Hippocampal and entorhinal cortex high-frequency oscillations (100-500 Hz)in human epileptic brain and in kainic acid-treated rats with chronic seizures

Purpose: Properties of oscillations with frequencies >100 Hz were studied i n kainic acid (KA)-treated rats and compared with those recorded in normal and kindled rats as well as in patients with epilepsy to determine differen ces associated with epilepsy. Methods: Prolonged in vivo wideband recordings of electrical activity were made in hippocampus and entorhinal cortex (EC) of (a) normal rats, (b) kind led rats, (c) rats having chronic recurrent spontaneous seizures after intr ahippocampal KA injections, and (d) patients with epilepsy undergoing depth electrode evaluation in preparation for surgical treatment. Results: intermittent oscillatory activity ranging from 100 to 200 Hz in fr equency and 50-150 ms in duration was recorded in CA1 and EC of all three a nimal groups, and in epileptic human hippocampus and EC. This activity had the same characteristics in all groups, resembled previously observed "ripp les" described by Buzsaki et al., and appeared to represent field potential s of inhibitory postsynaptic potentials (IPSPs) on principal cells. Unexpec tedly, higher frequency intermittent oscillatory activity ranging from 200 to 500 Hz and 10-100 ms in duration was encountered only in KA-treated rats and patients with epilepsy. These oscillations, termed fast ripples (FRs), were found only adjacent to the epileptogenic lesion in hippocampus, EC, a nd dentate gyrus, and appeared to represent field potential population spik es. Their local origin was indicated by correspondence with the negative ph ase of burst discharges of putative pyramidal cells. Conclusions: The persistence of normal-appearing ripples in epileptic brain support the view that inhibitory processes are preserved. FRs appear to be field potentials reflecting hypersynchronous bursting of excitatory neuron s and provide an opportunity to study the role of this pathophysiologic phe nomenon in epilepsy and seizure initiation. Furthermore, if FR activity is unique to brain areas capable of generating spontaneous seizures, its ident ification could be a powerful functional indicator of the epileptic region in patients evaluated for surgical treatment.