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Abstract:
Previous PET studies have suggested there is a right
extrastriate occipital region specialized for visual form, which
appeared to be more active for false-font character strings than
consonant strings or words (reviewed in Liotti et al., J. Clin.
Neurophys., 1994; Buckner et al., OHBM Mtg., 1995). We have
investigated this phenomenon with ERPs (n=9), using a block-design
approach as would be done with PET, and found a clear, focal,
effect over right occipital cortex peaking at ~180 msec ("ROcc180")
for strings of nonletter symbols (e.g., pound signs, parentheses)
relative to consonant strings and words. Moreover, in a second
experiment (n=11) with randomly interspersed stimulus types (words,
consonant strings, nonletter-symbol strings, and false-font
strings, matched for physical characteristics), we again found this
effect, for both symbol and false-font strings relative to
consonant strings and words. The distributions of the symbol and
false-font effects differed somewhat, but each was well modeled by
a single dipole. The estimated dipole location for the false font
effect was within a cm of the previously identified right-occipital
PET effect (Residual Variance(RV)=5.8%); the symbol effect dipole
was more dorsal and medial (RV=2.8%). These effects would appear to
reflect enhanced processing in these right occipital regions and to
provide the timing characteristics of the previously found PET
effects.
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