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Download Vision. Measurement of visual acuity. Color vision []
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Examination of vision Outline • theoretical background – the eyeball – the optical system of the eye – the visual field • practical examination - visual acuity - refractive errors - accomodation - adaptation - blind spot - visual field - color vision - optical illusions The visual system • is the part of the central nervous system which enables organisms to process visual detail • it interprets information from visible light to build a representation of the surrounding world • assimilate information from the environment (70-75%) • the resulting perception: eyesight, sight, or vision • 3 segments: • receptor level (eyeball) • visual pathways • central level (cortical and subcortical structures) The visual system • complex tasks: – reception of light – formation of monocular representations – construction of a binocular perception from a pair of two dimensional projections – identification and categorization of visual objects – assessing distances to and between objects – guiding body movements in relation to visual objects – non-image forming functions (e.g. pupillary light reflex, circadian photoentrainment) The eyeball • complex role: optical device, signal transducer, neural processor • structure: three coats – enclosing transparent structures The eyeball Outer layer: • sclera – white, fibrous, protective - contains collagen and elastic fibers - muscles attached to the sclera • cornea – transparent, covers the iris, pupil, and anterior chamber - refracts light (accounts for approximately 2/3 of the eye's total optical power, fixed focus) - avascular, rich sensitive innervation • the cornea and sclera are connected by a ring called the limbus The eyeball Middle layer: • choroid - the vascular layer of eye - contains connective tissue - provides oxygen and nourishment to the outer layers of the retina - pigmented, reduces the reflexions of light • ciliary body - roles: accommodation, aqueous humor production - anchors the lens in place • iris - regulates the amount of light entering the eye: muscles attached to the iris expand or contract the aperture at the center of the iris (pupil) The eyeball Middle layer: • zonule of Zinn - ring of fibrous strands - connects the ciliary body with the crystalline lens of the eye • lens - flexible - changes the focal distance of eye - thus the eye can focus on objects at various distances (accommodation) The eyeball Inner layer: • retina - neural structure - senses light - receptor cells: rods and cones (S, M, L) - neurons: bipolar and ganglion cells - gets its circulation from the vessels of the choroid as well as the retinal vessels - can be seen in an ophthalmoscope The eyeball within these coats: • aqueous humor: a clear fluid - is contained in two areas: - the anterior chamber between the cornea and the iris and exposed area of the lens - the posterior chamber, behind the iris, in front of the zonule and rest of the lens • vitreous body - clear jelly - is bordered by the sclera, zonule, and lens The eyeball • geometry: not properly a sphere, rather it is a fused two-piece unit – smaller frontal unit, more curved (r = 8 mm) – larger posterior unit (r = 12 mm) – fundus (area opposite the pupil) shows the characteristic pale optic disk (papilla), where vessels entering the eye pass across and optic nerve fibers depart the globe. The optical system of the eye • image formation: dioptre (diopter): unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (that is, 1/metres) • cornea: 42 D, lens: 18 D, total ~ 60 D Visual pathways Visual field Outline • theoretical background – the eyeball – the optical system of the eye – the visual field • practical examination - visual acuity - refractive errors - accomodation - adaptation - blind spot - visual field - color vision - optical illusions Visual acuity • acuteness or clearness of vision • tested using an optotype (Snellen chart) read from a distance of 5 (minimal accomadation) • expressed as a vulgar fraction: – numerator: the distance between the subject and the chart – denominator: the distance at which the lines that make up those letters would be separated by a visual angle of 1 arc minute – e. g. normal: 5/5 • lenses of varying powers are used to precisely correct for refractive errors Visual acuity Tumbling E Chart Visual acuity Landolt C (Landolt ring) Tumbling E Chart Refractive (refraction) errors • an error in the focusing of light by the eye • a frequent reason for reduced visual acuity • no refractive error when viewing distant objects: emmetropia or emmetropic eye • the eye can focus parallel rays of light (light from distant objects) on the retina, without using any accommodation; a distant object in this case is defined as an object 5 meters or further away from the eye • refractive error when viewing distant objects: ametropia or ametropic eye: when not using accommodation, the eye cannot focus parallel rays of light (light from distant objects) on the retina • types of ametropia: categorized as spherical errors and cylindrical errors • spherical errors occur when the optical power of the eye is either too large or too small to focus light on the retina Myopia • myopia (nearsightedness): when the optics are too powerful for the length of the eyeball; can arise from a cornea with too much curvature (refractive myopia) or an eyeball that is too long (axial myopia) • correction: Hyperopia • hyperopia (farsightedness): when the optics are too weak for the length of the eyeball; can arise from a cornea with not