Chapter 15 (2) Terms

Trace the visual pathway to the visual cortex, and briefly describe the steps in visual processing

Visual pathway: - Axons of retinal ganglion cells form optic nerve - Medial fibers of optic nerve decussate at optic chiasma - Most fibers of optic tracts continue to lateral geniculate body of thalamus - Fibers from thalamic neurons form optic radiation and project to primary visual cortex in occipital lobes Visual processing - Retinal cells split input into channels (color, brightness, angle, direction, speed of movement of edges job of amacrine and horizontal cells) - Lateral geniculate nuclei of thalamus (process for depth perception, cone input emphasized, contrast sharpened) - Primary visual cortex (striate cortex) • Neurons respond to dark and bright edges, and object orientation • Provide form, color, motion inputs to visual association areas

Compare and contrast light and dark adaptation

Light adaption: move from darkness into bright light (both rods and cones are both strongly stimulated, pupils constrict Large amounts of pigments broken down instantaneously, producing glare Visual acuity improves over 5-10 minutes as: - Rod systems turns off - Retinal sensitivity decreases - Cones and neurons rapidly adapt Dark adaptation: move from bright light into darkness - Cones stop functioning in low-intensity light - Rod pigments bleached; system turned off - Rhodopsin accumulates in dark - Transducin returns to outer segments - Retinal sensitivity increases within 20-30 minutes - Pupils dilate Nyctalopia: rod degeneration could be cause by retinitis pigmentosa (degenerative retinal diseases that destroy rods)

Outline the causes and consequences of astigmatism, myopia, hyperopia, and presbyopia

Myopia: nearsightedness, focal point in front of retina, e.g. eyeball too long Corrected with a concave lens Hyperopia: farsightedness, focal point behind retina, e.g. eyeball too short Corrected with a convex lens Presbyopia: loss of accommodation over age 50 Astigmatism: unequal curvatures in different parts of cornea or lens, corrected with cylindrically ground lenses or laser procedures

Compare and contrast the roles of rods and cones in vision

Rods and cones: modified neurons and receptive regions called outer segments (contain visual pigments (photopigments) Rods: very sensitive to light, best suited for night vision and peripheral vision, contain single pigment (perceived as gray tones), pathways converge, causing fuzzy, indistinct images Cones: need bright light for activation (low sensitivity), react more quickly, have one of three pigments for colored view, nonconverging pathways result in detailed high-resolution vision, color blindness (lack of one or more cone pigments)

Trace the pathway of light through the eye to the retina and explain how light is focused for distant and close vision

Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of retina, photo receptors Distant vision: Eyes best adapted for distant vision, far point vision (distance beyond which no change in lens shape needed for focusing) Ciliary muscles relaxed Lens stretched flat by tension in ciliary zone Close vision: light from close objects (<6 m) diverges as approaches eye (requires eye to make active adjustments using three simultaneous processes) accommodation of lenses, constriction of pupils, convergence of eyeballs Accommodation: changing lens shape to increase refraction - Near point vision (closest point on which the eye can focus Constriction: accommodation pupillary reflex constricts pupils to prevent most divergent light rays from entering eye Convergence: medial rotation of eyeballs toward object being views Close vision contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge