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Abstract:
Neuroimaging studies of visual word processing have not
successfully distinguished orthographic from phonologic processes.
We attempted to delineate these components during implicit word
processing by incorporating difficult-to-pronounce but
orthographically rich letter strings. 26 normal adults underwent
functional MRI while performing a feature (ascender) detection task
on consonant strings, difficult-to-pronounce strings of
high-frequency bigrams (e.g., curng, ewonl, scogt), easily
pronounceable pseudowords (e.g., deach, grume, noops), and words.
Subtraction of consonant strings from word and pseudoword
conditions replicated previous findings, showing left-lateralized
activation in prefrontal, lateral temporal and supramarginal
cortices. Phonological processes, represented by subtraction of
conditions with mainly orthographic information (bigram strings)
from conditions with easily pronounceable stimuli (pseudowords and
words), were associated with left-lateralized activation in pars
triangularis (BA45), pars orbitalis (BA47), anterior superior
temporal gyrus (BA22) and posterior middle temporal gyrus (BA21).
In contrast, orthographic processes, represented by subtraction of
consonant strings from difficult-to-pronounce bigram strings, were
associated with left-lateralized activation in pars opercularis
(BA44/6) and inferior temporal gyrus (BA37), and bilateral
activation in supramarginal gyrus and intraparietal sulcus
(BA40/7). These findings suggest that cortical networks involved in
orthographic and phonologic processes, though highly interactive
under normal circumstances, can be modulated independently by
manipulating statistical properties of stimulus strings.
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