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Abstract:
Target stimuli elicit event-related brain potentials (ERPs)
and alter the background EEG. Time-domain signal averaging isolates
time-locked ERPs, whereas frequency domain averaging isolates
event-related difference spectra (ERDISPs) that include ERP
activity as well as phase-varying evoked activity and modulations
in background EEG rhythms. We compared target-evoked ERPs and
ERDISPs in a visual oddball task in control subjects (N=16) and
patients with unilateral lesions of the frontal lobe (N=10),
temporal/parietal junction (N=7), and mesial temporal lobe
including the hippocampus (N=7). In control subjects, ERPs showed
N2 and P3 components. ERDISPs showed target-related increases in
delta, theta and gamma power, and target-related reductions in
alpha and beta power. Different patterns of lesion-induced
abnormalities were seen in ERPs and EEG spectra. For example,
hippocampal lesions did not affect parietal P3b amplitudes, but
markedly diminished target-related theta enhancements. Post-target
alpha desynchronization was reduced in hippocampal patients and
selectively diminished over the lesioned hemisphere in
temporal/parietal and frontal patients. The results suggest that
ERDISP and ERPs index the functioning of distinct neural networks
involved in target detection.
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