Date of Graduation

8-2013

Document Type

Dissertation (PhD)

Program Affiliation

Neuroscience

Degree Name

Doctor of Philosophy (PhD)

Advisor/Committee Chair

Nitin Tandon, MD

Committee Member

Edgar T Walters, PhD

Committee Member

Steve Cox, PhD

Committee Member

Valentin Dragoi, PhD

Committee Member

Arne Ekstrom, PhD

Committee Member

Harel Shouval, PhD

Abstract

Recognition and naming of objects and actions are fundamental components of language. They involve several different systems working in coordination to accomplish a complex behavior. During visual naming, sensory and semantic processing are carried out by dedicated cortical substrates in the temporal and occipital lobes, while response selection and articulatory planning are handled by prefrontal cortex. Despite decades of research using lesion analysis, functional MRI and electro-encephalography, the precise dynamics involved remain unknown due to the inadequate spatio-temporal resolution of these methodologies. Of particular interest is the organization of semantic knowledge and the degree of serial and parallel organization of the language production system. To better understand these issues, we studied epilepsy patients undergoing electro-corticography during visual naming of nouns and verbs. Employing novel methods for grouped data analysis, we found that initial processing concurrently activated ventral temporo-occipital cortex, which implies that this level of semantic processing occurs in parallel. However, we found significant differences, both in extent and location, between noun and verb processing in the visual ventral (noun) and dorsal (verb) processing streams. This suggests that slightly different networks are involved in the storage and retrieval of separate grammatical categories. To characterize subsequent activity within prefrontal cortex, we used measures of information flow to investigate the network dynamics of speech production. Unlike sensory processing, we found distinct processing stages during which separate operations are executed in parallel. Specifically, pars orbitalis of left inferior frontal gyrus controlled the onset of response selection and phonological planning, which were executed at the same time. Processing was subsequently terminated through ascending input from motor cortex. The interplay of these control signals, both ascending and descending, resulted in distinct state transitions, implying a degree of seriality to speech production following the parallelism of semantic retrieval. The differences in processing architecture between retrieval and articulation are possibly related to the dissociations of function – memory storage and access are largely associative, whereas motor actions are tightly controlled over time.

Keywords

Language, electrocorticography, visual naming, fMRI, information flow, semantics, functional connectivity