© 2016 Pearson Education, Inc.

15.2 Focusing and Light Overview: Light and Optics
Wavelength and color Electromagnetic radiation: all energy waves, from long radio waves to short X rays; visible light occupies a small portion in the middle of the spectrum Light has wavelengths between 400 and 700 nm Eyes respond only to visible light © 2016 Pearson Education, Inc.

(109 nm=) 1 m 10−5 nm 10−3 nm 1 nm 103 nm 106 nm 103 m Micro- waves
Figure 15.10a The electromagnetic spectrum and photoreceptor sensitivities. (109 nm=) 1 m 10−5 nm 10−3 nm 1 nm 103 nm 106 nm 103 m Gamma rays Micro- waves X rays UV Infrared Radio waves © 2016 Pearson Education, Inc.

Overview: Light and Optics
Wavelength and color (cont.) Light: packets of energy (photons or quanta) that travel in wavelike fashion at high speeds When visible light passes through spectrum, it is broken up into bands of colors (rainbow) Red wavelengths are longest and have lowest energy, and violet are shortest and have most energy Color that eye perceives is a reflection of that wavelength Grass is green because it absorbs all colors except green White reflects all colors, and black absorbs all colors © 2016 Pearson Education, Inc.

Visible light Blue cones (420 nm) Green cones (530 nm) Red cones
Figure 15.10b The electromagnetic spectrum and photoreceptor sensitivities. Visible light Blue cones (420 nm) Green cones (530 nm) Red cones (560 nm) Rods (500 nm) 100 Light absorption (percent of maximum) 50 400 450 500 550 600 650 700 Wavelength (nm) © 2016 Pearson Education, Inc.

Overview: Light and Optics (cont.)
Refraction and lenses Refraction: bending of light rays Due to change in speed of light when it passes from one transparent medium to another and path of light is at an oblique angle Example: from liquid to air Lenses of eyes can also refract light because they are curved on both sides Convex: thicker in center than at edges Concave: thicker at edges than in center © 2016 Pearson Education, Inc.

Overview: Light and Optics (cont.)
Refraction and lenses (cont.) Convex lenses bend light passing through it, so that rays converge at focal point Image formed at focal point is upside-down and reversed from left to right Concave lenses disperse light, preventing light from being focused © 2016 Pearson Education, Inc.

Figure 15.12 Light is focused by a convex lens.
Point sources Focal points Focusing of two points of light. The image is inverted—upside down and reversed. © 2016 Pearson Education, Inc.

Focusing Light on the Retina
Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of retina, and finally photoreceptors Light is refracted three times along path: (1) entering cornea, (2) entering lens, and (3) leaving lens Majority of refractory power is in cornea; however, it is constant and cannot change focus © 2016 Pearson Education, Inc.

Focusing Light on the Retina (cont.)
Lens is able to adjust its curvature to allow for fine focusing Can focus for distant vision and for close vision © 2016 Pearson Education, Inc.

Figure 15.13a Focusing for distant and close vision.
The ciliary muscle and the ciliary zonule focus an image by changing the shape of the lens. • They are arranged sphincterlike around the lens. • Ciliary muscle contraction loosens the ciliary zonule fibers and relaxation tightens them. View Ciliary muscle Lens Ciliary zonule (suspensory ligament) © 2016 Pearson Education, Inc.

Focusing for distant vision Eyes are best adapted for distant vision Far point of vision: distance beyond which no change in lens shape is needed for focusing 20 feet for emmetropic (normal) eye Cornea and lens focus light precisely on retina at this distance Ciliary muscles are completely relaxed in distance vision, which causes a pull on ciliary zonule; as a result, lenses are stretched flat © 2016 Pearson Education, Inc.

Figure 15.13b Focusing for distant and close vision.
The lens flattens for distant vision. Sympathetic input relaxes the ciliary muscle. This tightens the ciliary zonule and flattens the lens. Relaxed ciliary muscle Tightened ciliary zonule Flattened lens Nearly parallel rays from distant object Image © 2016 Pearson Education, Inc.

Focusing for close vision Light from close objects (<6 m) diverges as approaches eye Requires eye to make active adjustments using three simultaneous processes: Accommodation of the lenses Changing lens shape to increase refraction Near point of vision Closest point on which the eye can focus Presbyopia: loss of accommodation over age 50 © 2016 Pearson Education, Inc.

