Separation of the ‘blue’ and ‘green’ mechanisms of foveal vision by measurements of increment thresholds

Abstract

The liminal brightness increment (l.b.i.) of man was detd. for a 1[degree] square of blue light of ap-prox. 470 millimicrons observed by foveal vision against a background of an 18[degree] circle of orange light of approx. 615 millimicrons, of intensity varying from 0 to 100,000 photons. Starting with l.b.i. of 0.017 to 0.27 photons of blue light at 0 orange intensity, curves for log (l.b.i.) vs. log (orange intensity) gradually curve upward (the "green" mechanism of the cones) from the horizontal to the range of 20-800 photons of orange intensity and 0.2-4 photons of blue light when a sharp break and flattening of the curve occurs with progressive upward curving (the "blue" mechanism of the cones). In approx. half of the cases, an upper limit to the "blue" mechanism curve occurred at l.b.i. 9-24 photons. From 1/3-2/3 of the cases showed evidence of a "rod" component curve at l.b.i. lower than the "green" mechanism curve when the orange intensity was 1 to 10 photons. Fatiguing the rod mechanism for 1200 sec. by exposure for 10 min. to 770 candles/ft.2 at 8 cm. allowed for re-measurement of the cone mechanism after 500 sec. The rod component curve may be partly due to foveal rods and partly due to failure of foveal fixation.