The influence of a rod-detected mask of illuminance IR on the threshold illuminance of a cone-detected test flash (ICth) was assessed while rods were recovering from the effects of a bleach, and when rods were selectively light adapted. Providing that IR was restricted to within 2 log10 units of rod mask threshold (IRth), results show that ICth/IC0 = K (square root (IR/IRth) + D), where IC0 is cone absolute threshold, D is a dark noise term and K is a proportionality constant. These data were used to obtain 'equivalent background functions' or 'Crawford (1947) transforms' (illuminance of a background field plotted against time in the dark). The same Crawford transform was obtained when either IRth or ICth (in the presence of a fixed illuminance IR) were used as equating variables. All of the foregoing results could be predicted by considering both the influence of light adaptation on rods (see Fain, 1976) and the model developed by Bauer, Frumkes & Nygaard (1982). Under dark-adapted conditions, 40' and 60' diameter rod masks of equal illuminance have very similar influences on ICth. When rods are selectively light adapted 60' masks have smaller influences on ICth. The foregoing results were used to extend the model developed in the previous paper. We suggest that rod adaptation has a distinct influence on neural loci designated E (the excitatory spatial summator) and I (the inhibitory spatial summator), and that E represents a site for both adaptation and spatial summation.
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