Neural Representations of Visual Words and Objects: A Functional MRI Study on the Modularity of Reading and Object Processing
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There have been several studies supporting the notion of a ventral-dorsal distinction in the primate cortex for visual object processing, whereby the ventral stream specializes in object identification, and the dorsal stream is engaged during object localization and interaction. There is also a growing body of evidence supporting a ventral stream that specializes in lexical (i.e., whole-word) reading, and a dorsal stream that is engaged during sub-lexical reading (i.e., phonetic decoding). Here, we consider the extent to which word-reading processes are located in regions either intersecting with, or unique from, regions that sub-serve object processing along these streams. Object identification was contrasted with lexical-based reading, and object interaction processing (i.e., deciding how to interact with an object) was contrasted with sub-lexical reading. Our results suggest that object identification and lexical-based reading are largely ventral and modular, showing mainly unique regions of activation (parahippocampal and occipital-temporal gyri function associated with object identification, and lingual, lateral occipital, and posterior inferior temporal gyri function associated with lexical-based reading) and very little shared activation (posterior inferior frontal gyrus). Object interaction processing and phonetic decoding are largely dorsal, and show both modular regions of activation (more lateralized to the dorsal-frontal right hemisphere for pseudohomophone naming, and more to the dorsal-frontal left hemisphere for the object interaction task) as well as significant shared regions of processing (precentral gyri, left inferior frontal cortex, left postcentral gyrus, left lateral occipital cortex, and superior posterior temporal gyri). Given that the perceptual experimental conditions show primarily modular and very little shared processing, whereas the analytical conditions show both substantial modular and shared processing, we discuss a reconsideration of “modularity of mind” which involves a continuum between strictly modular processing and varying degrees of shared processing, and which also depends on the nature of the tasks compared (i.e., perceptual versus analytical).
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