Letter representations in the mind and brain
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Date
2015-10-20
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Johns Hopkins University
Abstract
Letter identification imposes numerous challenges on the brain's visual system. Identification processes must be flexible enough to recognize that ear and Ear refer to the same word while being precise enough to recognize that lend me your ear and lend me your car have different meanings. At the core of this dissertation is the assumption that the flexibility and precision which enables us to effortlessly recognize letters is based on our mental representations of letters. The experiments presented focus on two types of letter representations: font-invariant allographs and amodal abstract letter identities (ALIs). First, a set of behavioral experiments demonstrated that stored, font-invariant letter shape (allograph) representations influenced the visually similarity judgments participants made to pairs of letters presented in an atypical font. Following this result, an fMRI experiment was performed in which the neural response to visually presented single letter stimuli was analyzed using Representational Similarity Analysis (RSA). This analysis yielded evidence for allograph representations encoded within the left middle occipital gyrus and left fusiform gyrus. Finally, MVPA RSA analyses were employed to compare the patterns of neural responses to visually presented letter shapes and aurally presented letter names. This experiment revealed a region in the left fusiform gyrus that represented amodal ALIs (e.g., a, A, and /eI/ access the same amodal ALI). The research in this dissertation furthers our understanding of the representations that mediate letter identification and the results touch upon fundamental issues about the nature of information processing in cognitive science.
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Keywords
fMRI, MVPA, Cognitive Science, Cognitive Neuroscience