Special Visual Function PDF

Special Visual function What is special visual function? Not so special actually…. Visual acuity The most widely used measure of visual resolution. For clinical diagnosis and evaluation and for legal screening and selection. For inter-professionals communications. The value...

Special Visual function What is special visual function? Not so special actually…. Visual acuity The most widely used measure of visual resolution. For clinical diagnosis and evaluation and for legal screening and selection. For inter-professionals communications. The value of visual acuity measurements is well proven for correcting refractive errors. Under some conditions, however, individual variation in standard measurements of visual acuity often is not able to predict individual variation in performance on some visual tasks, such as target detection and identification. Objectives Contrast sensitivity function Dark adaptation Dynamic vision Glare Contrast Sensitivity Contrast Lmin) = (Lmax – Lmin) / (Lmax + Is an important parameter in assessing vision. Visual acuity measurement in the clinic use high contrast, that is, black letters on a white background. In reality, objects and their surroundings are of varying contrast. Therefore, the relationship between visual acuity and contrast allows a more detailed understanding of our visual perception. Contrast Sensitivity (C) Where C can have a value between 0.0 and 1.0; sometimes C is called the modulation, Raleigh or Michelson contrast. The luminance of contrast gratings vary in a sinusoidal manner. This allows the contrast of the grating to be altered without changing the average luminance of the screen displaying the gratings. Sine wave gratings and SF The size of the bars of the grating can be expressed in terms of the number of cycles (one cycle consists of one light bar plus one dark bar of the grating) per degree subtended at the eye. This is called the spatial frequency (SF) of the grating and can be thought of as a measure of the fineness or coarseness of the grating. The units can be cycles per degree Spatial frequency and contrast In Modulation Transfer Function (MTF), i.e., the degree to which different frequencies are amplified or attenuated by the Optical system (Discussed in optics). The behavioral of eye analogy to the MTF is the contrast sensitivity function (CSF) which describes how sensitive an observer is to sine wave gratings as a function of their spatial frequency. This is measured using a contrast detection experiment wherein one determines the minimum contrast required to detect sine wave gratings of various spatial frequencies. As usual, sensitivity is defined as 1/(threshold contrast) (so if threshold is low, sensitivity is high). Contrast Sensitivity Function (CSF) Under photopic conditions, contrast sensitivity measurements reveal a band- pass function when using sinusoidal gratings. The peak of the CSF function is in the mid-spatial range and only under high contrast conditions, is resolution at its maximal level. Factors for the shape of CSF The shape and critical parameters of the CSF depends on a number of factors including: The mean luminance of the grating, whether the luminance profiles of the gratings are sinusoidal or square waveforms, the level of defocus, and the clarity of the optics of the eye. At low light levels, maximum contrast sensitivity is approximately 8% and maximum resolution is approximately 6 cycles per degree. As mean light levels increase, the peak of the contrast sensitivity function is now close to 0.5% contrast and the high spatial frequency cut off is at about 50 to 60 cycles per degree (~6/3 or 20/10). Examples of how the CSF is altered due to refractive error or disease The contrast sensitivity function provides a more thorough representation of the visual system. Not only will certain disease/disorders of the eye reduce visual acuity, contrast sensitivity will also be affected B. Patients with multiple sclerosis will have mid to low contrast sensitivity losses C. Patients with cataracts will have an overall reduction in contrast sensitivity D. Mild refractive error or mild amblyopia, will lead to a CSF C. With more severe refractive errors or severe amblyopia, resulting in a CSF similar to curve C. Multiple spatial frequency channels The CSF is typically not thought of as the MTF of a single kind of neuron, but rather an envelope of sensitivity over several underlying mechanisms, each corresponding to neurons with differing preferred spatial frequencies (i.e., with different sizes of receptive field; larger = lower spatial frequency preference).  Stare at a particular sine wave grating (e.g., 8 cycles/degree) for an extended period of time. The visual system adapts to that pattern, and any neurons or mechanisms that were sensitive to that pattern become desensitized temporarily. If one re-measures the CSF while in that adapted state After effect explanation Schematic illustration of the size of receptive fields in the parafoveal region (7° eccentricity) (a) and in the peripheral retina (35° eccentricity) (b). Tests on contrast sensitivity function Functional Acuity Contrast Test (F.A.C.T.