Constriction of the pupils Accommodation pupillary reflex involves constriction of pupils to prevent most divergent light rays from entering eye Mediated by parasympathetic nervous system Convergence of the eyeballs Medial rotation of eyeballs causes convergence of eyes toward object being viewed Controlled by somatic motor neuron innervation on medial rectus muscles © 2016 Pearson Education, Inc.

Figure 15.13c Focusing for distant and close vision.
The lens bulges for close vision. Parasympathetic input contracts the ciliary muscle. This loosens the ciliary zonule and allows the lens to bulge. Contracted ciliary muscle Loosened ciliary zonule Bulging lens Divergent rays from close object Image © 2016 Pearson Education, Inc.

Clinical – Homeostatic Imbalance 15.8
Problems associated with refraction related to eyeball shape: Myopia (nearsightedness) Eyeball is too long, so focal point is in front of retina Corrected with a concave lens Hyperopia (farsightedness) Eyeball is too short, so focal point is behind retina Corrected with a convex lens Astigmatism Unequal curvatures in different parts of cornea or lens Corrected with cylindrically ground lenses or laser procedures © 2016 Pearson Education, Inc.

Figure 15.14-1 Problems of refraction.
Emmetropic eye (normal) Focal point is on retina. Focal plane Myopic eye (nearsighted: eyeball too long) Uncorrected Focal point is in front of retina. Eyeball too long Corrected Concave lens moves focal point further back. © 2016 Pearson Education, Inc.

Figure 15.14-2 Problems of refraction.
Emmetropic eye (normal) Focal point is on retina. Focal plane Hyperopic eye (farsighted: eyeball too short) Uncorrected Focal point is behind retina. Eyeball too short Corrected Convex lens moves focal point forward. © 2016 Pearson Education, Inc.

15.3 Phototransduction Functional Anatomy of Photoreceptors
Photoreceptors (rods and cones) are modified neurons that resemble upside-down epithelial cells Consists of cell body, synaptic terminal, and two segments: Outer segment: light-receiving region Contains visual pigments (photopigments) that change shape as they absorb light Inner segment of each joins cell body Inner segment is connected via cilium to outer segment and to cell body via outer fiber © 2016 Pearson Education, Inc.

Functional Anatomy of Photoreceptors (cont.)
Cell body is connected to synaptic terminal via inner fibers Plasma membrane of outer segment folds back to form many discs Photopigments are embedded in discs © 2016 Pearson Education, Inc.

Functional Anatomy of Photoreceptors (cont.)
Photoreceptors are vulnerable to damage Degenerate if retina detached Destroyed by intense light Vision is maintained because outer segment is renewed every 24 hours Tips fragment off and are phagocytized © 2016 Pearson Education, Inc.

Figure 15.15a Photoreceptors of the retina.
Process of bipolar cell Light Light Light Synaptic terminals Inner fibers Rod cell body Rod cell body Nuclei Cone cell body Mitochondria Outer fiber Connecting cilia Inner segment Apical microvillus Outer segment Discs containing visual pigments Pigmented layer Discs being phagocytized Pigment cell nucleus Melanin granules Basal lamina (border with choroid) The outer segments of rods and cones are embedded in the pigmented layer of the retina. © 2016 Pearson Education, Inc.

Comparing Rod and Cone Vision
Rods are very sensitive to light, making them best suited for night vision and peripheral vision Contain a single pigment, so vision is perceived in gray tones only Pathways converge, causing fuzzy, indistinct images As many as 100 rods may converge into one ganglion © 2016 Pearson Education, Inc.

Comparing Rod and Cone Vision (cont.)
Cones have low sensitivity, so require bright light for activation React more quickly than rods Have one of three pigments, which allow for vividly colored sight Nonconverging pathways result in detailed, high-resolution vision Some cones have their own ganglion cell, so brain can put together accurate, high-acuity resolution images © 2016 Pearson Education, Inc.

Table 15.1 Comparison of Rods and Cones
© 2016 Pearson Education, Inc.

Color blindness: lack of one or more cone pigments Inherited as an X-linked condition, so more common in males As many as 8–10% of males have some form The most common type is red-green, in which either red cones or green cones are absent Depending on which cone is missing, red can appear green, or vice versa Rely on different shades to get cues of color © 2016 Pearson Education, Inc.

