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Abstract:
Prior work (Cog. Brain Res. 5:283, 1997), using a continuous
recognition task, demonstrated highly similar variation in accuracy
and reaction time as a function of number of intervening items for
nonverbalizable, colored images and four-letter English words. This
suggests that these materials, putatively engaging predominantly
right versus left cerebral hemispheres, respectively, somehow
utilize similar mnemonic processes. Essentially the same task was
thus performed by 6 subjects during four stages of fMRI scanning,
30 sec per stage: 1) fixating a small point; 2) observing a
200-msec presentation of a word or an image; 3) endeavoring to
remember similarly presented words or images; 4) pressing a button
to indicate whether presented words or images were "NEW", or had
been seen before, "OLD". Blocks of words or of images were repeated
three times. Performance was comparable to the earlier study. For
the encoding phase, i.e., stages 2 (since encoding of visual input
is unavoidable!) and 3, the activation patterns showed little
overlap: right dorsolateral prefrontal cortex and bilateral
striate/extrastriate and fusiform cortex for images; left ventral
extrastriate for words. Retrieval (stage 4), however, generated
patterns with a high degree of commonality: right dorsolateral
prefrontal cortex, cingulate gyrus, bilateral insula, premotor
cortices, thalamus and fusiform gyri. There thus appears to be a
sharing of mnemonic processes primarily during retrieval.
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