Cognition and paroxysmal EEG activities: from a single spike to electrical status epilepticus during sleep.

Epileptic EEG paroxysms can interfere with cognitive processes producing transitory effects, such as those related to a single spike, as well as long-lasting effects, such as in electrical status epilepticus during slow-wave sleep (ESES). Focal spike-related disruption of cortical functions can produce transitory cognitive impairment, with neuroanatomical specificity between the site of the epileptic focus and the impaired cognitive tasks. ESES represents a model of the long-lasting effects of continuous spike-wave activity on higher cortical functions. The duration of ESES and the localization of interictal foci seem to play a major role in influencing the degree and type of cognitive dysfunction, suggesting that the ESES clinical picture results from a localized disruption of EEG activity caused by focal epileptic activity during sleep. Recently, Giulio Tononi's group reported that a local increase of slow-wave activity (SWA) during sleep after learning is associated with improved performance of the learned task after sleep (Huber et al., Nature 2004;430:78-81). On the basis of these findings, we can speculate that prolonged focal epileptic activity during sleep (as occurring in ESES) interferes with local SWA at the site of the epileptic focus, impairing the neural processes and, possibly, the local plastic changes associated with learning and other cognitive functions.