Abstracts

Neural mechanisms of conscious attention require thalamocortical control of widespread neocortical networks. Absence seizures cause a momentary loss of consciousness, during which the subject is inactive and unresponsive. However, during these seizures there is a remarkable sparing of not only global arousal and postural control but the contents of ongoing memory., Traditional clinical electroencephalographic (EEG) observations of absence seizures have been interpreted to indicate these seizures are [ldquo]generalized,[rdquo] with a sudden and diffuse onset across the entire brain. In applying dense-array (256-channel) EEG to examine a patient[apos]s absence seizures, we found that, although seizure onset appeared to generalize rapidly, both the wave and the spike features of the pathological discharges could be localized to orbital frontal and temporal networks, both during the onset of the seizure and the propagation of stereotyped spike-wave patterns. To test the generality of this finding, we asked four additional patients with absence seizures to volunteer for one-hour dense-array EEG recordings to examine both spatial resolution (256-channel) and temporal resolution (1000 samples per second) of the neurophysiological activity., Twenty-five absence seizures were recorded in the five patients. For each seizure in each patient, source analysis of the dense-array EEG showed that the large discharges of the spike-wave cycle engaged networks of frontal and temporal cortex, with the abrupt wave-spike transition producing maximal current distributions over orbital frontal cortex. Although there were substantial differences shown by individual patients (particularly in the slow wave and oscillatory EEG changes prior to seizure onset), each seizure progressed to a stereotyped spike-wave pattern that was localized to these temporal and frontal networks., These observations suggest that the neurophysiological models of frontal lobe control of thalamocortical projections that were developed in animal research may explain absence episodes in humans. Absence seizures thus imply dual mechanisms of conscious experience, as they isolate frontothalamic mechanisms of voluntary attention from corticolimbic mechanisms of ongoing memory.,