Visual Pigments Retinal: key light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments Synthesized from vitamin A Four opsins are rhodopsin (found in rods only), and three found in cones: green, blue, red (depending on wavelength of light they absorb) Cone wavelengths do overlap, so same wavelength may trigger more than one cone, enabling us to see variety of hues of colors Example: yellow light stimulates red and green cones, but if more red are triggered, we see orange © 2016 Pearson Education, Inc.

Visual Pigments (cont.)
Retinal isomers are different 3-D forms Retinal is in a bent form in dark, but when pigment absorbs light, it straightens out Bent form called 11-cis-retinal Straight form called all-trans-retinal Conversion of bent to straight initiates reactions that lead to electrical impulses along optic nerve © 2016 Pearson Education, Inc.

Figure 15.15b Photoreceptors of the retina.
Rod discs Visual pigment consists of • Retinal • Opsin Rhodopsin, the visual pigment in rods, is embedded in the membrane that forms discs in the outer segment. © 2016 Pearson Education, Inc.

Phototransduction Phototransduction: process by which pigment captures photon of light energy, which is converted into a graded receptor potential Capturing light Deep purple pigment of rods is rhodopsin Arranged in rod’s outer segment Three steps of rhodopsin formation and breakdown: Pigment synthesis, pigment bleaching, and pigment regeneration Similar process in cones, but different types of opsins and cones require more intense light © 2016 Pearson Education, Inc.

Phototransduction (cont.)
Capturing light (cont.) Pigment synthesis Opsin and 11-cis retinal combine to form rhodopsin in dark Pigment bleaching When rhodopsin absorbs light, 11-cis isomer of retinal changes to all-trans isomer Retinal and opsin separate (rhodopsin breakdown) Pigment regeneration All-trans retinal converted back to 11-cis isomer Rhodopsin is regenerated in outer segments © 2016 Pearson Education, Inc.

Figure 15.16 The formation and breakdown of rhodopsin.
Slide 2 11-cis-retinal 2H+ 1 Pigment synthesis: Oxidation 11-cis-retinal, derived from vitamin A, is combined with opsin to form rhodopsin. Vitamin A 11-cis-retinal Rhodopsin Reduction 2H+ Dark Light Opsin and All-trans-retinal All-trans-retinal © 2016 Pearson Education, Inc.

Slide 3 11-cis-retinal 2H+ 1 Pigment synthesis: Oxidation 11-cis-retinal, derived from vitamin A, is combined with opsin to form rhodopsin. Vitamin A 11-cis-retinal Rhodopsin Reduction 2 2H+ Pigment bleaching: Light absorption by rhodopsin triggers a rapid series of steps in which retinal changes shape (11-cis to all- trans) and eventually releases from opsin. Dark Light Opsin and All-trans-retinal All-trans-retinal © 2016 Pearson Education, Inc.

Slide 4 11-cis-retinal 2H+ 1 Pigment synthesis: Oxidation 11-cis-retinal, derived from vitamin A, is combined with opsin to form rhodopsin. Vitamin A 11-cis-retinal Rhodopsin Reduction 2 2H+ Pigment bleaching: Light absorption by rhodopsin triggers a rapid series of steps in which retinal changes shape (11-cis to all- trans) and eventually releases from opsin. Dark Light 3 Pigment regeneration: Enzymes slowly convert all-trans-retinal to its 11-cis form in cells of the pigmented layer; requires ATP. Opsin and All-trans-retinal All-trans-retinal © 2016 Pearson Education, Inc.

Phototransduction (cont.)
Light transduction reactions Light-activated rhodopsin activates G protein transducin Transducin activates PDE, which breaks down cyclic GMP (cGMP) In dark, cGMP holds cation channels of outer segment open Na+ and Ca2+ enter and depolarize cell In light cGMP breaks down, channels close, cell hyperpolarizes Hyperpolarization is signal for vision! © 2016 Pearson Education, Inc.

Figure 15.17 Events of phototransduction.
Slide 2 G protein signaling mechanisms are like a molecular relay race. Light (1st messenger) Receptor G protein Enzyme 2nd messenger Retinal absorbs light and changes shape. Visual pigment activates. 1 Ca2+ Ca2+ Visual pigment Na+ Na+ Light cGMP cGMP GMP 11-cis-retinal Transducin (a G protein) © 2016 Pearson Education, Inc.

