The similarity structure of distributed neural responses reveals the multiple representations of letters

Most cognitive theories of reading and spelling posit modality-specific representations of letter shapes, spoken letter names, and motor plans as well as abstract, amodal letter representations that serve to unify the various modality-specific formats. However, fundamental questions remain regarding the very existence of abstract letter representations, the neuro-topography of the different types of letter representations, and the degree of cortical selectivity for orthographic information. We directly test quantitative models of the similarity/dissimilarity structure of distributed neural representations of letters using Multivariate Pattern Analysis-Representational Similarity Analysis (MVPA-RSA) searchlight methods to analyze the BOLD response recorded from single letter viewing. These analyses reveal a left hemisphere ventral temporal region selectively tuned to abstract letter representations as well as substrates tuned to modality-specific (visual, phonological and motoric) representations of letters. The approaches applied in this research address various shortcomings of previous studies that have investigated these questions and, therefore, the findings we report serve to advance our understanding of the nature and format of the representations that occur within the various sub-regions of the large-scale networks used in reading and spelling.