Abstracts

Rationale: High-frequency oscillations (HFOs) at 80 Hz and above can be observed on electrocorticography (ECoG) in humans. A widely-proposed hypothesis is that spontaneous HFOs during slow-wave sleep are likely to be epileptiform in patients with focal epilepsy, whereas HFOs driven by sensorimotor or cognitive tasks are physiological in nature. In this study, we determined the presence of physiological HFOs spontaneously emerging without external sensory stimuli. Methods: We studied 10 patients with focal epilepsy undergoing extraoperative subdural ECoG recording. We determined whether HFOs are spontaneously generated by the non-epileptogenic occipital cortex during interictal slow-wave sleep. We then compared the spectral and spatial characteristics of such spontaneous occipital HFOs with those of HFOs driven by a visual task. We also explored whether the spectral frequency and amplitude of such spontaneous occipital HFOs differed from those of presumably epileptogenic HFOs emerging from the seizure focus outside of the occipital lobe. Results: We identified spontaneous HFOs at ?80 Hz with a mean duration of 350 msec intermittently arising from the non-epileptogenic occipital lobe (Figure 1). The spectral frequency band of these spontaneous occipital HFOs was similar to that of visually-driven HFOs. Spontaneous occipital HFOs were spatially sparse and confined to smaller areas, whereas visually-driven HFOs involved larger occipital areas and were propagated to the rostral direction more extensively. Neither spectral frequency band nor amplitude of spontaneous occipital HFOs of physiological nature were significantly different from those of epileptogenic HFOs arising from non-occipital seizure focus. Spontaneous occipital and epileptogenic HFOs differed in the way their occurrence was coupled to the phases of delta-oscillations. Epileptogenic HFOs were approximately equally and strongly locked to the phase of delta activity in the range from 1.0 to 3.0 Hz. As opposed to that, for spontaneous occipital HFOs, the strength of delta-phase coupling decayed from 1.0 to 3.0 Hz (Figure 2). Conclusions: HFOs of physiological nature are spontaneously generated by the occipital cortex during slow-wave sleep; this observation should be taken into consideration when epileptogenic HFOs are utilized to determine the extent of cortical resection in the context of epilepsy surgery. Coupling of spontaneous delta and HFOs may increase the understanding of the significance of delta-oscillations during slow-wave sleep. This study was supported by NIH grants: NS47550 & NS64033 (to E. Asano).