Slide 3 G protein signaling mechanisms are like a molecular relay race. Light (1st messenger) Receptor G protein Enzyme 2nd messenger Retinal absorbs light and changes shape. Visual pigment activates. 1 Ca2+ Ca2+ Visual pigment Na+ Na+ All-trans-retinal Light cGMP cGMP GMP 11-cis-retinal Transducin (a G protein) Visual pigment activates transducin (G protein). 2 © 2016 Pearson Education, Inc.

Slide 4 G protein signaling mechanisms are like a molecular relay race. Light (1st messenger) Receptor G protein Enzyme 2nd messenger Retinal absorbs light and changes shape. Visual pigment activates. 1 Phosphodiesterase (PDE) Ca2+ Ca2+ Visual pigment Na+ Na+ All-trans-retinal Light cGMP cGMP GMP 11-cis-retinal Transducin (a G protein) Visual pigment activates transducin (G protein). 2 Transducin activates phosphodiesterase (PDE). 3 © 2016 Pearson Education, Inc.

Slide 5 G protein signaling mechanisms are like a molecular relay race. Light (1st messenger) Receptor G protein Enzyme 2nd messenger Retinal absorbs light and changes shape. Visual pigment activates. 1 Phosphodiesterase (PDE) Ca2+ Ca2+ Visual pigment Na+ Na+ All-trans-retinal Light cGMP cGMP GMP 11-cis-retinal Transducin (a G protein) Visual pigment activates transducin (G protein). 2 Transducin activates phosphodiesterase (PDE). 3 PDE converts cGMP into GMP, causing cGMP levels to fall. 4 © 2016 Pearson Education, Inc.

Slide 6 G protein signaling mechanisms are like a molecular relay race. Light (1st messenger) Receptor G protein Enzyme 2nd messenger Retinal absorbs light and changes shape. Visual pigment activates. 1 Phosphodiesterase (PDE) Ca2+ Ca2+ Visual pigment Na+ Na+ All-trans-retinal Light cGMP cGMP cGMP-gated cation channel open in dark cGMP-gated cation channel closed in light GMP 11-cis-retinal Transducin (a G protein) Visual pigment activates transducin (G protein). 2 Transducin activates phosphodiesterase (PDE). 3 PDE converts cGMP into GMP, causing cGMP levels to fall. 4 As cGMP levels fall, cGMP-gated cation channels close, resulting in hyperpolarization. 5 © 2016 Pearson Education, Inc.

Information Processing in the Retina
Photoreceptors and bipolar cells generate only graded potentials (EPSPs and IPSPs), not APs When light hyperpolarizes photoreceptor cells, they stop releasing inhibitory neurotransmitter glutamate to biopolar cells Bipolar cells (no longer inhibited) depolarize, release neurotransmitter onto ganglion cells Ganglion cells generate APs transmitted in optic nerve to brain © 2016 Pearson Education, Inc.

Figure 15.18-1 Signal transmission in the retina.
Slide 2 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the dark Light cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes. 1 Na Ca2 Photoreceptor cell (rod) 40 mV Ca2 Bipolar cell Ganglion cell © 2016 Pearson Education, Inc.

Slide 3 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the dark Light cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes. 1 Na Ca2 Photoreceptor cell (rod) Voltage-gated Ca2+ channels open in synaptic terminals. 2 40 mV Ca2 Bipolar cell Ganglion cell © 2016 Pearson Education, Inc.

Slide 4 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the dark Light cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes. 1 Na Ca2 Photoreceptor cell (rod) Voltage-gated Ca2+ channels open in synaptic terminals. 2 40 mV Ca2 Neurotransmitter is released continuously. 3 Bipolar cell Ganglion cell © 2016 Pearson Education, Inc.

Slide 5 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the dark Light cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes. 1 Na Ca2 Photoreceptor cell (rod) Voltage-gated Ca2+ channels open in synaptic terminals. 2 40 mV Ca2 Neurotransmitter is released continuously. 3 Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results. 4 Bipolar cell Ganglion cell © 2016 Pearson Education, Inc.

Slide 6 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the dark Light cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes. 1 Na Ca2 Photoreceptor cell (rod) Voltage-gated Ca2+ channels open in synaptic terminals. 2 40 mV Ca2 Neurotransmitter is released continuously. 3 Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results. 4 Bipolar cell Hyperpolarization closes voltage-gated Ca2+ channels, inhibiting neurotransmitter release. 5 Ganglion cell © 2016 Pearson Education, Inc.

