The inferior occipitotemporal brain areas, especially in the left hemispher
e, have been shown to be involved in the processing of written words and le
tter strings. This processing probably occurs within 200 ms after presentat
ion of the letter string. It has also been suggested that this activation m
ay differ between fluent and dyslexic readers, Using whole-head magnetoence
phalography, we studied the spatiotemporal dynamics of brain processes evok
ed by visually presented letter strings in 12 healthy adult subjects. Our a
chromatic stimuli consisted of rectangular patches in which single letters,
two-letter syllables, four-letter words, or symbol strings of equal length
were embedded and to which variable noise was added, This manipulation dis
sociated three different response patterns, The first of these patterns too
k place similar to 100 ms after stimulus onset, originated in areas surroun
ding the V1 cortex and was distributed along the ventral visual stream, ext
ending laterally as far as V4v, This response was systematically modulated
by noise but was insensitive to the stimulus content, suggesting involvemen
t in early visual analysis. The second pattern took place similar to 150 ms
after stimulus onset and was concentrated in the inferior occipitotemporal
region with left-hemisphere dominance. This activation showed a preference
for letter strings, and its strength and timing correlated with the speed
at which the subjects were able to read words aloud. The third pattern also
occurred in the time window similar to 150 ms after stimulus onset, but or
iginated mainly in the right occipital area, Like the second pattern, it wa
s modulated by string length, but showed no preference for letters compared
with symbols. The present data strongly support the special role of the le
ft inferior occipitotemporal cortex in visual word processing within 200 ms
after stimulus onset.