5. The Form Sense I 2223.pptx

The Form Sense I Introduction. Objectives and Readings. Spatial resolution and visual acuity. Criteria used to specify visual acuity. Measuring visual acuity thresholds. Notations and chart construction. SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Define th...

The Form Sense I Introduction. Objectives and Readings. Spatial resolution and visual acuity. Criteria used to specify visual acuity. Measuring visual acuity thresholds. Notations and chart construction. SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Define the term visual acuity (VA). • Describe the different criteria that can be used to measure VA. • Know how to specify VA e.g. Optometry: Science, Techniques and clinical management. Rosenfield and Logan. • Chapter 12 – most of this chapter is relevant. Snellen, MAR, decimal, logMAR. • Know the advantages of the logMAR (ETDRS) format over the Snellen chart. • Understand the concept of threshold in terms of visual acuity measurements. SFO1004 • Schwartz: Chapter 7 (Spatial Vision) https://webvision.med.utah.ed u/ • Part VIII: Visual acuity Dr Sarah J Waugh© Spatial resolution and visual acuity (VA) • Spatial resolution is the ability of the visual system to resolve detail. • Visual acuity (VA) can then be defined as the smallest visible detail that can be seen. • Usually specified in angular terms (e.g. minutes of arc). • VA is probably the single most important measurement obtained by optometrists (or if not, it is SFO1004 certainly the most Dr Sarah J Waugh© Visual acuity or vision? • Vision – the smallest size letter that a patient reads off a letter chart without any corrective lenses (glasses or contact lenses). • Visual acuity – the smallest size letter that a patient reads off a letter chart using their best corrective lenses (glasses or contact SFO1004 lenses). • All other terms are related. • Aided vision – usually indicates the patient is wearing glasses or contact lenses. • Unaided vision or unaided visual acuity – same as vision. • BCVA – sometimes used to describe the ‘best corrected’ visual acuity. But visual acuity always assumes the best correction is used. • Sometimes visual acuity is abbreviated just as ‘acuity’. Dr Sarah J Waugh© Measuring visual acuity • Different ways to specify visual acuity but the principle for measuring is the similar. • Observer (patient) views a target (lets say a letter chart) of a certain size at a fixed distance (lets say 6m). • The size of the target is varied. In clinics, this is usually done by varying the size of the letters on the chart – as in the figure. • An alternative would be to change the viewing distance. • SFO1004 Remember it is all about Dr Sarah J Waugh© Specification of visual acuity • Visual acuity is often thought of as something measured using a letter chart (which is true for much of clinical practice). • However, the capacity of the visual system to resolve detail is dependent on the conditions and methods used in measurement. • Four methods or criteria for assessing visual acuity can SFO1004 be described – all can be • Detection – Ability to detect an object. • Relative localisation – Ability to localise features of an object. • Resolution – Resolve the details of an object. • Recognition – Recognise an object. Dr Sarah J Waugh© Visual acuity – detection • The detection of a feature or object in the visual field. • This could be a small dot or thin line presented to a patient. • A typical target used in experimentation is the ability to detect a thin line against a white background (see figure). • An observer is asked to identify in which of the 2 SFO1004 cards the line is present. Detection of a line Dr Sarah J Waugh© Visual acuity – detection Detection of a line • With such a 2-alternative task, the observer has a 50:50 chance, by guessing, which of the 2 cards has the line. • By presenting the cards repeatedly, and in a random fashion, an estimate can be derived of whether the observer actually saw the line on each presentation. • The distance of the observer from the cards is varied and SFO1004 the presentations repeated. Vary test distance Dr Sarah J Waugh© Visual acuity – detection Detection of a line • At close distances, the observer would correctly identify the line on 100% of the presentations. • As the distance from the observer increases, the percentage of correct responses start to decrease as it becomes harder for the observer to see the line. • At some distance, the line becomes so hard to see that the observer’s responses fall to a chance level. • In this case it would be 50% SFO1004 correct. Why? Vary test distance Dr Sarah J Waugh© Visual acuity as a threshold measure • If