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Abstract:
Patients with pure alexia due to left hemisphere damage may
appear to recognize words they cannot pronounce, suggesting
residual lexical access by the mute right hemisphere.
Alternatively, these patients may use sublexical information to
classify words. The aim of this study was to distinguish lexical
from orthographic processing in three severe alexics. The patients
had large left occipitotemporal infarctions with resulting right
hemianopia and complete alexia precluding even letter-by-letter
reading. Patients performed at chance on reading tasks requiring
semantic access, such as word-picture matching. Despite absent
phonological or semantic access, all patients could discriminate
words from nonwords. Sensitivity to orthographic information was
demonstrated by successful detection of very low frequency bigrams
on an odd-out task. On a choice lexical decision task, subjects
preferred nonwords composed of high-frequency bigrams to nonwords
with low-frequency bigrams. In the critical experiment, subjects
were required to decide which of two stimuli was a word. The
word-nonword pairs were constructed so that the lexical factor,
word frequency, and an orthographic factor, mean bigram frequency,
were assessed independently in a crossed design. Results showed no
effect of word frequency but a large effect of bigram frequency on
lexical decision outcome (chi-sq(1) = 31.56, p<.0001). These
results suggest an absence of lexical access by the isolated right
hemisphere, which can, however, perform lexical decision on the
basis of orthographic information.
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