Slide 7 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the dark Light cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes. 1 Na Ca2 Photoreceptor cell (rod) Voltage-gated Ca2+ channels open in synaptic terminals. 2 40 mV Ca2 Neurotransmitter is released continuously. 3 Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results. 4 Bipolar cell Hyperpolarization closes voltage-gated Ca2+ channels, inhibiting neurotransmitter release. 5 No EPSPs occur in ganglion cell. 6 Ganglion cell © 2016 Pearson Education, Inc.

Slide 8 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the dark Light cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes. 1 Na Ca2 Photoreceptor cell (rod) Voltage-gated Ca2+ channels open in synaptic terminals. 2 40 mV Ca2 Neurotransmitter is released continuously. 3 Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results. 4 Bipolar cell Hyperpolarization closes voltage-gated Ca2+ channels, inhibiting neurotransmitter release. 5 No EPSPs occur in ganglion cell. 6 Ganglion cell No action potentials occur along the optic nerve. 7 © 2016 Pearson Education, Inc.

Slide 2 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light Light cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes. 1 Light Photoreceptor cell (rod) 70 mV Bipolar cell Ca2+ Ganglion cell © 2016 Pearson Education, Inc.

Slide 3 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light Light cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes. 1 Light Photoreceptor cell (rod) 70 mV Voltage-gated Ca2+ channels close in synaptic terminals. 2 Bipolar cell Ca2+ Ganglion cell © 2016 Pearson Education, Inc.

Slide 4 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light Light cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes. 1 Light Photoreceptor cell (rod) 70 mV Voltage-gated Ca2+ channels close in synaptic terminals. 2 No neurotransmitter is released. 3 Bipolar cell Ca2+ Ganglion cell © 2016 Pearson Education, Inc.

Slide 5 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light Light cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes. 1 Light Photoreceptor cell (rod) 70 mV Voltage-gated Ca2+ channels close in synaptic terminals. 2 No neurotransmitter is released. 3 Lack of IPSPs in bipolar cell results in depolarization. 4 Bipolar cell Ca2+ Ganglion cell © 2016 Pearson Education, Inc.

Slide 6 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light Light cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes. 1 Light Photoreceptor cell (rod) 70 mV Voltage-gated Ca2+ channels close in synaptic terminals. 2 No neurotransmitter is released. 3 Lack of IPSPs in bipolar cell results in depolarization. 4 Bipolar cell Depolarization opens voltage-gated Ca2+ channels; neurotransmitter is released. 5 Ca2+ Ganglion cell © 2016 Pearson Education, Inc.

Slide 7 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light Light cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes. 1 Light Photoreceptor cell (rod) 70 mV Voltage-gated Ca2+ channels close in synaptic terminals. 2 No neurotransmitter is released. 3 Lack of IPSPs in bipolar cell results in depolarization. 4 Bipolar cell Depolarization opens voltage-gated Ca2+ channels; neurotransmitter is released. 5 Ca2+ EPSPs occur in ganglion cell. 6 Ganglion cell © 2016 Pearson Education, Inc.

Slide 8 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. In the light Light cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes. 1 Light Photoreceptor cell (rod) 70 mV Voltage-gated Ca2+ channels close in synaptic terminals. 2 No neurotransmitter is released. 3 Lack of IPSPs in bipolar cell results in depolarization. 4 Bipolar cell Depolarization opens voltage-gated Ca2+ channels; neurotransmitter is released. 5 Ca2+ EPSPs occur in ganglion cell. 6 Action potentials propagate along the optic nerve. 7 Ganglion cell © 2016 Pearson Education, Inc.

Light and Dark Adaptation
Rhodopsin is so sensitive that bleaching occurs even in starlight In bright light, bleaching occurs so fast that rods are virtually nonfunctional Cones respond to bright light So, activation of rods and cones depends on: Light adaptation Dark adaptation © 2016 Pearson Education, Inc.

Light and Dark Adaptation (cont.)
Light adaptation When moving from darkness into bright light we see glare because: Both rods and cones are strongly stimulated Large amounts of pigments are broken down instantaneously, producing glare Pupils constrict Visual acuity improves over 5–10 minutes as: Rod system turns off Retinal sensitivity decreases Cones and neurons rapidly adapt © 2016 Pearson Education, Inc.

