Brief periods of fast oscillations are prominent components of the sensory-evoked potential recorded in the cortex and thalamus of anesthetized and awake animals (Barth, 2003), including humans (Curio, 2000). Consistent evidence indicates that these fast oscillations reflect the synchronized firing of local neural elements. However, how this synchronization is brought about and what exactly is the biophysical correlate of fast oscillations have been matters of intense discussion. It is generally accepted that high frequency synchronization is confined to small neuronal space (Buzsaki & Draguhn, 2004). In this study, by recording in the isthmi nuclei of the pigeon, we show that a confined, high frequency synchrony event can spread to a larger neuronal surface by an organized divergent efferent projection. The nucleus isthmo pars parvocellularis (Ipc) and the nucleus isthmo pars magnocellularis (Imc) are laminar structures reciprocally connected to the optic tectum. Imc neurons, presumably GABAergic, also project to Ipc with widely ramified axonal fields covering most of the nucleus (Wang et al. 2004a). The isthmi constitute a neural loop that modulates the visual responses of tectal cells (Wang et al. 2004b). We recorded neuronal activity in Ipc and Imc in response to flashing and visual motion stimulation. We did single and dual microelectrode recordings using tungsten and fine pipettes in anesthetized pigeons (ketamine/xylazine, 5/50 mg/kg, i.m.; anaesthesia was maintained by supplementary doses every 2 h). Ipc neurons responded with trains of high frequency bursts to visual stimulation presented in a 15-20 deg excitatory receptive field (RF). In addition to this response, in every Ipc recording loci, visual stimulation of discrete regions outside the RF elicited a high frequency, 600 Hz, negative wave of variable amplitude (100-400 μV). Recording from two widely separated regions (0.7-1 mm) in Ipc revealed that, while each region has a distinct excitatory RF, both regions share the locations in visual space from where this high frequency oscillation was elicited. Notably, the oscillations recorded from both electrodes almost totally overlapped. Intracellular staining with biocytin, demonstrated that Imc neurons and Imc terminals in Ipc, with RFs overlapping these visual regions, fired in high frequency barrages phase-locked to this wave. These results indicate that a local ensamble of Imc neurons can get sinchronized in the sub-millisecond range and induce a phase locked oscillation in their terminal fields throughout Ipc. Consequently, separated regions in Ipc would receive a common input drive synchronized at the same timescale.