Abstracts

High-frequency oscillations ([gt]60 Hz, HFO) have been associated with interictal discharges (IID) and seizure onset. This association may provide an important clue in revealing the mechanisms of epileptiform activity. To more completely understand the generation of HFO we performed laminar microelectrode recordings in neocortex of epileptic patients. Linear arrays of 24 closely spaced microelectrodes (intercontact distance 150[mu]) were inserted perpendicular to the pia beneath a subdural electrode grid in patients undergoing evaluation for medically intractable epilepsy. We recorded spatial potential gradient (PG) and multi-unit activity (MUA). Population transmembrane currents were estimated from PG as Current Source Density (CSD). These data allow assessment of synaptic, transmembrane and cellular activity with respect to different cortical layers. IID were selected based on a characteristic morphology of a fast ([lt]200 ms), followed by slower component. IIDs were analyzed for frequency components using wavelets and FFT. 30-536 IIDs were recorded in each of 8 subjects. The spike or sharp-wave component of the IID was characterized by a prominent sink and increased MUA, consistent with synchronous, local, neuronal depolarization. In 3 subjects, low-amplitude 80-120 Hz ripples followed the initial current sink of the event (Fig A). In 2 of these cases, bursts were seen only in a subset of IIDs. In 2 subjects, 100-250 Hz oscillations were observed during, and sometimes beyond, the rising phase of a prolonged initial component (Fig B). In both situations HFO had greatest power in upper cortical layers. Events in 3 subjects, however, showed no obvious, sustained HFO.
No HFO were observed before discharge onset. In fact, IID were usually preceded by a brief period ([sim]50-250 ms) in which broadband power was decreased (Fig C arrow). A similar decrement followed the discharge (small arrow). HFO are present in IID of some patients with neocortical epilepsy. These oscillations do not precede the event but form a component of the discharge itself. Spectral power may decrease before and after discharges. These data suggest that HFO are not necessary in the generation of all neocortical discharges but may play a role in its maturation under certain circumstances.[figure1] (Supported by NIH-grants NS18741, NS44623 to EH; R.S. Morrison Fellowship to SSC.)