The Eye by Britannica Educational Publishing

Author:Britannica Educational Publishing
Language: eng
Format: epub
Tags: The Human Body
Publisher: Britannica Educational Publishing
Published: 2011-03-14T16:00:00+00:00

POSITION IN RELATION TO OBSERVER

The recognition of the directions of objects in relation to the observer is more complex. The direction of projection of a retinal point is constantly modified to take into account movements of the eye; this may be called psychological compensation. Correct projection is achieved by projecting the stimulated retinal point through the nodal point of the eye. Movements of the eye caused by movements of the head must be similarly compensated. As a result, any point in space remains fixed in spite of movements of the eye and head. Given this system of compensated projection, the recognition of direction in relation to the individual is feasible. For example, D may be said to be due north or, more vaguely, “over there”; when the head is turned, since D is perceived to be in the same place, it is still due north or “over there.” In some circumstances, the human subject makes an error in projecting his or her retinal image, so that the object giving rise to the image appears to be in a different place from its true one; the image is said to be falsely projected. If the eye is moved passively, for example, by pulling on the conjunctiva with forceps, the subject has the impression that objects in the outside world are moving in a direction opposite to that of the eye.

The apparent movement of an afterimage, when the eye moves, is an excellent illustration of psychological compensation. A retinal stimulus, being normally projected through the nodal point, is projected into different points in space as the eye moves; an afterimage can be considered to be the manifestation of a continued retinal impulse, and its projection changes as the eye moves. The afterimage thus appears to move in the same direction as that of the movement of the eye. Whether the drift of an afterimage across the field of view is entirely due to eye movements is difficult to say. One certainly has the impression that the eye is chasing the afterimage.

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