PSYCHOPHYSICAL EVIDENCE

The close relationship between amount of bleached photopigment and sensitivity obtained during long-term dark adaptation strongly indicated that the sensitivity regulation mechanisms were mainly located in the receptors. Both Hecht and Wald had accepted this basic assumption. Rushton (1965a, b), however, strongly opposed this view. Thus, he held that both the signals from bleached photopigments and the signals generated by background light regulated sensitivity centrally to the receptors in what he termed ‘an automatic gain control-summation pool’ (AGC pool). He based his position on two different lines of evidence.

Firstly, he found that a background light so weak that less than 10% of the rods could have caught a single quantum of light raised the threshold three-fold. Apparently, the 10% of the rod receptors that had received a single photon hit had markedly reduced the sensitivity of the 90% that were quite unaffected by the adapting light.

Secondly, Rushton (1965a) could provide strong supporting evidence in favour of his view by both light- and dark-adaptation experiments.

In the light-adaptation experiment, Rushton took advantage of an ingenious technique where he stabilized the light stimuli on the retina. The background was a black-red grating of period 0.5º (presumed to be less than the receptive field of the summation pool) and the test flash, a black-green similar size grating (presumed to activate rod receptors). The test flash superimposed on the background was alternately in- and out-phase.