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.
Article
Brain Basis of Blindsight
Holly Bridge
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Neuroimaging
Chelsea Ekstrand
The growing field of neuroimaging has offered exciting insights into the inner workings of the human brain in health and disease. Structural neuroimaging techniques provide detailed information about the physical properties and anatomy of the brain and nervous system, including cerebrospinal fluid, blood vessels, and different types of tissue. The most commonly used structural neuroimaging techniques are computed tomography (CT) and structural magnetic resonance imaging (MRI). CT uses X-rays to create a two-dimensional representation of neural tissue, whereas MRI quantifies differences in tissue density by manipulating molecules using magnetic fields, magnetic field gradients, and radio waves. Functional neuroimaging techniques provide a measure of when and where activity is occurring in the brain by quantifying underlying physiological processes. Functional neuroimaging techniques fall into two broad categories: measures of direct brain activity, including electroencephalography (EEG) and magnetoencephalography (MEG), and measures of indirect brain activity, such as positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and functional near-infrared spectroscopy (fNIRS). Different functional neuroimaging techniques can be used to examine different physiological changes, including electrical activity, magnetic field changes, metabolic and neurotransmitter activity, and indirect measures of blood flow to offer insight into cognitive processing. Structural and functional neuroimaging have made a profound impact on understanding the brain both during normal functioning and in clinical pathology. Overall, neuroimaging is a powerful tool for both research and clinical practice and offers a noninvasive window into the central nervous system of humans in both health and disease.