®) Pelli Robson contrast Chart Pelli-Robson test measures contrast sensitivity using a single large letter size (20/60 optotype), with contrast varying across groups of letters. Bailey lovie low contrast chart  Mobility: Pelli-Robson chart performance is a good predictor of time for patients with age-related macular degeneration to complete an obstacle course & number or errors (collisions). Low vision patients: Reading speed is affected by spatial CSF losses Possible new treatments: selective contrast enhancement at certain spatial frequencies to boost face recognition Contrast enhancement in ARMD  Contrast theory in myopia (2023 updated) The contrast theory for myopia hypothesises that high retinal contrast leads to overstimulation of the retina; this overstimulation is then thought to signal to the eye to continue growing. Low retinal contrast, on the other hand, signals the eye to slow or stop growing. Contrast theory suggests that this near work disrupts emmetropization and causes myopia, by presenting a large amount of high contrast information across the retina. Rappon J, Chung C, Young G, Hunt C, Neitz J, Neitz M, Chalberg T. Control of myopia using diffusion optics spectacle lenses: 12-month results of a randomised controlled, efficacy and safety study (CYPRESS). Br J Ophthalmol. 2022 Sep 1:bjophthalmol-2021-321005. DARK  Dark adaptation The eye operates over a large range of light levels. The sensitivity of our eye can be measured by determining the absolute intensity threshold, Thus, the minimum luminance of a test spot required to produce a visual sensation. This can be measured by placing a subject in a dark room, and increasing the luminance of the test spot until the subject reports its presence. Consequently, dark adaptation refers to how the eye recovers its sensitivity in the dark following exposure to bright lights.  Comparing the size of the human pupil in bright and dark conditions. In these images from a medical scanner the pupil is outlined in red. In daylight the pupil is typically around 2 mm in diameter. In dark conditions the pupil expands to a diameter of approximately 5 mm in older adults and up to 7 mm in younger people, increasing the amount of light entering the eye by a factor of approximately ten times. Duplicity Theory Dark adaptation forms the basis of the Duplicity Theory Above a certain luminance level (about 0.03 cd/m2), the cone mechanism is involved in mediating vision; photopic vision. Below this level, the rod mechanism comes into play providing scotopic (night) vision. The range where two mechanisms are working together is called the mesopic range, as there is not an abrupt transition between the two mechanism.  The dark adaptation curve The dark adaptation curve shown below depicts this duplex nature of our visual system. The first curve reflects the cone mechanism. The sensitivity of the rod pathway improves considerably after 5-10 minutes in the dark and is reflected by the second part of the dark adaptation To produce a dark adaptation curve, subjects gaze at a pre-adapting light for about five curve. minutes, then absolute threshold is measured over time. Pre-adaptation is important for Factors Affecting Dark Adaptation Intensity and duration of pre-adapting light Size and location of the retina Wavelength of the threshold light Rhodopsin regeneration Intensity of pre-adapting light Duration of pre-adapting light  Size and location of the retina The retinal location used to register the test spot during dark adaptation will affect the dark adaptation curve due to the distribution of the rod and cones in the retinal Test spot location on the retina When a small test spot is located at the fovea (eccentricity of 0o), only one branch is seen with a higher threshold compared to the rod branch. When the same size test spot is used in the peripheral retina during dark adaptation, the typical break appears in the curve representing the cone branch and the rod branch. Test spot size on the retina When a small test spot is used during dark adaptation, a single branch is found as only cones are present at the fovea. When a larger test spot is used during dark adaptation, a rod-cone break would be present since the test spot stimulates both cones and rods. As the test spot becomes even larger, incorporating more rods, the sensitivity of the eye in the dark is even greater, reflecting the larger spatial summation characteristics of the rod pathway.  Wavelength of the threshold light When stimuli of different wavelengths are used, the dark adaptation curve is affected. A rod-cone break is not seen when using light of long wavelengths such as extreme red. This occurs due to rods and cones having similar sensitivities to light of long wavelengths. On the other hand, when light of short wavelength is used, the rod-cone break is most prominent as the rods are much more sensitive to short wavelengths than cones once the rods have dark adapted.  