Investigating the neural code for single-word reading
Glezer, Laurie Jeanne Schwarz.
Thesis (Ph.D.)--Georgetown University, 2009.; Includes bibliographical references.; Text (Electronic thesis) in PDF format. Successful reading requires the brain to correctly recognize printed individual words. It has been proposed that single-word reading can be viewed as a special case of visual object recognition with its dual goals of achieving specificity and invariance. This hypothesis allows us to leverage theories of object recognition to provide hypotheses for the neural bases of single-word reading, in particular the nature of the representation of visual words, how this representation is shaped by expertise, and is modulated dynamically by attentional effects. The specific aims of this project were to a) probe the selectivity of neurons in an area in the inferior temporal cortex, the visual word form area (VWFA), b) examine the evidence for a hierarchical organization of the visual word form representation along the ventral visual stream, and c) to examine hemispheric specialization in word form processing by comparing the neural representation in the left and right VWFA. The technique used in this project provides an innovative way to examine the neural correlates of single word reading in the occipitotemporal cortex, allowing us to probe neuronal tuning more directly. Here, we provide evidence that the left VWFA holds a neural representation that is finely tuned to whole real words, akin to an orthographic lexicon. We futher find evidence for hierarchical processing along the left ventral visual stream from sublexical in posterior areas to lexical in the the VWFA. And finally, we provide support for theories of left hemispheric specialization in word form processing showing the the left but not the right hemisphere contain neurons tightly tuned to whole real words. Given the cultural recency of reading and the variability of lexica across languages, reading arguably needs to depend on neural representations that are acquired through experience with written words. Our results therefore provide strong support for theories of experience-driven plasticity of the neural representations in the left hemisphere underlying reading establishing that this learning does not just apply to lower level representations, for characters and sublexical letter combinations but also to whole words. This "simple-to-complex" hierarchy of single-word reading fits well with general theories of object recognition in cortex and also provides a powerful framework to not only investigate neural plasticity at the general level by using orthographic plasticity as a well controlled example, but also specifically in the case of reading acquisition. It also provides an important first step in understanding orthographic representations that can now be applied to studying higher levels of the reading process in the typical and atypical population.
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