OPT 404 Visual Acuity PDF

OPT 404 notes - Visual Perception Visual Acuity Size and Visual Angle - - The size of the object image on the retina is more important than the physical size of the object - Referencing to the objects size in terms of the visual angle measured in degrees of the visual angle - The image on th...

OPT 404 notes - Visual Perception Visual Acuity Size and Visual Angle - - The size of the object image on the retina is more important than the physical size of the object - Referencing to the objects size in terms of the visual angle measured in degrees of the visual angle - The image on the retina decreases in size with distance from the observer - This can be seen in the diagrams below Different kinds of ACUITY - Minimum visible resolution - DETECTION ACUITY A thin line on background < 1 arc sec (=1/60 arc min); anything less than this results in blurred images on the retina - Minimum angle of resolution (MAR) = RESOLUTION ACUITY Discrimination of a feature; letter optotypes, two point resolution 30-60 arc seconds (6/3 - 6/6) - Minimum discriminable angle = HYPERACUITY Very good - can be 2-10 arc sec If 90%) with a good level of illumination - The smallest spatial detail that can be resolved - Depends on the optics of the eye as well as the visual system - Decreases with increasing eccentricity (in peripheral vision) › - Measure of the spatial resolution ie the smallest letter size that the visual system can identify (5’ letter size for acuity 6/6) TUMBLING E - MAR - Minimal angle of resolution - The smallest spatial detail (gap) that can be resolved (1’ gap size for acuity 6/6) LogMAR and Bailey-Lovie - LogMAR: Logaritm to the base 10 of the minimum angle of resolution (MAR) - The original charts were designed by Bailey and Love in 1976, the alternative being the Thomson Test chart - The 6/60 line gives a score of 10 - Each line below will give a score of 0.1 less than the line above - because of the balanced distribution of the letters which are graded in difficulty as how easy they are to see - For each of the five letters in each line, count for a score of 0.1/5 = 0.02 - As each letter is worth 0.2, if a patient reads on the 0.4 line plus 3 letters of the 0.3 line they will have a score of 0.34 - This is a result of the five letters of line 0.4 minus the score for each letter read from the 0.3 line [0.4 - (0.02 x 3) = 0.34] - NOTE - The 6/6 line is 0.0 logMAR and the next line below where there are smaller letters is -0.1logMAR and the one above where there are larger letters is 0.1logMAR where -Logarithmic progression of size (ie constant factor) is equal perceptual steps - Equal numbers of letters on each line with equal spacing between letters and rows proportional to letter size -Equal average legibility at each acuity line What are other ways of measuring acuity - Patients who are unable to communicate (children etc) - Hundreds and Thousands - hold outside child’s reach, the child grasping at the hundreds and thousands implies an acuity of - 6/12 - 6/24 - Optokinetic Nystagmus - Eyes will naturally follow a moving stimulus and move back - Eg watching road from a car or train - Use rotating drum with different width stripes - OKN only evoked if stripes are more than the minimum angle of resolution - Preferential Looking - 2 AFC (grating either on left or right) - Child will look at interesting/novel stimulus (grating) for longer - Change the width of the stripes (spatial frequency) until they no longer look at the striped region - Average electrical activity in primary visual cortex measured by an EEG or Visual Evoked Potential (VEP) - Present grating to patient, then reduced size until no electrical activity change in response to stimulus What limits Visual Acuity: - OPTICAL LIMITS - Blur (dioptric blur, aberrations, accommodation state) - reduces the contrast of the key features needed to recognise the letter, and the impact of the blur is much greater for small letters than larger ones - Pupil Size (diffraction) - the pupil is adjusted to give the most focussed image, when the light level reduces significantly, there may not be enough light reaching the retina - so the pupil opens wider, sacrificing image quality for increased light hitting the retina - small pupils increase VA, which improved discrimination of fine stimuli - and large pupils increase light influx which improved detection of faint stimuli - NEURAL LIMITS - Sampling density (cone mosaic) - Luminance level - Eccentricity - Adaptation state - EXTERNAL LIMITS (stimulus characteristics or other extrinsic factors) - Luminance level - Constrast - Orientation - Age - Amblyopia (lazy eye) - Binocularity - focusing on one object with both eyes Chromatic Aberration - Different wavelengths are refracted differently - Longitudinal chromatic aberration means that if the eye is focused on one wavelength, it is blurred for others - Chromatic Aberration is significant in human optics but we do not notice it - Longitudinal Chromatic Aberration is used in the DUOCHROME TEST - used to finesse optical correction - Green and red lights should be equally blurred or clear so that prescription is best for light at the peak of wavelength (YELLOW) The circle of least confusion - the point at which the image is in the most focus as the rays do not fall perfectly to a point, rather than a small area Bichromatic tests (duochrome) assume that the visual system focuses yellow light (570nm) This is for an emmetropic eye - perfect 20/20 vision Hyperopia - LONG