Quantitative Study of Visual After-Images
C. A. Padgham · British Journal of Ophthalmology · 1953
MUCH has been written on the persistence of visual sensation after the light stimulus has been removed-the so-called visual after-image (Helmholtz, 1924; Duke-Elder, 1932).The after-image may be positive, in which the light and dark parts correspond to those of the object, or negative, in which the light and dark parts are reversed.Similarly, in the positive after-image of a coloured object, the colours roughly correspond to those of the object, whereas in the negative after-image complementary colours are said to appear.With a very bright stimulus, the after-image may persist for several minutes or perhaps longer, whereas under more normal conditions the duration is short, and may be studied by experiments on flicker and persistence of vision.Whilst the short-period phenomena have received considerable attention, the long-period after-images have mainly been studied qualitatively, and no comprehensive quantitative experimental work appears to have been done.Craik (1940), and later Cibis and Nothdurft (1948), have described experi- ments from which they conclude that after-images are of retinal origin, and Misiak and Lozito (1951) show that some central modification of after- images occurs.To decide, however, on the mechanism of after-images, and on their relation to other phenomena of vision seems premature until more experimental data is available.Pannevis (1947; 1948) has made measure- ments on the brightness variation of long-period after-images, although the amount of work is not great.His method was to re-stimulate the letina and induce a negative after-image equal in apparent brightness to the positive after-image, so that no resultant image could be seen.The assump- tion that the positive after-image is not affected by re-stimulation is probably unjustifiable.Probably the reason for the lack of adequate quantitative data is due to the difficulty of evolving a technique for making adequate measurements.The author has found that, by using the binocular matching technique of Wright (1946), measurements could be made of the retinal illumination of one eye which caused a sensation of brightness equal to that of the positive after-image in the other eye.Also he has found that by taking a sufficient number of observations, significant quantitative data can be obtained.Work has been done with white lightt stimuli only, on a dark-adapted eye.