Light and Dark Adaptation (cont.)
When moving from bright light into darkness, we see blackness because: Cones stop functioning in low-intensity light Bright light bleached rod pigments, so they are still turned off Pupils dilate Rhodopsin accumulates in dark, so retinal sensitivity starts to increase Transducin returns to outer segments Sensitivity increases within 20–30 minutes © 2016 Pearson Education, Inc.

Nyctalopia (night blindness): condition in which rod function is seriously hampered Ability to drive safely at night is impaired Due to rod degeneration, commonly caused by prolonged vitamin A deficiency If administered early, vitamin A supplements restore function Can also be caused by retinitis pigmentosa Degenerative retinal diseases that destroy rods Tips of rods are not replaced when they slough off © 2016 Pearson Education, Inc.

15.4 Processing and Relaying of Visual Information
Visual Pathway to the Brain Axons of retinal ganglion cells form optic nerve Medial fibers from each eye cross over at the optic chiasma then continue on as optic tracts, which means each optic tract: Contains fibers from lateral (temporal) aspect of eye on same side and medial (nasal) aspect of opposite eye, and Each carries information from same half of visual field © 2016 Pearson Education, Inc.

Visual Pathway to the Brain (cont.)
Most fibers of optic tracts continue on to lateral geniculate nuclei of thalamus From there, thalamic neurons form optic radiation, which projects to primary visual cortex in occipital lobes Conscious perception of visual images occurs here © 2016 Pearson Education, Inc.

Visual Pathway to the Brain (cont.)
Other optic tract fibers send branches to midbrain One set ends in superior colliculi, area controlling extrinsic eye muscles A small subset of ganglion cells in retina contains melanopsin (circadian pigment), which projects to: Pretectal nuclei: involved with pupillary reflexes Suprachiasmatic nucleus of hypothalamus: timer for daily biorhythms © 2016 Pearson Education, Inc.

Figure 15.19a Visual pathway to the brain and visual fields, inferior view.
Both eyes Fixation point eye Left only Right only eye Right eye Left eye Optic nerve Supra- chiasmatic nucleus Optic chiasma Pretectal nucleus Optic tract Uncrossed (ipsilateral) fiber Lateral geniculate nucleus of thalamus Crossed (contralateral) fiber Optic radiation Superior colliculus Primary visual cortex (occipital lobe) The visual fields of the two eyes overlap considerably. Note that fibers from the lateral portion of each retinal field do not cross at the optic chiasma. © 2016 Pearson Education, Inc.

Figure 15.19 Visual pathway to the brain and visual fields, inferior view.
Both eyes Fixation point eye Left only Right only eye Right eye Left eye Optic nerve Supra- chiasmatic nucleus Optic chiasma Pretectal nucleus Optic tract Lateral geniculate nucleus Superior colliculus (sectioned) Uncrossed (ipsilateral) fiber Lateral geniculate nucleus of thalamus Crossed (contralateral) fiber Optic radiation Superior colliculus Primary visual cortex (occipital lobe) Corpus callosum The visual fields of the two eyes overlap considerably. Note that fibers from the lateral portion of each retinal field do not cross at the optic chiasma. Photograph of human brain, with the right side dissected to reveal internal structures. © 2016 Pearson Education, Inc.

Depth Perception Both eyes view same image from slightly different angles Visual cortex fuses these slightly different images, resulting in a three-dimensional image, which leads to depth perception Requires input from both eyes © 2016 Pearson Education, Inc.

Loss of an eye or destruction of one optic nerve eliminates true depth perception entirely Peripheral vision on damaged side is also affected If neural destruction occurs beyond optic chiasma, then part or all of opposite half of the visual field is lost Example: stroke can affect left visual cortex, which leads to blindness in right half of visual field © 2016 Pearson Education, Inc.

Visual Processing Retinal cells split input into channels that include information about: Color and brightness, but also complex info such as angle, direction, and speed of movement of edges (sudden changes in brightness or color) Lateral inhibition decodes “edge” information Job of amacrine and horizontal cells © 2016 Pearson Education, Inc.

Visual Processing (cont.)
Ganglions pass information to lateral geniculate nuclei of thalamus to be processed for depth perception, with cone input emphasized Primary visual cortex contains topographical map of retina Neurons here respond to dark and bright edges and to object orientation Provide form, color, motion inputs to visual association areas © 2016 Pearson Education, Inc.

Visual Processing (cont.)
Info is also passed on to temporal, parietal, and frontal lobes, where objects are identified and location in space determined © 2016 Pearson Education, Inc.

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