, as well as the visual cortex. Lesions in the temporal, occipital and parietal lobes of the right hemisphere are particularly likely to lead to disturbances in visual perception.
The process of recognition begins in the cortex through the analysis of simple characteristics such as borders, colors, and contrasts. After the discrimination of these characteristics, the whole shape (Gestalt) starts to form a three-dimensional representation, which is finally identified through knowledge stored in the memory. "Visual agnosia" means that the person can see but does not recognize what he or she sees. The difficulty in recognition may affect both pictures and objects. The visual agnosias are traditionally classified into two major categories: associative agnosias arising from a failure in recognition or generic knowledge, and apperceptive agnosias due to problems in perception. The specific type of agnostic disorder depends on the lesion site and the particular functions involved. Special forms of agnosia include difficulties in recognizing and differentiating faces and difficulties with colors.
"Visuospatial functions" include the ability to understand visual representations and their spatial relationships. The term “spatial” refers to perceiving space, distances, and directions.
Performance of tasks involving construction combine perceptual processes with motor activity and always have a spatial component. Patients with constructional disturbances tend to take a piecemeal, fragmented approach, losing the overall gestalt of the construction task.
The phenomenon of "visual inattention", also called "visual neglect", involves lack of awareness of visual stimuli on one side of the field of vision, usually opposite to the side of the brain lesion