COLOR VISION: THE APPROACH THROUGH INCREMENT-THRESHOLD SENSITIVITY.

I am most sensible of the honor of addressing this meeting and of the generosity of the National Academy of Sciences in bringing me here as their guest. The invitation to contribute to the present symposium came just as I had finished a rather elaborate study of the color-matching properties of the average observer. However, that work I believe to be of more calorimetric interest, and I have elected, therefore, to deal with a quite different approach to color vision by a method which does not rest on actual color-matching-the method of increment thresholds. Consider, to take first the simplest case, a subject whose eyes are fully adapted to a completely dark field. The increment threshold is then just the smallest perceptible quantity of a certain test light which is applied at a particular part of the visual field. For convenience, I use the expression "smallest perceptible quantity," although, of course, thresholds are definable only statistically, for example as critical values of the stimulus variable at which the chance of a perception equals 50 per cent. In the general case, the increment threshold is still the smallest perceptible quantity of a certain test light, but now this is added to another stimulus-the conditioning stimulus-which consists of any distribution of brightness and color to which we may choose to expose the eye. The conditioning stimulus need not be an unvarying one: the increment threshold is, in fact, often measured while the eye is adapting itself to a change from one constant stimulus to another constant stimulus, the whole spatial and temporal pattern of stimulation being considered to form the conditioning stimulus. The increment threshold (or, better, its reciprocal, the threshold sensitivity) is a measure of visual sensitivity which can be varied in different ways to bring out particular properties of the visual response system. We may, for example, vary the angular size and exposure time of the test stimulus, to determine how well the visual system integrates over area and over time. Most important, we may change the spectral composition of the test stimulus, in order to determine the spectral sensitivities that are active in the response system. So far, only changes in the parameters of the test stimulus have been mentioned. In addition, any number of different conditioning stimuli may be used. However, there is evidence that the modifications of the increment threshold by more complex patterns of brightness and color are similar to those obtained with a very simple pattern, namely, a uniform stimulus which extends over and around the area of the field where the test stimulus is applied. We should regard the action of a more stimulus, including stimuli applied at some distance from the test area, as equivalent to that of a suitably chosen uniform stimulus of the kind mentioned if, for all possible test stimuli (of various sizes, colors, exposure times, etc.), the observed increment threshold was the same for the two conditions. The equivalence is probably not generally complete, but it has been found to hold approximately in some cases, and we are justified in concentrating, in the first instance, on the simple stimulus pattern. This has the advantage that much