PREFERENTIAL ACCESS TO OBJECT SEMANTICS VIA LEXICAL PROCESSING IN THE VENTRAL STREAM OF THE BRAIN
Date
2019-08-23
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ORCID
Type
Thesis
Degree Level
Masters
Abstract
Converging evidence supports a distributed-plus-hub view of semantic processing in the brain, in which there are distributed modular semantic sub-systems (e.g., for shape, colour, and action) connected to an amodal semantic hub. Furthermore, object semantic processing of colour and shape, and lexical reading and identification, are processed mainly along the ventral stream, while action semantic processing occurs mainly along the dorsal stream. In Experiment 1, participants read a prime word that required imagining either the object or action referent, and then named a lexical word target. In Experiments 2 and 3, participants performed a lexical decision task (LDT) with the same targets as in Experiment 1, in the presence of foils that were legal nonwords (NWs; Experiment 2; allows orthography, phonology, and semantics to contribute to responding) or pseudohomophones (PHs; Experiment 3; allows only orthography to contribute to responding). Semantic priming was similar in effect size regardless of prime type for naming and the LDT with NW foils, but was greater for object primes than action primes for the LDT with PH foils, suggesting a shared-stream advantage when the task demands focus on orthographic lexical processing. Experiment 4 used functional magnetic resonance imaging (fMRI) and identified the potential loci of shared-stream processing to regions in the ventral stream anterior to colour sensitive visual area V4 cortex and anterior to lexical and shape sensitive regions in the left fusiform gyrus, as well as in cerebellar lobule VI. Action priming showed more activation than object priming in dorsal stream motion related regions of the right parietal occipital junction, right superior occipital gyrus, and bilateral visual area V3. Experiment 5 identified structural connectivity using diffusion tensor imaging (DTI), and implicated connections from the cerebellar lobule VI to the anterior temporal lobe (ATL) semantic hub via the thalamus, supporting that this cerebellar region may act as a visual object semantic sub-system of the semantic network. The behavioural experiments demonstrate that object semantic and lexical processing are temporally shared, and the fMRI activation supports the theory that spatially shared-stream activation occurs in the ventral stream during object (but not action) priming of lexical processing. The DTI connectivity analysis supports the theory that lobule VI may act as an additional object semantic sub-system. This research suggests that shared-stream processing occurs between lexical identification and object semantic processing in the ventral stream, providing preferential access to object semantics via lexical processing. This shared-stream processing has implications for models of reading and the semantic system, which currently do not delineate between different modalities of semantic processing. The shared-stream regions identified may prove useful for pre-surgical localization of important intersections between the reading and semantic networks. These results also provide predictions that pure alexia and surface dyslexia patients with comorbid semantic deficits may be disproportionately affected by object semantic deficits compared to action semantic deficits.
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Keywords
fMRI, reading and lexical identification, semantic priming, object and action semantics, ventral and dorsal visual processing streams
Citation
Degree
Master of Arts (M.A.)
Department
Psychology
Program
Psychology