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Abstract:
Single-cell recordings from the temporal lobe of monkeys
viewing stimuli show that cells may be highly selective, responding
for example to particular complex non-objects (e.g. Fujita et al.,
1992) or to objects such as faces (e.g. Perrett et al., 1992).
However, stimulus-selective cells may be inhibited by non-preferred
stimuli (e.g. Perrett et al., 1991). Can such inhibitory
"sharpening" mechanisms be detected in human visual cortex? The
fusiform gyrus generates N200 ERPs to specific categories of
stimuli such as faces and words (e.g. Allison et al., 1999).
Laminar recordings and CSD analysis in animal cortex (e.g. Mehta et
al., 2000) suggest that the human N200 reflects excitatory
depolarizing potentials in apical dendrites of pyramidal cells. We
found that 26% of face-specific N200 sites generate a P200 ERP to
words. Conversely, 56% of word-specific sites generate P200s to
faces. P200 likely reflects hyperpolarizing inhibition of apical
dendrites. These results suggest that: cells responsive to one
stimulus category inhibit cells responsive to another category;
inhibition of word-specific cells by faces appears to be stronger
than the converse; inhibition also requires energy, hence it is
likely that fMRI studies of face and word processing activate areas
involved both in "center" excitation and "surround"
inhibition.
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