AFTER-IMAGE, AFTER-SENSATION and AFTER-PERCEPT are the terms used to denote the direct after-effects of the stimu lation of a sense organ. These after-effects occur in almost all of the sense departments. A brief stimulation of the sense organ gives a primary sensation, then a pause of a fraction of a second, followed by a secondary sensa tion of the same quality as the primary sensa tion. After-images of touch follow after brief contact. They do not appear under ordinary circumstances, but may be observed if special conditions are produced; for example, a gentle tap of a point of a needle will be followed by a pause, then an after-sensation which differs from the primary sensation in that it seems to be produced from within the body, not from without. The effects of a temperature stimu lus may persist for a time in the same quality as the primary sensation. After-taste and after-smell have been observed, but have not been studied. Auditory after-sensations, anal ogous to after-sensations of touch, are very weak and of brief duration.
After-images of vision are stronger and more permanent, consequently have been given much more attention by experimentalists. It has been found that after the retina has been stimulated by light for one second, or less, the primary image disappears quickly; an interval of less than two seconds is then followed by a posi tive after-image, that is, an after-image of the same quality as the primary image.
A stimulus of longer duration is followed immediately by the positive after-image, and this image may itself be followed by a negative after-image, that is, an image which differs very much in brightness from the primary image, or is of a different color. With some observers a brief stimulus is followed imme diately by a negative after-image, which fades away quickly to be followed after an interval by a more permanent positive after-image. Sev eral images may succeed each other immedi ately or be separated by an interval of time. A
stimulus of still greater duration is followed directly by a negative after-image. In such cases the after-image is usually of a color that is complementary to the color of the primary image, especially if observed with closed eyes or if projected upon — that is, seen while look ing at — a gray background. The duration of the after-image varies with the intensity, dura tion and area of the stimulus. The results of experiments, under conditions such that the in tensity of the light does not vary, have not as yet shown that any one color has more power to produce after-images than any other color. The greater the angular distance of the portion of the retina stimulated, from the fovea, the less distinct and the less durable is the after image. There seems to be no at an angular distance of 45 degrees or more from the fovea. The explanation for this fact may be physiological, or psychological, or both; that is, it may be due to the fact that the periphery of the retina is more easily fatigued than the fovea, or it may be due to lack of ability to attend to those portions of the retina which are not customarily attended to. When an ob ject occupies the attention, the eye is so directed toward it that the image falls over the fovea; the mind does not ordinarily attend to images that are not over or very near the fovea.
A blow on the head may cause the after image to become less intense or to cease en tirely. Electrical stimulation of the eye and optic nerve will change the character of the after-image and shorten the time of its dura tion. General fatigue will shorten the dura tion of the after-image; for example, it has been found that an after-image lasts about 30 per cent longer in the morning than in the even ing. The distraction of attention in any man ner has its effect on the course of the after image; when the attention is directed wholly upon the after-image the duration is one-third longer than when the attention is not concen trated upon it.