If you are using a 50 micron wire, it should be stiff enough to penetrate the brain once you have removed the dura.  If you extend the wire about 1.5mm the cannula can set on the top of the brain with the wire going into the hippocampus.  If the cannula goes into the brain, that's ok, but you do not want to damage the brain too close to recording site.  Use a silver ball as a reference.