15LightSlide 3Refraction and LensesSlide 5Slide 6Focusing Light on the RetinaFocusing for Distant VisionSlide 9Focusing for Close VisionSlide 11Slide 12Problems of RefractionSlide 14Slide 15Slide 16Functional Anatomy of PhotoreceptorsSlide 18RodsConesChemistry of Visual PigmentsSlide 22Excitation of RodsSlide 24Excitation of ConesPhototransductionSlide 27Slide 28Signal Transmission in the RetinaSlide 30Slide 31Light AdaptationDark AdaptationVisual PathwaySlide 35Slide 36Slide 37Depth PerceptionRetinal ProcessingSlide 40Thalamic ProcessingCortical ProcessingPowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal CollegeC H A P T E RCopyright © 2010 Pearson Education, Inc.15 The Special Senses: Part BCopyright © 2010 Pearson Education, Inc.Light•Our eyes respond to visible light, a small portion of the electromagnetic spectrum •Light: packets of energy called photons (quanta) that travel in a wavelike fashion •Rods and cones respond to different wavelengths of the visible spectrumCopyright © 2010 Pearson Education, Inc.Figure 15.10Wavelength (nm)Visible light(b)(a)Bluecones(420 nm)Rods(500 nm)Greencones(530 nm)Redcones(560 nm)X rays UV InfraredMicro-wavesRadio wavesGammaraysLight absorption (pervent of maximum)Copyright © 2010 Pearson Education, Inc.Refraction and Lenses•Refraction•Bending of a light ray due to change in speed when light passes from one transparent medium to another •Occurs when light meets the surface of a different medium at an oblique angleCopyright © 2010 Pearson Education, Inc.Refraction and Lenses•Light passing through a convex lens (as in the eye) is bent so that the rays converge at a focal point•The image formed at the focal point is upside-down and reversed right to leftCopyright © 2010 Pearson Education, Inc.Figure 15.12Point sources(a) Focusing of two points of light.(b) The image is inverted—upside down and reversed. Focal pointsCopyright © 2010 Pearson Education, Inc.Focusing Light on the Retina•Pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, neural layer of retina, photoreceptors•Light is refracted•At the cornea•Entering the lens•Leaving the lens•Change in lens curvature allows for fine focusing of an imageCopyright © 2010 Pearson Education, Inc.Focusing for Distant Vision•Light rays from distant objects are nearly parallel at the eye and need little refraction beyond what occurs in the at-rest eye•Far point of vision: the distance beyond which no change in lens shape is needed for focusing; 20 feet for emmetropic (normal) eye•Ciliary muscles are relaxed•Lens is stretched flat by tension in the ciliary zonuleCopyright © 2010 Pearson Education, Inc.Figure 15.13aLensInvertedimageCiliary zonuleCiliary muscleNearly parallel raysfrom distant object(a) Lens is flattened for distant vision. Sympatheticinput relaxes the ciliary muscle, tightening the ciliary zonule, and flattening the lens.Sympathetic activationCopyright © 2010 Pearson Education, Inc.Focusing for Close Vision•Light from a close object diverges as it approaches the eye; requires that the eye make active adjustmentsCopyright © 2010 Pearson Education, Inc.Focusing for Close Vision•Close vision requires•Accommodation—changing the lens shape by ciliary muscles to increase refractory power•Near point of vision is determined by the maximum bulge the lens can achieve•Presbyopia—loss of accommodation over age 50•Constriction—the accommodation pupillary reflex constricts the pupils to prevent the most divergent light rays from entering the eye•Convergence—medial rotation of the eyeballs toward the object being viewedCopyright © 2010 Pearson Education, Inc.Figure 15.13bDivergent raysfrom close object(b) Lens bulges for close vision. Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge.InvertedimageParasympathetic activationCopyright © 2010 Pearson Education, Inc.Problems of Refraction•Myopia (nearsightedness)—focal point is in front of the retina, e.g. in a longer than normal eyeball•Corrected with a concave lens•Hyperopia (farsightedness)—focal point is behind the retina, e.g. in a shorter than normal eyeball•Corrected with a convex lens•Astigmatism—caused by unequal curvatures in different parts of the cornea or lens•Corrected with cylindrically ground lenses, corneal implants, or laser proceduresCopyright © 2010 Pearson Education, Inc.Figure 15.14 (1 of 3)FocalplaneFocal point is on retina.Emmetropic eye (normal)Copyright © 2010 Pearson Education, Inc.Figure 15.14 (2 of 3)Concave lens moves focalpoint further back.Eyeballtoo longUncorrectedFocal point is in front of retina.CorrectedMyopic eye (nearsighted)Copyright © 2010 Pearson Education, Inc.Figure 15.14 (3 of 3)Eyeballtoo shortUncorrectedFocal point is behind retina.CorrectedConvex lens moves focalpoint forward.Hyperopic eye (farsighted)Copyright © 2010 Pearson Education, Inc.Functional Anatomy of Photoreceptors•Rods and cones•Outer segment of each contains visual pigments (photopigments)—molecules that change shape as they absorb light•Inner segment of each joins the cell bodyCopyright © 2010 Pearson Education, Inc.Figure 15.15aProcess ofbipolar cellOuter fiberApical microvillusDiscs containingvisual pigmentsMelaningranulesDiscs beingphagocytized Pigment cell nucleusInner fibersRod cell bodyCone cell bodySynaptic terminalsRod cell bodyNucleiMitochondriaConnectingciliaBasal lamina (borderwith choroid) The outer segments of rods and cones are embedded in the pigmented layer of the retina. Pigmented layerOuter segmentInnersegmentCopyright © 2010 Pearson Education, Inc.Rods•Functional characteristics•Very sensitive to dim light•Best suited for night vision and peripheral vision•Perceived input is in gray tones only•Pathways converge, resulting in fuzzy and indistinct imagesCopyright © 2010 Pearson Education, Inc.Cones•Functional characteristics •Need bright light for activation (have low sensitivity)•Have one of three pigments that furnish a vividly colored view•Nonconverging pathways result in detailed, high-resolution visionCopyright © 2010 Pearson Education, Inc.Chemistry of Visual Pigments•Retinal• Light-absorbing molecule that combines with one of four proteins (opsin) to form visual pigments•Synthesized from vitamin A•Two isomers: 11-cis-retinal (bent form) and all-trans-retinal (straight