Evidence for two sites of adaptation affecting the dark-adapted ERG of cats and primates

The present study compared the effects of full-field steady adapting backgrounds on the sensitivity of the scotopic threshold response (STR) of the dark-adapted ERG and scotopic PII (b-wave and d.c.-component) to Ganzfeld flashes in cats (n = 4), macaque monkeys (n = 2), and one human subject. In cats, the sensitivity of the STR was reduced by a factor of 2 by backgrounds that were 500 times weaker than backgrounds reducing PII; and for the primates, the STR was reduced by backgrounds almost 100 times weaker than those reducing PII. Since the STR is generated more proximally in the retina than PII, these results provide evidence for proximal and more distal retinal sites of postreceptoral light adaptation. A practical implication is that dim scattered room light can remove the STR from the ERG while hardly affecting PII.