Our lab has published a lot of studies in which we used a glass microelectrode (silver wire in 3M NaCL solution) for extracellular intracranial microstimulation (ICMS) in the anesthetized rat. We used a screw placed into the skull over the cerebellum (behind lambda) as a ground and were able to see isolated whisker twitches using this method. However, for the study I'm starting now, I want to limit the current spread by using a concentric bipolar stimulating electrode. The goal is to stimulate just one or a few callosal projection neurons in frontal motor cortex of one hemisphere, and record (extracellular, single-unit) the response of neurons in the corresponding area of the other hemisphere. The problem is that there are a lot of specifications that I have to make when ordering these electrodes. The only one I'm sure about is that I want a rounded tip...can you suggest guidelines for choosing diameters of the inner and outer poles, etc.?