AREA AND THE INTENSITY-TIME RELATION IN THE PERIPHERAL RETINA

For short flashes of light and small areas of illumination the product of intensity (I) and exposure time (t) is constant for threshold excitation of the human eye (Bloch, 1885; Pi&on, 1920; Braunstein, 1923). This relationship does not hold for exposure times beyond ca 0.05 second, and for a greater range of exposure times a more complicated expression is required. Blonde1 and Rey (1911) state that the equation It = a + bt (where a and b are constants) gives a good description of the intensitytime effect over a range extending from 0.001 to 3.0 seconds. In examining the discharge of impulses in a single fiber of the optic nerve of Limulus, Hartline (1934) determined that below a certain critical duration, the energy (I X t) necessary to produce a constant frequency of impulses is constant (C). Above this critical duration intensity alone seems to be effective and I = Constant. The presence of the critical duration had been demonstrated earlier in experiments by the same author (Hartline, 1928) on the grasshopper eye and by Adrian and Matthews (1927) on the eel eye. In the experiments on Limulus the transition, at the critical duration, from the relation It = C to I = Const is fairly abrupt. On the other hand the data for the human eye show a continuous increase in threshold energy, so that the condition It = C for short durations goes over gradually into I = Const for long durations. It is significant that the experiments on Limulus dealt with a single sense cell with no central connections, while in the human eye we are of course dealing with effects due to the activity of a large population of sense cells. It is reasonable to expect that any sudden change in slope of the intensity-time relation which might be apparent for a single sense cell will be lost in the statistically determined effect from a great number of sense cells. For in the first place there is a distribution of properties among the individual sense cells (Hecht, 1927~28), and in the second place there exists a considerable degree of