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Abstract:
McCarthy et al. (1997) showed that infrequent and irregularly
presented visual targets activate prefrontal cortex (PFC),
primarily within the right middle frontal gyrus (MFG). Kirino et
al. (1997) replicated this finding and further demonstrated that
equally infrequent novel visual stimuli (pictures of ordinary
objects) did not activate the MFG. This latter result was
surprising, as event-related potential (ERP) studies have shown
that novel stimuli evoke a P3a component that is prominent over
frontal scalp. Many ERP studies have used novel sounds, such as
gunshots or dog barks, to evoke P3a, suggesting that the sounds'
alerting qualities may be an important factor. Here we report a
study of auditory target processing in PFC in which Standard
(low-frequency tones, ~94%), Target (high-frequency tones, ~3%),
and Novel sounds (e.g., sirens, breaking glass, ~3%) were compared.
A region-of-interest analysis measured signal intensity within the
cingulate, inferior (IFG), middle, and superior frontal gyri. As in
Kirino et al., Targets evoked significant PFC activation, primarily
within the right MFG. Novels evoked little or no activation in MFG,
but did activate the IFG. These results suggest a functional
differentiation in PFC: the MFG processes Target stimuli that are
mapped to specific responses, while the IFG processes Novel stimuli
that, while task-irrelevant, have orienting qualities.
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