chap 4 - Chapter 4 Fundamental Visual Functions and...

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Chapter 4: Fundamental Visual Functions and Phenomena I. Scotopic and Photopic Vision and Dark Adaptation - Photopic Vision- vision performed with cones - Scotopic Vision- vision accomplished with rods PHOTOPIC SCOTOPIC RECEPTOR Cones Rods RETINAL LOCATION Concentrated in fovea Peripheral retina FXNL LUMINANCE LVL Daylight Night light PEAK WAVELENGTH 550 nm 500 nm COLOR VISION YES NO DARK ADAPTATION Rapid Slow SPATIAL RES High acuity, low sensitivity Low acuity, high sensitivity - Dark Adaptation- process of adjusting to a dimly illuminated environment o Cones adapt in 5-10 minutes o Rods require 20-30 minutes - Light Adaptation- sensitivity of the retina, exposure to the light decreases - Rhodopsin- light absorbing pigment that when stimulated marks the first stage in response to light energy - Rhodopsin is an unstable chemical readily altered by light energy. When exposed to light it is broken down, or bleaches. It regenerates in darkness. The regeneration is the basic photochemical process underlying dark adaptation - In regeneration portion of the cycle, when the eye is kept in the dark, vitamin A joins with opsin to reconstitute rhodopsin. Equilibrium is established between the decomposition of rhodopsin by light and its subsequent regeneration from the elements in dark - Night blindness- a critical lack of vitamin A in the diet may produce a pathological difficulty in seeing at night or in dim illumination - Cones adapt to dark more swiftly than rods, but are less sensitive.
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