Vision PDF

Vision Asghar Ghasemi  Vision (sight) is the perception of objects in the environment by means of the light they emit or reflect. Vision, the act of seeing, is extremely important to human survival More than half of the sensory receptors in the human body are located in the eyes A large part...

Vision Asghar Ghasemi  Vision (sight) is the perception of objects in the environment by means of the light they emit or reflect. Vision, the act of seeing, is extremely important to human survival More than half of the sensory receptors in the human body are located in the eyes A large part of the cerebral cortex is devoted to processing visual information  Light is visible electromagnetic radiation Human vision wavelengths: 400 to 700 nm (visible light) Most solar radiation of shorter and longer wavelengths is filtered out by ozone, carbon dioxide, and water vapor in the atmosphere Light velocity in vacuum: 300,000,000 m/sec  Light refraction Refraction is the bending, that is, the change of direction in the path of a ray of light when it passes from one medium to another (for example, from air to water).  Refractive index The ratio of speed of light in air to that in given medium is called the refractive index of that medium. : μ or The of vacuum/air is arbitrarily set at 1 The refractive index of water is 1.33 Amount of refraction depends on 3 factors: (1) Δμ (difference in refractive index) between the two media (2) Angle at which the light rays strike the interface (3) Wave length: the eye lens bends blue rays >red rays  Two main types of lens Focal point: The point where parallel rays passing through the lens would converge (in the case of a convex lens) or appear to diverge (in the case of a concave lens) Focal length (distance): The distance between the optical center of a lens and the focal point Convergence of parallel light rays by a convex lens convex lens Divergence of parallel light rays by a concave concave lens lens Focal length - +  Dioptric power of the lens The power of a lens is expressed in diopter (D) and is the reciprocal of its focal length in meters The power of a convex lens with focal length of + 25 cm is + 4 diopter The power of a concave lens of − 50 cm is − 2 diopter Eye structure The eyeball is a sphere ~ 24 mm in diameter  Optical component of the eye The optical components of Optical system of the the eye are transparent eye creates an image on elements that admit light the retina by 3 rays, bend (refract) them, mechanisms: and focus images on the retina: 1. Refraction of light rays by cornea and 1. Cornea lens 2. Aqueous humor 2. Accommodation of 3. Lens lens by ciliary muscle 4. Vitreous body 3. Change in pupil size Images focused on the retina are by small, iris muscles inverted, and right-to-left reversed Refraction  Reduced eye If all the refractive surfaces of the eye µAir=1 are added together algebraically and considered as a single lens, the optics of the eye is simplified and represented as a reduced eye (μ=1.336) In the reduced eye, a single refractive surface is considered to exist, with its central point 17 mm in front of the retina Total refractive power of reduced eye is 59 D when we have distant vision (> 6 m)  Reduced eye Refractive power of the eye= 59 D Cornea= two-thirds=40 D Lens= one-third=20 D Most of the refractive power of the eye is provided by the anterior surface of cornea because of greater difference of refractive index with its adjacent medium (i.e., air) The importance of the internal lens is that in response to nervous signals from the brain, its curvature can be changed between 13-26 D (accommodation) Δ=0.07 Δ=0.38  Errors of refraction Eyeball is too Eyeball is too short long em = in; metr = = excessive; = = closed; = eye; measure; opia = vision eye; = condition = condition Astigmatism ( = not; = point; = condition) Inability to simultaneously focus light rays that enter the eye on different planes Focusing on vertical lines, such as the edge of a door, may cause horizontal lines, such as a tabletop, to go out of focus Caused by a deviation in the shape of the cornea so that it is shaped like the back of a spoon rather than part of a sphere Corrected with cylindrical lenses, which refract light more in one plane than another Accommodation  Emmetropia is a state in which the eye is relaxed and focused on an object >6 m away, the light rays coming from that object are parallel, and the rays are focused on the retina without effort The near response, or adjustment to close-range vision, involves three processes to focus an image on the retina (near vision triad): 1. Accommodation of the lens 2. Constriction of the pupil 3. Convergence of the eyes Accommodation is a dynamic change in the dioptric power of the lens allowing the point of focus of the eye to be changed from distant to near objects Anchor lens to the ciliary muscles Far vision Near vision Gaze: a fixed intent look  The amplitude of accommodation Children= 14 D: The refractive power of the lens of the eye can be increased voluntarily from 20 to 34 diopters 45-50 years= 2 D 70 years= 0 (Presbyopia) = old; = eye; = condition Regulation of pupil size  Changes in pupillary size Proper imaging of light on the retina depends not only on the lens and cornea but also on the iris, which adjusts the amount of light that can enter the eye through the pupil. Pupil 1.5 mm The amount of light that enters the 8 mm diameter eye