PERSISTENT UNIHEMISPHERIC PERCEPTUAL IMPAIRMENTS IN HUMANS FOLLOWING FOCAL SEIZURES

Perception has been linked to a highly coordinated activation of corti cal regions whose functional organization and performance is subject t o plastic changes. We tested whether chronic repetitive disturbances o f the brain by focal epileptic activity have a long-standing detriment al effect on the perceptual performance in the affected hemisphere. Ni ne patients were examined who had a history of complex partial seizure s but no structural cerebral damage on magnetic resonance imaging and no evidence of ongoing epileptic activity on scalp electroencephalogra phy and who had clinically been without seizures for at least 3 days. The side of primary epileptic involvement was determined by seizure se miology (n = 2), focal electroencephalographic slowing (n = 3) or foca l abnormality during single photon emission topography (SPECT) (n = 4) . The computer controlled psychometric assessment of the somesthetic f requency discrimination revealed that the perception in the hand corre sponding to the affected hemisphere was impaired relative to the contr alateral hand (P < 0.01), and to the performance of a group of normal controls (P < 0.01). We conclude that mechanisms related to focal epil eptic activity can result in regional perceptual decrements even when there is no clinical or surface-electroencephalographic evidence of ep ileptic discharges. This in turn suggests that somatosensory testing m ay be of help in localizing, or at least lateralizing an epileptic foc us.