To assess brain function, we employed two highly reliable neurophysiological indices that can, unlike functional magnetic resonance imaging, be practically employed to study subjects whose body weights (>300 lbs) or trunk diameters (> 60 cm) exceed the physical limits of many research scanners. The indices were the P300a and P300b components of the event related electroencephalographic potential (ERP). The latencies of these components are inversely correlated with the integrity of white matter pathways connecting their generators (36) whereas their amplitudes are more closely related to the gray matter volumes of the generators themselves (37, 38). P300a is generated frontally. In contrast, the P300b has a diffuse distribution of frontal and non-frontal generators (39–41).