through the pupil is proportional to the of the pupil or to the of the pupil Light enters 1 30 The iris contains 1. radially (pupillary dilator- activated by the sympathetic) and 2. circularly/sphincter (pupillary constrictor- activated by parasympathetic fiber) oriented smooth muscle fibers that control the amount of light Circular/sphincter muscle entering the eye, in a way similar to that of the diaphragm of a camera Radial muscle  Changes in pupillary size Pupillary constriction and dilation occur in response to: 1. Emotions 2. Change in light intensity (photopupilary reflex/ pupillary light reflex) 3. A shift in one’ s gaze between distant and nearby objects (as part of near-vision triad)  Pupillary light reflex Direc t  Argyll Robertson pupil It is a disorder in a patient of neurosyphilis due to lesion of pretectal nucleus of midbrain which is one of the cell stations in light reflex pathway The disease is characterized by narrow pupil with no reaction to light but reaction to accommodation.  Near-vision triad adduction of the eyes: orients the visual axis of each eye toward the object to focus its image on each fovea  Fluid system of the eye Aqueous humor Vitreous humor Place lies in front of the lens between the posterior surface of the lens and the retina Flow of fluid High Very low (gelatinous mass) Function Maintaining the intraocular Maintaining spherical shape of pressure (15±2 mm Hg) eye Rate of production (2-3 µL/min) and reabsorption (2-3 µL/min) of aqueous humor equals Retina  Retina The retina is the light-sensitive portion of the eye that contains at least 5 types of cells: 1. Photoreceptors: Cones: for color vision Rods: for detection of dim light (black and white vision and vision in the dark) 2. Bipolar cells 3. Horizontal cells 4. Amacrine cells 5. Ganglion cells Pigment layer of the retina Contains that prevents light reflection Stores which is an important precursor of the photosensitive chemicals of the rods and cones  Blood Supply of the Retina 1. Internal layers of the retina: central retinal artery, which enters the eyeball through the center of the optic nerve and then divides 2. Outermost layer of the retina, including the outer segments of the rods and cones (near the diffusion from the choroid blood vessels  Photoreceptors: Rods and Cones Each retina contains about 100 million rods and 3 million cones Folding of membrane Free floating discs Rhodopsi Color/cone pigments n Cell body Light-sensitive photochemical proteins are found in the outer segment. In general, the rods are narrower and longer than the cones  Fovea Fovea (fovea = pit, depression) (1 mm2), a depression in the center of the retina Fovea is the region of the retina with the highest visual resolution (acuity) Acuity refers to the ability to see details Fovea has a high number of cones The light from the fixation point is focused on the fovea A major function of eye movements is to bring objects of interest into view on the fovea Central fovea (0.3 mm2) Is composed almost entirely of cones (35000 slender cones) Has the highest visual acuity Cones are better for detail vision Less Convergence Causes the Cones to Have Better Acuity Than the Rods  Why visual acuity is high in the fovea? 1. Cone density is maximal in the fovea 2. Foveal cones are long and thin, allowing for high packing density 3. In the foveal region, the blood vessels and other retinal layers are all displaced to one side rather than resting directly on top of the cones, and both image distortion and light loss are minimized 4. No convergence: 1:1 ratio of cones and bipolar cells Photoreceptor distribution in the Retina Rods are 30 to 300 times more sensitive to light than cones  Phototransduction (visual transduction) (the process by which light energy is converted into a receptor potential in the outer segment of a photoreceptor)  Detail of the rod outer segment One disc of the outer segment showing the membrane studded with pigment molecules. the isomer present in the absence of light A rod cell A pigment molecule in the membrane of the disc, with the protein moiety (opsin) the isomer produced when the and the vitamin A derivative (retinal) pigment absorbs a photon of light.  Photoreceptors in dark GC: Guanylate cyclase NKA: Na+-K+-ATPase PDE: Phosphodiesterase Light converts 11-cis retinal to all-trans retinal Rhodopsin (visual purple) in rods: 1. Scotopsin (protein): a GPCR 2. 11-cis retinal: Light-absorbing part Photoreceptor in light  Receptor potential in photoreceptors 1. Is a hyperpolarizing receptor potential 2. In cones, the light-induced hyperpolarization occurs 4 times as fast as in the rods 3. Receptor potential in photoreceptors is approximately proportional to the logarithm of the light intensity  Rods amplifies the effect of a single photon of light and has extreme sensitivity A single photon of light can cause a receptor potential of about 1 millivolt in a rod Only 30 photons of light will cause half-saturation of the rod How can such a small amount of light cause such great excitation? cGMP PDE Transduci n flow of > 1 million Isomerization of sodium ions is blocked The cones are about 30 to 300 times less sensitive to light than the Color vision Three kinds of cones Light-sensitive named for the absorption substance in cones peaks of their photopsins 1. Short-wavelength (S) cones Color/cone pigments (blue cones), with peak sensitivity at a wavelength 445 Protein: