AFTER IMAGE

AFTER IMAGES:

    After images refer to the visual sensations experienced following cessation of stimulation of the eye by light or patterns of light and dark. Though related to, but strictly speaking after images are not entopic phenomena. After images arise from outside, in contrast to the entopic phenomenon which arise from within the eye. After images were described hundreds of years ago and were studied comprehensively by Purkinje.

TYPES OF AFTER IMAGES:

Positive versus negative after images:

Positive After image:

    It is visualized similar to the original stimulus, ie. the dark areas of the stimulus appear dark and light areas of the stimulus appear light.

Negative After image:

    Negative after image is reversed, ie. the light areas of the stimulus appear dark and dark areas appear light.

Homochromatic versus Complementary after images:

    Homochromatic after image is of the same colour as the stimulus.

    Complementary after image is of the complementary to the colour of the stimulus.

VISUALIZATION OF AFTER IMAGES:

Negative after images:

    These can be observed after staring for a few moments at a bright source of light and then transferring    the gaze to a dimly illuminated even background. A dark image of the light will be seen surrounded by a light illuminated field. A coloured stimulus will be seen in its complementary colour.

    Negative after image fades in about 15 sec but can usually be brought back ultimately prolonged for about a minute, if the eyes are repeatedly closed for a few seconds and then reopened.

Positive after images:

    These can be visualized by staring at a dark pattern in very bright light for only 2 to 3 sec and then    occluding the eyes. A faint positive pattern similar to the original can be seen with closed eyes for about 5-10 seconds, since positive after images are more transient.

    Classical method for study of after images is with the help of disc of Bidwell. It produces afterimagery known as Bidwell's ghost. This method may be used to demonstrate formation of after images. Details of it are beyond the scope.

FACTORS AFFECTING NATURE OF AFTER IMAGES:

    A few important factors on which the nature of after imagery depends are as follows:

        1. Nature and intensity of stimulus.

        2. Occurrence of succeeding stimuli.

        3. Region of the retina stimulated.

        4. State of retinal adaptation.

PHYSIOLOGICAL BASIS OF AFTER IMAGES:

    It has been suggested that, perhaps the after images arise from photochemical reactions in the retina. However, the possibility that some phases of afterimagery have a central origin cannot as yet be ruled out. In general, after image phenomena are not so simple and clear cut as appears from their definition. Further, unfortunately relatively little has been learned of the visual system through the study of after images.

CLINICAL APPLICATIONS:

    Bieschowsky's after image test and the after image transfer test are used clinically in the diagnosis of retinal correspondence.