Cohort-Selective Gamma Rhythms Support Hierarchical Visual Processing During Word Recognition

Models of lexical retrieval suggest visual input during reading activates neural ensembles as soon as the viewer begins scanning segments of visual information (Marslen-Wilson[1987]). Support of these models is based on behavioral data from fast-reaction time tasks showing that the initial sequence of a word is instrumental in 1) activating the cohort of related word-forms and 2) selecting the form best matching the input. We tested this theory using electrocorticographic recordings (ECoG) of cortical local field potentials (LFP) taken from epilepsy patients while they viewed words and individual letters on a laptop computer. Lexical processing of rst-letter information was observed in the form of a letter-specific negative event related potential (ERP) at 200 ms (N200) in posterior cortex and a positive ERP after 300 ms in anterior cortex. Positive word form related ERPs were observed again at posterior sites after activation of anterior cortex, providing support for hierarchical models of object recognition. A spectral decomposition of intracranial EEG signals disclosed specific narrowband frequencies involved in both differentiation of individual letter identities and lexical processing of word first letter information. Similar patterns of of high-gamma band (65-128 Hz) activity in occipito-temporal cortex were found to persist when letters were presented in isolation or as the first part of a word. Broad tuning to letter stimuli in the form of high-gamma band selectivity to both individual letters and word primary letters illustrates a potential neural substrate for lexical cohorts in vision and association cortex during visual word recognition.M.S., Biomedical Engineering -- Drexel University, 201