enough curvature (refractive hyperopia) or an eyeball that is too short (axial hyperopia) • correction: Accommodation • the process by which the vertebrate eye changes optical power to maintain a clear image (focus) on an object as its distance changes • reflex, but can also be consciously controlled • by changing the form of the elastic lens using the ciliary body Accommodation •the amplitude of accommodation declines with age: by the fifth decade of life the accommodative amplitude has declined so the near point of the eye is more remote than the reading distance = presbyopia • emmetropic eye (does not require optical correction for distance vision) needs an optical aid for near vision • myopic eye (nearsighted, requires an optical correction for distance vision) – less correction needed • hyperopic eye (farsighted) - needs a correction for both distance and near vision Accommodation • curtain experiment: one can look through the window at a distant tree, or can adjust our eyes so as to see the curtain, bot never in the same time • Scheiner’s experiment: uses a card with two minute holes, separated from each other by less than the diameter of the pupil, one looks at a pin; at a short distance from the eye the pin appears double; as it is moved from the eye a point is found where it appears single, and beyond which it remains single for the emmetropic eye, but for the myopic eye it soon again becomes double 3.5 mm from centre to centre Holes 1.5 mm in diameter Accommodation • Purkinje-Sanson experiment: three images of the same object, produced by reflections from the surface of the cornea and the anterior and posterior surface of the lens, the images on the two anterior surfaces are virtual and noninverted, and the image on the posterior surface is real and inverted (the viewer adapts to this phenomenon and ignores these normal images) Adaptation • the human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude • pupillary light reflex, adjusting the • two mechanisms: amount of light that reaches the retina - myosis: contraction of the pupil - mydriosis: dilatation of the pupil • changes in the sensitivity of rods and cones in the eye Adaptation Recovery time of a retinal pigment: • a strong white light is projected in the eye for 3-4 sec, then is turned off and a weak light is turned on (one of the basis colors) • the time is measured from the moment of turning off the white light to the moment when the colored light is detected (normal: < 10 sec) Latency: • the time from the application of the light stimulus to the appearance of activity in the optic nerve (~1/10 sec) Persistance: • the sensation of light remains for ~0.15 sec after the end of the stimulus Flicker fusion threshold (or flicker fusion rate) • the frequency at which an intermittent light stimulus appears to be completely steady to the observer Astigmatism • cylindrical refraction error • the optical power of the eye is too powerful or too weak across one meridian (as if the overall lens tends towards a cylindrical shape along that meridian) • blurred vision - due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina • Placido keratoscope Blind spot • Mariott's test • close your left eye and stare at the square mark in the diagram with your right eye, off to the right you should be able to see the spot • slowly move toward the computer screen (or paper), keep looking at the square mark while you move • at a particular distance the spot will disappear (it will reappear again if you move even closer) • the spot disappears because it falls on the optic disc • for the left eye, stare at the spot and notice the disappearance of the square Visual field • perimetry is the systematic measurement visual field detection of the presence of test targets on a defined background Visual field Visual field loss • due to disease or disorders of the eye, optic nerve, or brain • scotoma Visual field loss • due to disease or disorders of the eye, optic nerve, or brain • hemianopia / quadrantanopia Color vision • the capacity of an organism or machine to distinguish objects based on the wavelengths (or frequencies) of the light they reflect, emit, or transmit • dyschromatopsia color vision deficiency is the decreased ability to perceive differences between some of the colors that others can distinguish Optical illusions Optical illusions • negative afterimage Stare, unfocused, at the red cross for 10 seconds then look at white wall! Optical illusions • negative afterimage Optical illusions • negative afterimage Stare, unfocused, at the flag for 10 seconds then look at white wall Optical illusions • negative afterimage Cyan Magenta Yellow Optical illusions • lilac chaser Optical illusions • cognitive illusions are assumed to arise by interaction with assumptions about the world • Necker cube Optical illusions • cognitive illusions are assumed to arise by interaction with assumptions about the world • Rubin vase Optical illusions • cognitive illusions are assumed to arise by interaction with assumptions about the world • ambiguous image Optical illusions • cognitive illusions are assumed to arise by interaction with assumptions about the world • Kanizsa triangle Optical illusions • cognitive illusions are assumed to arise by interaction with assumptions about the world Optical illusions • distorting illusions are characterized by distortions of size, length, or curvature Müller-Lyer illusion Optical illusions • distorting illusions are characterized by distortions of size, length, or curvature cafe-wall illusion Optical illusions Optical illusions • simultaneous contrast Optical illusions Optical illusions Optical illusions