The Human Visual Pathways and Skilled Reading

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Thought and emotion are processed in the brain in large part by cortical neurons, small cells located in a thin (3-5mm) sheet on the surface of the brain. These neurons combine signals from different parts of the brain. The inputs and outputs of these neurons, the brain's wiring, are the axons. Some axons make short-range connections between neurons. They carry signals only a few tenths of a millimeter. Other axons make long-range connections. They carry the signals several centimeters between neurons in widely separated parts of cortex. In recent years, for the first time in human history, it has become possible to measure in the living human brain the path traveled by the long-range axons. With funding from the National Science Foundation, Dr. Brian Wandell of Stanford University is developing methods for measuring and identifying, noninvasively, the path followed by axons that travel a few centimeters. The new methods are based on magnetic resonance imaging data and specialized mathematical algorithms. This project is implementing a mathematical method to test the accuracy and improve the spatial resolution of these methods. This methodological advance will make it possible to make new brain measurements to identify the axons in visual cortex that carry the signals essential for seeing and reading. Experiments are being conducted to identify all the major white matter fiber tracts in the human visual cortex and also to identify specifically the tracts from visual to reading areas of the brain.The long-range connections essential for vision are located in the posterior part of the brain. When we read, these visual signals are carried to more anterior parts of the brain. The neurons in these anterior regions interpret the shape of the letters and words, and are essential for skilled reading. The ability to resolve the reading pathways in individuals, including both the cortex and the axons, is within reach. This project is implementing new mathematical methods to identify these pathways in individual subjects. The first measurements are designed to understand the development of the long-range visual connections essential for seeing. A full wiring diagram of the relevant brain pathways and measurements of typical growth trajectories will make it possible to identify developmental abnormalities in individual children so that they can receive appropriate intervention. The methods will be applicable to many other parts of the brain, and there is a plan for wide dissemination of the analysis software.