the percentage of correct responses (where the observer correctly identified the line) was plotted (Y – axis) against the test distance (X- axis), the results could be something like shown. • At 100% performance the observer saw the line correctly on each presentation. At 50% performance, they were guessing. • We can connect the data by a line called a ‘psychometric function’. The psychometric function describes the human behaviour in this task. • Psychometric functions can be SFO1004 determined for all visual Dr Sarah J Waugh© Visual acuity – detection • Detection acuity is not something measured very often. • However, clinical tests using the criterion of minimum detectable have been developed as shown. • In this test, the patient has to detect the small black dot visible in the aperture. • The dot size is varied until the observer cannot SFO1004 detect the dot. Dot visual acuity test Dr Sarah J Waugh© Visual acuity – localisation • Visual acuity can also be specified with respect to the localisation of a feature in the visual field, relative to another. • An example would be a task where the observer identifies which one of 2 cards (shown) has the line with a small displacement or offset. • Under optimal conditions, thresholds of a few seconds of arc of visual angle have been measured. • We could use the method previously described to determine SFO1004 a threshold visual acuity for this Localisation of a line Dr Sarah J Waugh© Visual acuity – localisation • Localisation acuity has practical applications. • An example is the use in the calipers as shown. • Very fine measurements are possible due to the ability of the visual system localise to such fine levels. • This type of acuity is sometimes referred to as ‘Vernier’ acuity after the French mathematician that invented a type of scale. Vernier scale SFO1004 Dr Sarah J Waugh© Visual acuity – localisation • Clinical tests using localisation ability include tests of binocular vision and eye alignment. • One example as shown in the image, is a test of fixation disparity. • The patient has to say whether the line above and below the ‘X’ appear to be aligned. • For this test, the patient would wear special glasses so each eye would only see one of the lines (either the top or bottom) • Can you think of another test that uses localisation ability (you may have one during the induction SFO1004 session in your 1005 course)? Fixation disparity testing Dr Sarah J Waugh© Visual acuity – resolution • A resolution task requires the ability to discern the minimum separation visible between features of the target object. • The discrimination of two objects or that there is a gap (e.g. lines, spots, checkerboard patterns. • The threshold would be the smallest gap or SFO1004 separation that could be https://webvision.med.utah. edu/book/part-viii-psychoph ysics-of-vision/visual-acuity/ Dr Sarah J Waugh© Visual acuity – resolution • Resolution acuity measurements can be made using the Landolt C or ‘Tumbling’ E. The optotypes are presented at different orientations (so some consider Es and Cs as recognition targets rather than pure resolution targets). The patient does not have to recognise the letter. – The task is to identify the direction of the ‘gap’ in the C or the orientation of the ‘bars’ in the E (Up, down, right or left). • The minimum angle of resolution (or MAR) is specified as the smallest gap or SFO1004 bar thickness that can be resolved (min Dr Sarah J Waugh© Visual acuity – recognition • Most clinical tests are based on recognition acuity. • They require the correct recognition of letters, symbols, or numbers (known as optotypes). • Similar to resolution acuity, the stroke width (or thickness of the line) of the smallest optotype that can be seen is used to specify the minimum SFO1004 angle of resolution (MAR) * Dr Sarah J Waugh© Visual acuity – recognition • The creation of optotypes for letter charts is usually based on a grid pattern. • Non-serif (or sansserif) letters are preferred. • The minimum angle of resolution is the smallest gap or stroke thickness that can be resolved, specified in min of arc of visual SFO1004 angle. Dr Sarah J Waugh© Visual acuity – recognition • Visual acuity charts can use different letter sets. • British standard letters (5x4) – D, E, F, N, H, P, R, U, V, Z • Sloan letters (5x5) – C, D, H, K, N, O, R, S, V, Z • These letter groups were selected to have similar legibility (equally hard). SFO1004 Dr Sarah J Waugh© Visual acuity – Snellen fraction • Specifies the test distance and distance at which the letter would subtend 5’ (5 min arc). • e.g. a letter that is designated as 6/6 means that the letter, at 6m subtends an angle of 5’. • A 6/12 