The electroencephalogram (EEG) is the continuously varying difference in electrical potential between two points on the scalp. This potential difference is produced by postsynaptic potentials (not action potentials) in the underlying cortical neurons, principally large pyramidal cells in cortical layers 3, 4, and 5. Changes in neuronal function produce recognizable EEG patterns which have been analyzed and classified ( P^or..laQ.d...lMay.n§rd 1986; G.rUQ.,d.y..,199.5). An evoked potential is the reproducible change in electrical potential recorded from the appropriate sensory pathway, or sensory cortex, following a stimulus to sensory receptors (e.g. the eye for the visual evoked potential) or part of the pathway (e.g. a peripheral nerve for the somatosensory evoked potential). Therefore cortical evoked potentials are affected by lesions anywhere between the site of stimulation and the sensory cortex, and lesions can sometimes be localized by recording subcortical responses to the same stimulus.
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