important for the robustness of oscillations (Bartos et al., 2007; Vida et al., 2006). Two alternate mechanisms, "PING" (pyramidal-interneuron network gamma oscillations) and "ING" (interneuron network gamma oscillations) have been proposed for the role of inhibitory neurons in the generation of gamma oscillations (Tiesinga and Sejnowski, 2009; Whittington et al., 2000). PING is based on the reciprocal (feedback) connectivity between pyramidal cells and interneurons. Here, the oscillation is generated by the alternation in the firing of interneurons (excited by pyramidal cells) and pyramidal cells (as they re-emerge from the inhibition triggered by interneurons). The fact that individual basket cells contact a very large fraction of neighboring (i.e. within ~100 um) pyramidal cells, and that individual pyramidal cells in turn contact many local inhibitory neurons leads to the synchronous involvement of large populations of neurons in the oscillation. Furthermore, the decay time constant of inhibition of pyramidal cells sets the pace of the oscillation. The alternative mechanism, ING, is solely based on the reciprocal interactions between inhibitory neurons. Basket cells are interconnected via reciprocal inhibitory synapses. Given the right physiological conditions, these synaptically-coupled networks of inhibitory neurons can generate fast synchronous oscillations (Van Vreeswijk et al., 1994). In this model, the
