Abstract
A standard model of word reading postulates that visual information
is initially processed by occipitotemporal areas contralateral to
the stimulated hemifield, from whence it is subsequently transferred
to the visual word form (VWF) system, a left inferior temporal region
specifically devoted to the processing of letter strings. For stimuli
displayed in the left visual field, this transfer proceeds from the
right to the left hemisphere through the posterior portion of the
corpus callosum. In order to characterize the spatial and temporal
organization of these processes, reading tasks with split-field presentation
were performed by five control subjects and by two patients suffering
from left hemialexia following posterior callosal lesions. The subjects'
responses were studied using behavioural measures and functional
brain imaging techniques, providing both high spatial resolution
(functional MRI, fMRI) and high temporal resolution (high-density
event-related potentials, ERPs). Early visual processing was revealed
as activations contralateral to stimulation, located by fMRI in the
inferior occipitotemporal region and presumably coincident with area
V4. A negative wave occurring 150-160 ms post-stimulus, also strictly
contralateral to stimulation, was recorded over posterior electrodes.
In contrast with these hemifield-dependent effects, the VWF system
was revealed as a strictly left-hemispheric activation which, in
control subjects, was identical for stimuli presented in the left
or in the right hemifield and was located in the middle portion of
the left fusiform gyrus. The electrical signature of the VWF system
consisted of a unilateral sharp negativity, recorded 180-200 ms post-stimulus
over left inferior temporal electrodes. In callosal patients, due
to the inability of visual information to pass across the posterior
part of the corpus callosum, the VWF system was activated only by
stimuli presented in the right visual field. Similarly, a significant
influence of the word/non-word status on ERPs recorded over the left
hemisphere was discernible for either hemifield in controls, while
it affected only right-hemifield stimuli in callosal patients. These
findings provide direct support for the main components of the classical
model of reading and help specify their timing and cerebral substrates.
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