The size of rod signals

1. This investigation is based upon Alpern's (1965) contrast flash observations. The threshold for the test flash λ (Fig. 2 a ) is raised if a second flash ϕ falls on the annular surround. Moreover, if λ excites rods at threshold, it is only the rods in the surround that contribute to the threshold rise. 2. The possibility that the rise in λ threshold might be due to light physically scattered from surround to centre we exclude by several different experiments. We conclude (Fig. 1 b ) that the ϕ flash sets up a nerve signal N which is conducted to some place C where it inhibits the signal from the centre. 3. If the luminous surround, instead of being a full circle (Fig. 2 a ) consists only of the sectors shown black in Fig. 2 b , that occupy 1/ m of the surround area, it is found (in the physiological range) that the light/area on those sectors must be m times as great to produce the same threshold rise at centre, i.e. the total surround illumination must remain the same. 4. This result would obviously follow if N , the inhibitory nerve signal, were proportional to the total surround illumination. We have established the converse; the signal must be proportional to the quantum catch. 5. Light can be increased indefinitely, nerve signals cannot. When ϕ increases sufficiently, N saturates in the same way that S ‐potentials and receptor potentials saturate, namely according to N = ϕ/(ϕ + σ) where σ, the semi‐saturation constant is about 200 td sec, or 800 quanta absorbed per rod per flash. 6. Thus the nerve signal N is proportional to the quantum catch over 4 log units in the physiological range, namely from 1 quantum per 100 rods to 100 quanta per rod per flash. Above this for another 2 log units N continues to increase, but now more slowly, after the manner of S ‐potentials and receptor potentials.