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Chapter 9

NROC64-Chapter 9.docx

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Matthias Niemeier

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NROC64-Chapter 9 Introduction:  sensitivity to light enables animals, including humans to detect prey, predators, and mates  due to bouncing of light in eyes from objects around us in order to make sense of complex object  light is an electromagnetic property that is emitted in the form of a wave  due to nature of electromagnetic waves and interaction with the environment, the visual system can extract information about the world  requires a lot of neural machinery  vision is provided new ways to communicate, give brain mechanisms for preparing for trajectory of objects and space and time allowed for new forms of mental imagery and abstract  half of human cerebral is involved in analyzing the visual world  mammalian visual system begins with eye at the back of the eye is the retina: contains photoreceptors specialized to convert light energy into neural activity  eye automatically adjusts to differences in illumination and automatically fixates on objects of interest  eye can track moving objects (by movement) and keep its transparent surfaces clean (by blinking)  each eye had to overlapping retinas: one specialized for low light levels that we encounter from dusk to dawn and another for higher light levels and for detection from sunrise to sunset  the output of the retina is not faithful reproduction of the intensity of light falling  retina is specialized to detect differences in the intensity of falling on different parts of it  axons in the retinal neurons are bundled in optic nerves, which distinguish visual information (in the form of action potentials) to several brain structures that preform different functions  some targets of the optic nerve involve regulating biological rhythms, which are synchronized with light-dark daily cycle  others are involved in eye position and optics  first synaptic relay in the pathway that serves where perception occurs in a cell groups of the dorsal thalamus lateral geniculate nucleus or LGN  from the LGN, visual information ascends to cerebral cortex, where it is interpreted and remembered  electromagnetic radiation is all around and comes from radio antennas, mobile phones, X-ray machines, and the sun  light is electromagnetic radiation that is visible to the eye  electromagnetic radiation can be described as a wave of light  has a wavelength: distance between successive peaks and troughs  frequency: # of waves per second  amplitude: difference between wavelength and peak  radiation emitted at a high frequency (short wavelengths) has highest energy content (i.e. gamma radiation in X-rays ) 10 (<1m)  radiation emitted at lower frequencies (longer wavelengths) has less energy (i.e. radar and radio waves) > 1mm  visible light consists of wavelengths of 400-700nm  “hot” colour like red or orange consist of light with longer wavelength and so less energy, than cool colours like blue or violet  in a vacuum, a wave of electromagnetic radiation will travel in as a straight line and be described as a ray  light in our environment also travel in straight lines until they interact with atoms and molecules in the atmosphere and objects on the ground  interaction include reflection, absorption, and refraction  optics: study of light rays and their interactions  reflection: is the bouncing of light rays off a surface  manner in which a mirror of light is reflected depends on the angle at which it strikes the surface (i.e. a ray strikes mirror perpendicularly is reflected 180° back)  absorption: is the transfer of light to a particle or surface (i.e. light o skin warms you up)  some compounds absorb light energy only in a limited range of wavelengths, then reflect the remaining wavelengths (i.e. blue pigment absorbs long wavelengths, but reflects short wavelengths of 430 nm, which is blue)  lights sensitive photoreceptors cells in the retina contain pigments and use energy absorbed from light to generate changes in membrane potential  refraction: images are formed in the eye when the bending of light rays that can occur when they have moved from one transparent medium to the other  Ex: light passing from air into a pool of water, if its strikes at an angle, it will bend toward a line that is perpendicular to the surface  bending of light occurs, because the speed of light differs in the two media, light passes through air more rapidly than water  * the greater the difference between the speed of the two media, the greater the angle of refraction  pupil: is the opening that allows light to enter the eye and reach the retina  the pupil appears dark, because of the light-absorbing pigments in the retina  iris: surrounds the pupil- pigmentation of it is eye colour  iris contains two muscles that can vary the size of the pupil, one makes it smaller when it contracts and the other makes its larger  pupil iris are covered by glassy transparent external surface eye, the cornea  cornea is continuous with the sclera: “white of the eye” which forms a tough wall of the eyeball  eye’s orbit: the eyeball sits in a bony eye socket in the skull  extrocular muscles: 3 pairs of muscles inserted into the sclera, which move the eyeball in the orbit  muscles normally are not visible, because they le behind conjunctiva: a membrane that folds back from inside of the eyelids and attaches to the sclera  optic nerve: carrying axons from the retina, exists the back of the eye, passes the orbit, and reaches the base of the brain near the pituitary gland  ophthalmoscope: a device that enables