Tonic and phasic retinal ganglion cells. Typical examples of phasic (A) and tonic (B) voltage responses to 500-ms step current stimuli. Amplitude of stimulating current step is indicated above each trace. Records were taken from two different RGCs with whole cell capacitances 32.6 pF(A) and 38.5 pF (B). (C) and (D) are plots of the instantaneous firing frequencies as a function of time for cells shown in (A) and (B) respectively.