Abstracts

RATIONALE: We investigated voltage versus depth profiles and unit correlates of two types of high frequency oscillations in the entorhinal cortex (EC) of mesial temporal lobe epilepsy (MTLE)patients. The first type of oscillation, called ripples (R),has a frequency range of (80-160 Hz). The second type of oscillation, called fast ripples (FR)has a frequency range of (250-500 Hz). METHODS: Wide band recordings of local field potentials and unit activity were caried out in the EC of patients with MTLE using microelectrode arrays with tips spaced 500 m in vertical separation.Using offline analyses, voltage versus depth profiles were constructed, and crosscorrelation analysis between unit activity and local field oscillations were carried out. RESULTS:_Both R and FR were observed on the same microelectrode array. They occurred predominantly during drowsiness and/or slow wave sleep preceding or coinciding with IIS. Voltage versus depth profiles showed that R do not show inversion in different layers nor do they reverse in superficial layers whereas FR show inversion in the middle layers of the EC. Although the synchronization of multiunit activity increases during both R and FR generation, the degree of synchronization of multiunit activity during FR was significantly higher than during R. CONCLUSIONS:_ On the basis of our experiments we conclude that the substrates of R and FR generation can be the same while the mechanisms of generation of these events can be different. R appear to be generated due to simultaneous activation of a network of principal cells and interneurons. FR are generated by synchronization occurring specifically within a network of principal cells. Supported by NIH grants NS 08208 and NS 33310.