Photopsin (differ nm from rods) 2. Medium-wavelength (M) cones = light; = vision (green cones), which peak at Blue-sensitive 535 nm pigments 3. Long-wavelength (L) cones (red Green-sensitive cones), which peak at 570 nm pigmen Red-sensitive pigment Rods peak at 505 nm 11-Cis retinal (same as the rods)  Our perception of colors is based on a mixture of nerve signals representing cones with different absorption peaks Peak sensitivity 445 nm 535 nm 570 nm → R:G:B Orange (580 nm): 99:42:0 Blue (450 nm): 0:0:97 About equal stimulation of the R, G, and B cones gives one the sensation of seeing white Black is the sensation produced by the absence of light Retinal Neurons  Responses of retinal neurons to light The only retinal neurons that always transmit visual signals via action potentials are the ganglion cells. Ganglion cells, even when unstimulated, transmit continuous impulses (5 and 40/sec) Cell Response to light Neurotransmitter Conduction Photoreceptor Hyperpolarization Glutamate Electrotonic (rods and cones) Horizontal cells Hyperpolarization GABA Electrotonic Bipolar cells Hyperpolarization/ Glutamate Electrotonic Depolarization Amacrine cells Depolarization Inhibitory Electrotonic* neurotransmitter: GABA, glycine, dopamine, ACh, indolamine Ganglion cells Depolarization Glutamate Action potential * Occasionally, action potentials have also been recorded in amacrine cells, although the importance of these action potentials is questionable Which cells provide lateral inhibition in the retina? 1. Horizontal cells: connect laterally between the synaptic bodies of the rods and cones and with the dendrites of the bipolar cells and provide lateral inhibition 2. Interplexifotm cells: help horizontal cells for lateral inhibition 3. Some of the amacrine cells  Amacrine cells About 30 types of amacrine cells have been identified: 1. Act as apart of the direct pathway for rod 2. Signal the onset of a continuing visual signal 3. Signal the offset of visual signals 4. Signal change in illumination 5. Signal movement of a spot across the retina in a specific direction (i.e., directionally sensitive)  Ganglion cells in cats X Y W Size Moderate Largest Small % 55 5 40 Velocity Moderate High Slow Receptive fields in Small Large Large the retina function Color vision and Respond to rapid Directional fine details of changes in visual sensitivity and visual image images crude rod vision under dark conditions  Ganglion cells in humans P (Parvocellular) M (Magnocellular) Other names Beta cells; midget Alpha cells; parasol cells ganglion cells (in central retina) Projection to LGN Parvocellular layers Magnocellular layers (small cells) (large cells) Receptive field Small Large Conduction velocity Slow Fast Response Sustained Transient Color sensitivity Yes No Function Color vision and Detecting low-contrast detecting details stimuli and rapid movement visual signals  Melanopsin-containing ganglion cells Send signals mainly to non-visual areas of the brain, particularly the suprachiasmatic nucleus (SCN) of the hypothalamus, the master circadian pacemaker These signals help control circadian rhythms that synchronize physiological changes with night and day Convergence in retina Scotopic and photopic vision = dark; = vision = light; = vision  Scotopic/rod/old vision Photopic/cone/new vision Peripheral retina/low resolution Central retina (Fovea)/high resolution  Scotopic vision Photopic vision (peripheral retina) (central retina) Circuit Rod→ Bipolar→ Amacrine→ Gan Cone→Bipolar→ Ganglion glion Lateral inhibition Horizontal cell Horizontal cells Neurons and nerve Smaller Larger fibers Conduction velocity Slower Faster (2-5 times) Convergence High Low (200 rods: 1 ganglion cells (1 cone: 1 ganglion cell in in peripheral retina) fovea) Sensitivity to light Higher Lower Color sensitivity NO Yes  Retinal Circuitry and Visual Sensitivity Large receptive field Small receptive field High convergence Low convergence Low-resolution system High-resolution system Visual pathway (CN. II) (SCN: to control circadian rhythm) (Geniculocalcarin e (to control pupil size) fibers ) (to control visually-guided eye movement) (LGN) V1: Calcarine fissure of the medial occipital lobe Visual cortex is in the occipital lobe  Fast Position and Motion Pathway Two Major Pathways for M pathway (dorsal pathway) Analysis of Visual Where is seen? Information Accurate Color Pathway P pathway (ventral pathway) What is seen? The primary visual cortex has 6 layers Color blobs, special columns of neurons, are the primary areas for deciphering color.  Cells in V1 that detect orientation and shape of figures Simple cells Simple cells in layer IV of the V1 detect orientation of lines and borders Complex Cells Complex cells detect line orientation when a line is displaced laterally or vertically in the visual field (detect movement) Hypercomplex cells Hypercomplex cells detect lines of specific lengths, angles, or other shapes Hypercomplex cells are located in the outer layers of the primary visual columns, as well as neurons in some secondary visual areas  Visual Adaptation is the ability of the retina to adjust its sensitivity according to ambient light to operate efficiently over a wide range of lighting conditions  is achieved by switching between the use of the cone (for bright light) and rod systems (for low-light) Dark adaptation  Thank you 1. 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