Network mechanisms for fast ripple activity in epileptic tissue

Fast ripples are high-frequency, 250-600Hz field potential oscillations which can be recorded from hippocampal or neocortical structures. In the neocortex, fast ripples occur during both sensory information processing and under pathological, epileptic conditions. In the hippocampus and entorhinal cortex, fast ripples are exclusively associated with epilepsy and perhaps even mark the epileptogenic focus. In contrast to ripples, which regularly also occur in normal tissue and which are thought to reflect population spike bursts at 100-200Hz paced and synchronised by recurrent inhibition, the fast ripple frequency range exceeds the maximal firing frequency of most neurones. Hence, particularly in the hippocampus, fast ripples must emerge as a network phenomenon and cannot reflect the activity of single spiking neurones. In this review, current views on the mechanisms and processes underlying fast ripples are discussed.