Rhodopsin regeneration Dark adaptation also depends upon photopigment bleaching. Bleaching rhodopsin by 1% raises threshold by 10 (decreases sensitivity by 10), It can be seen that, bleaching 50% of rhodopsin in rods raises threshold by 10 log units while the bleaching 50% of cone photopigment raises threshold by about one and a half log units. Therefore, rod sensitivity is not fully accounted for at the receptor level and may be explained by further retinal processing. The important thing to note is that bleaching of cone photopigment has a smaller effect on cone thresholds. Summary Red light at night, why? Dark adaptation curve measurem ent Clinical application of dark adaptation curve Retinitis pigmentosa Clinical application of dark adaptation curve Dynamic visual acuity Dynamic visual acuity (DVA) is defined as the ability to identify the details of visual targets when there are relative movements between the subjects and objects. Dynamic visual acuity test (DVAT) plays a key role in the assessment of vestibular function, the visual function of athletes, as well as various ocular diseases. Palidis DJ, Wyder-Hodge PA, Fooken J, Spering M. Distinct eye movement patterns enhance dynamic visual acuity. PLoS One. 2017;12(2):e0172061  Vestibulo–ocular reflex Dynamic visual acuity (DVA) provides an overall functional measure of visual stabilization performance that depends on the vestibulo-ocular reflex (VOR) Potential application DVA measurements have practical applications in vision training with binocular vision issues, sports vision, low vision and traumatic or acquired brain injury. It affects everyday activities from driving, to locating items on a supermarket shelf while walking. Standardized norms or structured methods of DVA acquisition are as yet not widely accepted, but the ability to rapidly detect deficiencies and appropriately retrain or enhance proficiency is possible. Implementing DVA as an integral part of the eye examination may be encouraged.  Kinetic VA Kinetic VA (KVA) is the ability to identify approaching objects Dynamic VA is the ability to identify objects moving horizontally or vertically. In sports such as baseball that involve many horizontal and vertical movements, DVA is often used. When driving an automobile, however, drivers often must recognize signs moving forward and backward, requiring the use of KVA for traffic safety.  Kinetic VA Senior citizens account for over 50% of all traffic fatalities, and accident location analysis reveals that over 50% of fatal traffic accidents occur at intersections ( 十字 路口 ). Wu, Jinglong & Lu, Shengfu & Miyamoto, Shuhei & Hayashi, Yuji. (2009). NEW DEFINITIONS OF KINETIC VISUAL ACUITY AND KINETIC VISUAL FIELD AND THEIR AGING EFFECTS. IATSS Research. 33. 27-34. 10.1016/S0386-1112(14)60234-X. Glare Glare Glare is a visual phenomenon in which one feels either discomfort and/ or exhibits a lower visual performance. This happens when a relatively bright source of light (the source of glare) is placed within the visual field which is sufficient to cause an unpleasant sensation, temporary blurring of vision as well as eye fatigue. The magnitude of the sensation of glare depends upon such factors as the size, position, and luminance of a source, the number of sources and the luminance to which the eyes are adapted. Glare classification By origin direct glare indirect (reflected) glare By effect on people disability glare/ veiling glare discomfort glare  Direct glare is caused by bright areas, such as luminaires, ceilings and windows that are Direct Glare/ Indirect directly in the field of view. Indirect glare is caused by light that is reflected to the eye from Glare surfaces of the task area which are in the visual field. Discomfort Glare Discomfort glare causes uncomfortable sensation but does not result in a decrease in vision. This occurs when illumination of the visual field is much brighter than the level of illumination for which the retina is adapted.  Disability glare may not cause any discomfort, but it affects vision and reduces visual performance. Disability Glare This occurs when high luminance is present in a low luminance scene. Light from the source is scattered within the eye thereby forming a haze of veiling effect and this reduces retinal image contrast, thus diminishing the visual performance. Glare testing Glare testing is very useful for quantifying vision loss associated with light scatter. A lens opacity caused by cataracts, or a corneal scar can cause incoming light to the eye to scatter, showering stray light rays across the retina. This light scatter reduces image contrast. In many cases, patients who have significant losses in vision due to light scatter are able to retain good acuity because standard acuity tests vision using high contrast black-on-white letters. Causes of Glare Cataracts Refractive surgery, such as LASIK Aging Common eye problems such as nearsightedness, farsightedness or astigmatism Pupil dilation during an eye exam Artificial lens implants used to treat cataracts (intraocular lenses, or IOLs) Glare solutions Modify the working environment Lens corrections Cataract surgery Thank You