Another visual phenomenon that brings out the importance of inhibition is the sensation evoked when a visual stimulus is repeated rapidly; for example, one may view a screen that is illuminated by a source of light the rays from which may be intercepted at regular intervals by rotating a sector of a circular screen in front of it. If the sector rotates slowly, a sensation of black followed by white is aroused; as the speed increases the sensation becomes one of flicker—i.e., rapid fluctuations in brightness; finally, at a certain speed, called the critical fusion frequency, the sensation becomes continuous and the subject is unaware of the alterations in the illumination of the screen.
At high levels of luminance, when cone vision is employed, the fusion frequency is high, increasing with increasing luminance in a logarithmic fashion—the Ferry-Porter law—so that at high levels it may require 60 flashes per second to reach a continuous sensation. Under conditions of night, or scotopic, vision, the frequencies may be as low as four per second. The difference between rod and cone vision in this respect probably resides in the power of the eye to inhibit activity in cones rapidly, so that the sensation evoked by a single flash is cut off immediately, and this leaves the eye ready to respond to the next stimulus. By contrast, the response in the rod lasts so much longer that, when a new stimulus falls even a quarter of a second later, the difference in the state of the rods is insufficient to evoke a change in intensity of sensation; it merely prolongs it. One interesting feature of an intermittent stimulus is that the intensity of the sensation of brightness, when fusion is achieved, is dependent on the relative periods of light and darkness in the cycle, and this gives one a method of grading the effective luminance of a screen; one may keep the intensity of the illuminating source constant and merely vary the period of blackness in a cycle of black and white. The effective luminance will be the average luminance during a cycle; this is known as the Talbot-Plateau law.
A-horizontal-cross-section-of-the-human-eye-showing-theA horizontal cross section of the human eye, showing the major parts of the eye, including the …[Credits : Encyclopædia Britannica, Inc.]
Horizontal-section-of-the-eyeFigure 1: Horizontal section of the eye.[Credits : Encyclopædia Britannica, Inc.]
Structure-of-the-retinaStructure of the retina
Muscles-of-the-right-eyeMuscles of the right eye
Myopia-or-nearsightedness-can-be-corrected-with-glasses-that-haveMyopia, or nearsightedness, can be corrected with glasses that have concave lenses to allow near …[Credits : Encyclopædia Britannica, Inc.]
Hyperopia-or-farsightedness-can-be-corrected-with-glasses-that-containHyperopia, or farsightedness, can be corrected with glasses that contain convex lenses to reduce …[Credits : Encyclopædia Britannica, Inc.]
Visual-pathwaysVisual pathways
Perimeter-chart-showing-normal-visual-field-figures-on-the-perimeterFigure 2: Perimeter chart showing normal visual field; figures on the perimeter indicate degrees of …[Credits : Encyclopædia Britannica, Inc.]
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