Plasticity of Orthographic and Semantic Representations in the Human Brain

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The assignment of meaning to sensory stimuli lies at the foundation of human cognition. Reading written words is an excellent domain through which to study the neural bases of this cognitive process. With support from the National Science Foundation, Maximilian Riesenhuber, Ph.D., of Georgetown University, is integrating behavioral approaches with advanced brain imaging techniques to learn the answer to fundamental questions about how the brain processes written words and learns their meaning. Given the cultural recency of reading and the variability of lexica across languages, reading necessarily depends on specific neural representations that are acquired through experience with written words. For example, the written word 'apple' must be connected to semantic concept information learned previously, from seeing or tasting an actual apple. The underlying neural mechanisms that support the learning of orthographic and semantic information and their integration with perceptual learning are still poorly understood. While researchers have identified an area in the left half of the brain, the so-called 'visual word form area,' that appears to be crucial to reading, scientists debate the precise role of this area: Does it contain a collection of word fragments, a dictionary or an encyclopedia? Are the meanings of words and objects stored together in sensory cortex, or are they linked in higher cognitive areas? How does the brain learn that a particular word refers to a particular concept? How does the brain connect orthographic knowledge, learned in the left half of the brain, to perceptual information that has been shown to be predominantly represented in the right half? Resolving these questions is of key interest for cognitive models of reading and the broader understanding of how the brain makes sense of the world. The project builds on Riesenhuber's previous experimental and computational research on general object recognition and learning and recent results showing that the human brain contains a visual word dictionary in the 'visual word form area.' The first study is examining how learning new words refines the brain's visual word representation. The second is examining how novel words are linked to meaning in the brain, testing the hypothesis that the visual word dictionary contains only orthographic information, and that the meanings of words are stored in other brain areas. The third is investigating how orthographic, semantic, and perceptual information are integrated; and how the two brain hemispheres cooperate in linking pictures of objects to verbal labels. The project not only increases the understanding of normal visual word recognition, and how the brain learns to represent objects and associate semantic meaning; but it also provides a framework that can be used to study topics such as reading acquisition, second language learning, and disordered reading. In addition, the techniques developed for this line of research can be translated to study the neural representation of different writing systems, specifically logographic scripts such as Chinese, and the integration of perceptual and semantic information in sign language. The research project is leveraged extensively to train the next generation of scientists, at the high school, undergraduate, and graduate levels. The project includes a summer outreach program, emphasizing the involvement of underrepresented groups through a partnership with Howard University in Washington, DC. In addition, the project presents opportunities for high school students with an interest in cognitive neuroscience to gain hands-on research experience, through a partnership with Thomas Jefferson High School for Science and Technology in Alexandria, VA.