SIGHTEDNESS Myopia - SHORT SIGHTEDNESS Stiles-Crawford Effect - The phenomenon in which light entering the eye near the centre of the pupil appears brighter than light entering near the edge Optical Limits to Acuity - Diffraction and Aberration - Both cause blur - The extent of this is primarily dependant on pupil size - The relationship between blur and pupil size is opposite for these two caused of optical blur - Diffraction is a result of the wave nature of light - it is unavoidable in any optical system that has an aperture - When light passes through a small aperture (eg pupil) it spreads out behind it - The amount of spread depends on the size of the aperture - Using the pinhole aperture to cut out refractive error is sometimes clinically useful - When this is used, aberrations and refractive errors will be minimised allowing the clinician to tell if a reduction in visual acuity is due to neural factors rather than optical factors Neural Limits to acuity: Resolution of 2 points - To distinguish or resolve the two points, the retina must be able to measure the light level at 3 points: both peaks and the trough - There has to be one cone in between, which is not stimulated to resolve 2 spots MAR: cone spacing versus optical quality - Sampling of the image by the photoreceptors, and by subsequent retinal structures (bipolar cells and retinal ganglion cells) must be fine enough to capture the information provided by the image - Three receptors are in priniciple required to distinguish gap in eg Snellen Letter Eccentricity - Acuity will increase as eccentricity gets closer to 0 degrees (minus figures in the nasal field and positive figures in the temporal field) Differences in foveal and peripheral vision - MAR increases with retinal eccentricity - In the periphery: - Cone spacing increases - Spatial pooling of cone responses increases - Ganglion cell receptive field size increases - Foveal and peripheral vision are similar, the main difference is a change in the scale at which the visual system operates - At the fovea, the receptors sampling density is just fine enough to capture the fine detail that the eye’s optics can deliver - Any finer would be wasteful; any coarser would be ‘undersample’ the image and lead to loss of image information Luminance - Scotopic and photopic parts of the curve - Convergence onto retinal ganglion cells decreases with increasing luminance - Above -85cm/m2 OK Orientation - Acuity is better for vertical and horizontal than for oblique stimuli - More V1 neurones tuned to vertical and horizontal than to oblique angles - The so-called oblique effect Age - Very young and old have poorer acuity - 6/120 (MAR = 20’) at birth to adult levels at age 2 - Due to increased absorption of light by ocular media and decrease in photoreceptor function Amblyopia (lazy eye) - A reduced VA in one eye not correctable with prescription and no disease - Neural problem: disruption of visual information transfer to one eye (the amblyopic eye) to the brain - Usually caused by lack of appropriate visual input during critical period - Crowding effect is worse in amblyopia - Do not use single letters in children as you might miss an amblyopic eye - Preferably use logMAR chart or the crowding bars or box around the letters when using single letters Visual Deprivation - Abnormal early visual experience resulting in possible permanent consequences - Monocular vision from deprivation can cause massive changes in cortical physiology, resulting in devastating and permanent loss of spatial vision - Cataracts and strabismus (crossed eyes) can lead to serious problems but early detection and care can prevent such problems Binocularity - Binocular acuity usually better than monocular acuity - Probability Summation: Two eyes better than one, but NOT twice as good - Better by a factor of 1.4 (Square root of 2 - binocular summation) - If binocular VA is worse than monocular VA, suspect an uncompensated heterophoria or an unbalanced refractive error Minimum discriminable angle - Hyperacuity - localisation acuity - Easy to support we can’t distinguish 2 points because we can’t tell their locations apart - This is not true - we are much better at locating a spot than resolving 2 spots - 2 point resoltion is about 30” of arc - Positional accuracy is about 2” of arc, under ideal conditions - Positional accuracy can be measured using Verniers acuity experiments - Robust against optical degradation (blur from defocus or cataract) so good to assess the neural function - Below see Verniers acuity stimulus examples Is visual Acuity the only important measure of ability to see things? - NO - Acuity measures the ability to see small, high contrast features - It is measured because it is the main thing you can help with, but loss of acuity is usually caused by the optics of the eye - But vision rarely operates at high contrast levels - For instance, fog, rain, light levels affect contrast UNKNOWN WORDS - see underline ARC SEC - A circle is 360 degrees, each degree is split into 60 minutes and each minute is split into 60 seconds - an arc is a portion of the circumference of a circle So an arc sec is 1/3600 of a degree, in reference to ’20/20’ vision or 6/6 vision - means that visual acuity can resolve down to one arc minute - 1/60 of a degree Used to measure very small angles LETTER OPTOTYPES - Think Snellen chart etc, letters or symbols at a controlled distance and size to measure visual acuity CONE SPACING - The distance between cones in the retina