Duplicity Theory of Vision: From Newton to the Present

158 theories of sensitivity regulationlight did not raise the quantum demand at the receptor level (i.e.the threshold for rod-receptor signals did not increase) and that alladaptation occurred at the pool.This conclusion was also supported by experiments on darkadaptation where the bleaching light was sent by means of a briefelectronic flash either through a grating of 0.5º period to give astriped bleach or through a neutral filter to pass an equal amount oflight spread evenly on the retina. The test flash was under the lattercondition a grating of 0.5º period, while it was an equalizing neutraltest light in the former. Again, no significant differences between thetwo conditions were obtained.Rushton (1965a) argued that if sensitivity regulation occurredat the receptor level, the dark bars of the grating should have shieldedthe retina from the flash of the bleaching light. The retina should thushave been fully dark adapted at the shielded regions, while every rodin the bleached area should have been substantially bleached whenthe same total bleaching light was evenly distributed on the retina.Hence, from his results he concluded that bleaching with the gratingor with the same light spread evenly had about the same effect onthe threshold level, and also that the threshold level obtained by thetest flash was the same whether the test energy was concentratedby the grating or spread evenly – all in harmony with his AGC-poolhypothesis.20.2 Anatomical and electrophysiologicalevidenceAnatomical and electrophysiological evidence supporting Rushton’spost-receptor adaptation theory was presented by Dowling (1967).Together with Boycott he had examined the synaptic organizationof the retina in several types of vertebrate retina, including theprimate, using an electron microscope. Striking support for Rushton’sAGC-pool hypothesis was obtained. Thus, they found widespreadreciprocal synapses between bipolar cell terminals and amacrine-cellprocesses. The anatomical evidence indicated that bipolar cells made