one to peer through the pupil to the retina  it shows blood vessels on its surface  optic disk: a pale circular region where retinal vessels originate  sensation of light cannot occur at the optic disk, because there are no photoreceptors here  large blood vessels exist because he vessels cast shadows on the retina  not aware of any holes in our field of vision, because the brain fills in our perception of the areas  in the middle of each retina is darker-coloured region with a yellowish hue called macula (‘spot”), the part of the retina for central (as opposed to peripheral ) vision  maculae has relative absence of large blood vessels  the vessels are from the optic disk (front of the eye) to the macula (back of the eye), also the trajectory of the optic nerve  relative absence of large blood vessels in this region of the retina is one of the specializations that improves the quality of central vision  fovea: a dark spot about 2mm in diameter (“pit”)  retina is thinner in fovea than anywhere else –center of the retina  part of the retina that lies closer to the fovea is called nasal and the part that lies near the temple is called temporal , and the part of the retina above the fovea is called superior, and the part under it is called inferior Demonstrating the Blind Regions of Your Eye:  looking through the ophthalmoscope reveals that there is a sizable hole in the retina, optic disk (region where optic nerves axons exit the eye and retinal blood vessels enter the eyes)  optic disk does not have photoreceptors  blood vessels coursing across the retina are opaque and block the light from falling on photoreceptors beneath them  visual space where black circle disappears Is called blind spot  we are still able to view an uninterrupted, seamless image, despite light sensitive regions due to mechanisms in the visual cortex appear to “fill in” missing regions  cornea lacks blood vessels and is nourished by fluid behind it, called aqueous humour  behind this is transparent lens located behind iris  lens is suspended by ligaments (zonule fibers) attached to the ciliary muscles: which are attach sclera and form ring inside eye  shape of lens enable our eyes to adjust their focus to different viewing distances  also divides the interior of eye into two compartments containing slightly different fluids  aqueous humour: is a watery fluid that lies between cone and lens  vitreous humour: more viscous, jelly-like fluid, lies between the lens and the retina, its pressure serves to keep the eyeball spherical  * structures at the front of the eye serves to regulate amount of light allowed in and refract light on the retina at the back  bringing objects into focus requires combined refractive powers of cornea and lens  * cornea than lens is site of most refractive power of eyes  light passes into a medium where its speed is slowed, it will bend toward a line that is perpendicular to the border or interface between the media- light striking the cornea  curved surface of cornea bend so that when light rays strike, they can converge on the back of the eye  those that enter center of eye pass straight into the retina  distance from refractive surface to point where parallel light rays converge is called focal distance  focal distance depends on the curvature of the cornea- the tighter the curve, the shorter the focal distance  diopter: reciprocal of focal distance in meters  cornea has a refractive power of 42 diopters, so parallel light rays striking corneal surface will be 0.024 m (distance of cornea to the retina)  refractive power depends on slowing of light at the air cornea interface  if air is replaced with a medium that passes light at about same speed as eye, the refractive power of the cornea will be eliminated-reason for blurry looking thing underwater  lens also contributes another dozen diopters to the formation of a sharp image at a distant point  lens is involved in the crisp images of objects located closer than 9m from the eye  when objects approach, the light rays originating at a point can no longer be considered parallel  so light rays diverge and greater refractive power is required to bring them into focus on the retina  additional focusing power is due to changing shape of the lens- accomadation  *objects at distance require a little refraction, a flat lens and near objects require greater refraction by a fat (peripheral lens) Eye Disorders:  imbalance in extraocular muscles of the two eyes will cause eyes to point in different directions  strabismus: misalignment or lack of coordination between two eyes  esotropia: direction of gaze of two eyes cross (cross-eyed)  exotropia: direction of gaze diverge (wall-eyed)  strabismus is congenital and can be treated during childhood  treatment involves the use of prismatic glasses or surgery to extraocular muscles to realign in the eye  without treatment, conflicting images are sent to the brain from the two eyes, degrading depth perception and causing the person to suppress input from one eye  dominant eye is normal and suppressed eye is amblyopic: poor visual acuity  cataract: a clouding of the lens  many people over 65 years of age have some degree of cataracts and it impair vision  surgery is required, where lens is removed and replaced with an artificial plastic lens  artificial lens cannot adjust its focus like normal lens, but provides closer image that can be corrected with glasses  glaucoma: progressive loss of vision associated with elevated intraocular pressure, leading to blindness  aqueous humour maintains shape of the eye  as pressure increases, the entire eye is stressed, damaging the relatively weak point where the optic nerve leaves