Not only are the refractory period ranges for the two populations similar, the two distributions also have a similar anomaly: in each case, they show no behavioral improvement when C-T intervals are increased between .6 and .7 msec (39, 40). This suggests that there are two subpopulations of fibers contributing to each behavior: a small subpopulation of very fast fibers (refractory periods ranging from .4 to .6 msec) and a larger subpopulation of slower fibers (refractory periods ranging from .7 to 1.2 msec or perhaps a bit longer).