The change of visual sensitivity with time

Abstract

On changing instantaneously the external light stimulus applied to the eye, the latter does not also change instantaneously its state of adaptation to that corresponding to the new stimulus. No general definition of the state of adaptation of the eye has been given, or, perhaps, can be given. One can, however, imagine quite clearly that, when the eye is viewing some given external distribution of brightness, the concentration of photochemical substances in the various parts of the retina, the nervous messages sent from retina to brain and the consequent reactions in the latter will all be tending towards certain equilibrium values. When the external brightness distribution is changed, these equilibrium values will also change in the direction of a new set of equilibria. Unfortunately, it is difficult to measure any of these things directly and it is necessary to compromise. Amongst quantities more readily measurable the liminal or threshold responses of the eye occupy an important place. These quantities must be closely related to the actual physical state of the visual system and they are of considerable practical significance in themselves. The problem of the change of visual state with time has therefore been approached by measuring the change of visual sensitivity with time. The general experimental problem is to expose the eye to a certain primary or conditioning stimulus for a given period, then suddenly to change to a different stimulus, called for convenience the secondary stimulus, and to make at intervals thereafter a series of threshold measurements, the secondary stimulus remaining steady. This general specification of the problem may be subdivided as follows : 1—Primary and secondary stimuli the same in pattern. ( а ) Primary stimulus greater than secondary. ( b ) Primary stimulus less than secondary. 2—Primary and secondary stimuli different in pattern. This section may also be subdivided according to whether the secondary stimulation corresponds to ( a ) a higher, ( b ) a lower, or ( c ) the same equilibrium value of the visual test on the test area of the retina which is under investigation.