letter means that the letter would subtend 5’ at 12m distance and so forth. • Measurements in feet are still sometimes used (particularly in the US)’ – 6/6 is the same as 20/20, 6/12 = 20/40 6/18 = 20/60 etc. SFO1004 6/6 or 20/20 6m (20 feet) test A just seen, 6/6 letter subtends an overall size of 5’. Dr Sarah J Waugh© Visual acuity – M.A.R. • We can also specify the minimum angle of resolution – MAR (usually in minutes of arc). • MAR is the size of the critical feature (stroke width) of the smallest letter that can be seen. • For letter optotypes the critical detail is the stroke width which is 1/5th the size of the letter height. • The MAR is the reciprocal of the Snellen fraction e.g. – 6/6 then MAR is 1’ – 6/12 then the MAR is 2’, – 6/3 the MAR is 0.5’ etc. SFO1004 6/6 6m test A ‘just seen’ 6/6 letter subtends an overall size of 5’ and has a MAR of 1’ What angle would a ‘just seen’ 6/18 letter make at 6m? Dr Sarah J Waugh© Visual acuity – logMAR • Measurements of visual acuity are better expressed in logarithmic units. • This is because of how the eye responds to changes in size. • Such measurements are especially useful when moving test distances and, • When using charts based on logarithmic progression. • Logarithm (to the base 10) of the minimum angle of resolution (logMAR). • For a Snellen fraction of 6/6, the MAR is 1.0 min arc so the log10MAR = 0.0 • For visual acuity of lower than 6/6 the SFO1004 logMAR becomes negative. 6/6 6m test A just seen 6/6 letter subtends an overall size of 5’ and has a MAR of 1’ This would be logMAR = 0 Dr Sarah J Waugh© Visual acuity – Decimal notation • VA can also be specified in decimal notation. • Decimal notation is just the Snellen fraction as a decimal. • 6/6 = 1.0 • 6/12 = 0.5 • 6/24 = 0.25 • 6/60 = 0.1 etc. 6/6 or 20/20 6m (20 feet) test A just seen, 6/6 letter subtends an overall size of 5’. In decimal notation 6/6 = 1.0 SFO1004 Dr Sarah J Waugh© Snellen’s chart (1862) • Snellen proposed a chart to measure vision based on letters of a standard size. • Snellen specified the letter size based on the individual stroke width of the letters being equal to 1/5th the size of the letter. • Most modern charts still adhere to this convention. • Snellen also used ‘serifs’ on the letters (the small SFO1004 blocks) whereas most Early versions Final version Dr Sarah J Waugh© Green’s chart (1867/8) • American ophthalmologist worked with Donders and Snellen. • Proposed a new chart, first in 1867 (6 steps) then 1868 (10 steps). • Used a geometric (logarithmic) progression. • Each line increment represented the ratio of 0.1 logMAR. • Letter and line spacing was proportional to the letter size (equal to the letter width) (unlike Snellen’s chart). SFO1004 • Note non-serif letters Dr Sarah J Waugh© Bailey – Lovie logMAR (1976) • Proposed a new layout: • Logarithmic progression of letter size • Same number of letters per row • Letter spacing proportional between and within rows • Similar (average) letter legibility for letters in each row. • Introduced the term logMAR (logarithm of the minimum angle of resolution) to more easily describe the steps of this progression. • Also introduced new logMAR SFO1004 scoring method and credit Notice British Standard letters (5x4) and letter spacing of 1 optotype Dr Sarah J Waugh© ETDRS (1982) • National Eye Institute (NEI) in the USA combined the Sloan letter set with the BaileyLovie layout to create new charts for use in the Early Treatment of Diabetic Retinopathy Study (ETDRS). • These charts became known as ETDRS charts. • NEI required ETDRS charts for all later clinical studies which helped to popularize the new format and the logarithmic progression. • SFO1004 The ETDRS format is now the Notice Sloan letters (5x5) and letter spacing of 1 optotype Dr Sarah J Waugh© ETDRS and variations – there are many types! Sailoganathan A., Osuobeni E., & Siderov J. A new Hindi language logMAR visual acuity chart. Indian Journal of Ophthalmology, 2018; 66:634-640. SFO1004 Dr Sarah J Waugh© Conversion table Snellen fraction metres 6/3 6/3.8 6/4.8 6/6 6/7.5 6/9.5 6/12 6/15 6/19 6/24 6/30 6/38 6/48 6/60 6/75 6/95 6/120 SFO1004 logMAR MAR Decimal (rounded (rounded feet notation ) ) 20/10 -0.3 0.5 2.00 20/12.5 -0.2 0.6 1.60 20/16 -0.1 0.8 1.25 20/20 0.0 1.0 1.00 20/25 0.1 1.3 0.80 20/32 0.2 1.6 0.63 20/40 0.3 2.0 0.50 20/50 0.4 2.5 0.40 20/63 0.5 3.1 0.32 20/80 0.6 4.0 0.25 20/100 0.7 5.0 0.20 20/125 0.8 6.3 0.16 20/160 0.9 8.0 0.13 20/200 1.0 10.0 0.10 20/250 1.1 12.5 0.08 20/320 1.2 15.9 0.06 20/400 1.4 25.0 0.04 Dr Sarah J Waugh© Questions? • Remember you can ask questions via Brightspace discussion board or the chat function on Teams. SFO1004 Dr Sarah J Waugh©