the eye  optic nerve axons are compressed and vision is gradually lost from periphery inward  but when person notices loss of more central vision, damage is advanced and most of eye is permanently blind  professional boxers have detached retina: retina pulls from underlying wall of eye from a blow to the head or by shrinkage of virtuous humor  the fluid from the virtuous space flows through small tears in the retina resulting in trauma and causing retina to separate  symptoms in retinal detachment include abnormal perception of shadows and flashes of light  treatment involves laser surgery to scar the edge of retinal tear and reattaching the retina to the back of the eye  retinitis pigmentosa: progressive degeneration of photoreceptors  first sign is loss of peripheral vision and night vision, so total blindness may result, “tunnel vision”  cause of disease is unknown  but strong genetic component , 100 genes have been identified  no cure, but taking vitamin A may slow the progression  macular degeneration: loss of central vision only  affects 25% of over the age of 65  peripheral vision usually preserved, but inability to read, watch television, and recognize faces as central photoreceptors gradually deteriorate  laser surgery can minimize further vision loss, but disease has no cure Vision Correction:  ciliary muscles relaxed and lens is flat, the eye is emmetropic: parallel light rays form a distant point source are focused sharply on back of retina  emmetropic eye focuses parallel light rays on the retina without the need for accommodation  hyperopia or farsightedness: when eye is too short from the front to the back and light rays are focused at some point behind the retina and image f a point of light is a blurry spot on the retina  lens cannot accommodate enough on near points  corrected by placing a convex glass or plastic lens, because curved front edge of the lens, like the cornea, bends light toward center of retina  when light passes from glass to air as it exits the lens , the back of the lens also increases the refraction  eyeball is too long rather than too short, parallel rays will converge before retina, and again be imaged on the retina as a blurry circle- myopia or nearsightedness  amount o refraction provided by cornea and lens is too great to focus distant objects  concave lens must be used to move the point image back onto the retina  astigmatism: when eyes have irregularities, such that the curvature and refraction in the horizontal and vertical planes is different  corrected by using artificial lens that is curved more along than others  presbyopia: hardening of the lens that accompanies aging is explained by new lens cells are generated through life, but not lost  hardening of lens is less elastic, it is unable to change shape and accommodates sufficient focus on both near and far objects  presbyopia first corrected by Benjamin Franklin through bifocal lens  bifocals concave at top for far vision and convex at the bottom for near vision  in hyperopia and myopia, the amount of refraction provided by the cornea is either too little or too great for the eyeball  radial autonomy: a procedure to correct myopia, where a tiny incision through the peripheral portion of the cornea relax and soften central cornea, and reduces the amount of refraction and minimizing myopia  photorefractive keratomy (PRK): a laser is used to reshape the outer surface of the cornea by vapourizing thin layers  laser in situ atomelius (LASIK): a thin flap of the cornea is lifted so laser can reshape cornea from the inside  also people can use nonsurgical methods, a person can be fitted with special retainer contact lens or plastic cornea rings, which alter shape of cornea to correct refractive errors  during accommodation; the ciliary muscle contracts and swells in size, making the area inside the muscle smaller and decreasing tension in the suspensory ligaments  so the lens becomes rounder and thicker, because of its elasticity and this increases the curvature of the lens surface- increase in refractive power  realaxation of ciliary muscles increases tension in the suspensory ligaments and lens is stretched into flatter shape  an infant can focus objects just beyond their nose, but middle-aged adults cannot see objects closer than about an arm’s length  and treatment for middle-aged is artificial lens  pupil contributes to optical functioning of the edge by adjusting to ambient light levels  pupillary light reflex: involves connections between retina and neurons in the brain stem that control muscles that constrict the pupils  it is consensual: shining a light into only one eye causes the constriction of the pupils of both eyes  when pupils are not the same size (lack of consensual pupillary light reflex) is often taken as a serious neurological disorder in the brain stem  * constriction on the pupil has the effect of increasing the degree of focus just like decreasing the aperture size increases the f-stop of a camera lens  * when the eye accommodates to a closer image, the image of the farther point on the retina no longer forms, but a blurred circle  decreasing aperture (constricting the pupil) reduces size of the blurred circle so that the image proximate a point-makes it more n focus  the structure of the eyes and where they sit on our head limits image of the world that we can see at one time  visual field: total amount of space that can be viewed by the retina when the eye is fixated straight ahead  *the image is inverted on the retina, the left visual field is imaged on the right side of the retina and the right visual field is imaged on the l
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