The off-effect of the pure-cone electroretinogram of the squirrel was studied by means of double-flash stimuli. It was found that as the stimulus intensity was increased there was often a depression of the off-effect, a phenomenon attributed by Granit to postexcitatory inhibition. When a second flash was superimposed on the normal off-effect, there was an enhancement of the a wave and a depression of the b wave of the second response. If the second flash were superimposed on a depressed high-intensity off-effect, the increase of the second a wave was much less and the depression of the second b wave greater than with a normal off-effect. These results confirm Granit’s suggestion as to the nature of postexcitatory inhibition. There is apparently a reactivation of both PII and PIII at off in the pure-cone retina. The recovery of the b wave after off follows an exponential course and may well be due to the decay of the suppression process recently postulated by Arden, Granit, and Ponte.
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