Fig. 3.

(a) Extrapolation of the relation of response amplitude as a function of stimulus intensity gives the threshold of the impulse response to an excitatory stimulus. (b) Extrapolation of the measured rate constants to threshold gives the rate constant of zero at response amplitude of zero, which is evidence for the set point at which the background activity is stabilized by neurohumoral controls. That regulation is expressed in the limitations on neural firing rates imposed by their refractory periods. Adapted from Freeman (1974)