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The sensation of vision arises from the detection of photons of light at the eye, but in order to produce the percept of the world, extensive regions of the brain are required to process the visual information. The majority of information entering the brain via the optic nerve from the eye projects via the lateral geniculate nucleus (LGN) of the thalamus to the primary visual cortex, the largest visual area, having been reorganized such that one side of the brain represents one side of the world. Damage to the primary visual cortex in one hemisphere therefore leads to a loss of conscious vision on the opposite side of the world, known as hemianopia. Despite this cortical blindness, many patients are still able to detect visual stimuli that are presented in the blind region if forced to guess whether a stimulus is present or absent. This is known as “blindsight.” For patients to gain any information (conscious or unconscious) about the visual world, the input from the eye must be processed by the brain. Indeed, there is considerable evidence from functional brain imaging that several visual areas continue to respond to visual stimuli presented within the blind region, even when the patient is unaware of the stimulus. Furthermore, the use of diffusion imaging allows the microstructure of white matter pathways within the visual system to be examined to see whether they are damaged or intact. By comparing patients who have hemianopia with and without blindsight it is possible to determine the pathways that are linked to blindsight function. Through understanding the brain areas and pathways that underlie blindsight in humans and non-human primates, the aim is to use modern neuroscience to guide rehabilitation programs for use after stroke.

The left and right hemispheres of the brain process sensory information in different ways and function differently in controlling behavior. This lateralization of brain function, originally thought to be unique to humans, is now known to occur in a broad range of non-human vertebrates and even in invertebrates, indicating that it is an essential feature of both large and small brains. Some evidence indicates that lateralization of brain function improves cognitive capacity of the brain. Many, often unrelated, brain functions are lateralized. In humans, these include differential specialization of the hemispheres to process language and produce speech, express emotions, respond to faces, attend to spatial information, and control hand use. As now clear for handedness, both genetic expression and environmental influences are involved in complex ways. Since lateral division of function in the brains of non-human animals has sufficient similarity to that of humans, it is probable that at least some asymmetries were evolutionary precursors to language and speech. It is notable that lateralization is in no species totally in a single direction. Some evidence from studies of animal species provides support for the hypothesis that alignment of laterality in most individuals in a species occurs only for control of behavior that requires one lateralized individual to interact with another lateralized individual (i.e., in social interaction). In humans, individuals with non-right-handedness (left-handed and mixed-handed) are more prone to a number of psychiatric conditions or other behavioral conditions, including autism, schizophrenia, and dyslexia.