Psychophysical Aspects of Contrast Sensitivity*

S Afr Optom 2013 72(2) 76-85 Psychophysical aspects of contrast sensitivity* Anusha Y Sukhaa and Alan Rubinb a, b Department of Optometry, University of Johannesburg, PO Box 524, Auckland Park, 2006 South Africa a<[email protected]> b<[email protected]> Received 23 November 2012; revised version accepted 9 June 2013 Abstract demonstrate stereo-pair representation of contrast visual acuities in the context of diabetic eyes. This paper reviews the psychophysical aspects The doctoral research of the first author (AYS) of contrast sensitivity which concerns components that applies similar idea to understanding both of visual stimuli and the behavioural responses inter- and intra-ocular variation of contrast visual and methods used in contrast sensitivity testing. acuities. Some discussion is included of the different types of contrast sensitivity charts available as well Key words: contrast, contrast sensitivity, as a brief background on the different types of contrast visual acuity, vision science, vision graphical representations of contrast sensitivity psychophysics, stereo-pair scatter plots, and contrast visual acuities. Two illustrations also multivariate statistics Introduction (namely spatial frequency, contrast, spatial phase, and orientation) by means of Fourier analysis. Fourier Psychophysics is a scientific discipline designed analysis is an analytical method that calculates to measure internal sensory and perceptual responses simple sine-wave components whose linear sum to external stimuli1. Sensory stimuli and behavioural forms a given complex image2, 3. The visual stimuli responses are the defining or crucial concepts in typically used in contrast sensitivity testing consist of determining contrast thresholds (for example, visual sine-wave or square-wave gratings whose luminance stimuli are used in chart-based contrast sensitivity perpendicular to the bars is modulated in sinusoidal measurements). These two concepts (visual stimuli or square-wave form about a fixed mean level. and behavioural responses) together with different Sometimes other stimuli such as letters are used and types of contrast sensitivity tests and their graphical the limb or element width is used in converting to representations will be discussed here. spatial frequency (see Figure 1 below). Perception of these bars as a function of spatial frequency is used to 1. Visual stimuli determine contrast threshold2, 4. The four components Theoretically, any visual stimulus, pattern, or defining visual stimuli are as follows: image can be decomposed into sinusoidal components a MPhil b DPhil *This paper is from research towards a DPhil degree in Optometry at the University of Johannesburg with the supervision of Professor Alan Rubin. 76 The South African Optometrist ISSN 0378-9411 S Afr Optom 2013 72(2) 76-85 AY Sukha and A Rubin- Psychophysical aspects of contrast sensitivity (1) a. Spatial frequency Spatial frequency is defined as the number of To calculate contrast with non-periodic or letter image cycles that fall within a given spatial distance, charts the Weber formula applies: typically one degree of visual angle and measures L max − L min the fineness or coarseness of the grating3. One cycle Contrast = (2) consists of one light region (bar or line) plus one L max dark region or bar, for example, within a sine-wave where L min is the luminance of the lighter part, grating3. A grating of high spatial frequency contains and L min is the luminance of the darker part. By many narrow bars, that is, many cycles within each multiplying these ratios by 100, the percentage contrast degree (cpd) of visual angle. A grating of low spatial of a particular stimulus is represented. Although the frequency contains wide bars or few cycles per degree luminance has units such as candelas per square meter, of visual angle. Spatial frequency can also be defined contrast is a dimensionless number1, 6. in terms of viewing distance; therefore as the viewing distance decreases each bar casts a larger image, c. Spatial phase hence spatial frequency decreases1. Spatial phase refers to a grating’s position relative Snellen letter acuity can be expressed or converted to some landmark or reference. Edge-like and line-like into spatial frequency as illustrated in the figure below features result from maximal local phase coherence4. (Figure 1) 5. At a six (6) metre (or 20 ft) distance, the Snellen denominator is divided into 180 (or 600 d. Orientation ft). Thus 6/60 = 180/60 = 3 cpd and 6/6 (or 20/20) Orientation refers to the relative physical position converts to 30 cpd. of the gratings, specifically the tilt which can be horizontal, vertical or oblique4. 2. Behavioural responses and methods Behavioural responses and methods are used to determine contrast thresholds. Contrast-detection threshold is an important psychophysical method used to measure the sensitivity of the visual system1. Contrast threshold is defined as the statistical contrast boundary below which contrast is too low for an image to be reliably detected and above which contrast is high enough for frequent image detection3. Four Figure 1. Conversion of Snellen notation to spatial frequency types of variability of threshold measurements have (Figure reproduced with permission of Webvision) 5. been identified. They include random fluctuations in the visual stimulus, continual random fluctuations in b. Contrast neural activity when visual signals are carried from Contrast is a measure of how different a luminance the retina to the visual cortex, the subject’s level of level at some point in space or time is, compared to alertness or attention and the subject’s psychological 3, 5 some luminance reference. Spatial contrast compares bias . Behavioural methods and responses are some portion of the visual field with another portion, designed to minimize variability of threshold 3, 5 for example, the intensity difference between the light measurements . and dark bars6. Mathematically, contrast for sine-wave gratings is calculated using two parameters, maximum 1.1. Behavioural responses ( L max ) and minimum ( L min ) luminance, both of Behavioural responses are classified into criterion which can be measured physically. Sine-wave grating dependent and criterion-free methods. or periodic charts use the Michelson formula: a. Criterion dependent methods Awareness of the presence or absence of a signal L max − L min or stimulus, which engages the subject to respond Contrast = . L max 77 The South African Optometrist ISSN 0378-9411 S Afr Optom 2013 72(2) 76-85 AY Sukha and A Rubin- Psychophysical aspects of contrast sensitivity S Afr Optom 2013 72(2) 76-85 AY Sukha and A Rubin- Psychophysical aspects of contrast sensitivityz either affirmatively (that is, yes) or non-affirmatively chance of guessing a letter correctly). Other clinical (no) is classified as a criterion dependent method. charts may offer fewer alternatives and some charts Correct responses can range from 0% to 100%. may, for example, use 10 Sloan or British standard Results can be plotted as a percentage seen versus, letters which have similar legibility1, 5, 7. (for example), stimulus intensity resulting in an S-shape (ogive) psychometric function, which is also known as a frequency-of-seeing curve (see Figure 2). In criterion dependent methods a psychometric function can estimate the threshold (for stimulus intensity, for example, in candelas) as the 50% point on the curve1, 5, 7. Figure 3. A 2AFC psychometric function with the threshold at 75% is shown (Reproduced with permission from Webvision) 5. 1.2. Behavioural methods These behavioural procedures are classified into three types or methods: Figure 2. An ogive curve or psychometric function where a a. The method of adjustment criterion dependent method was applied is illustrated here - According to Norton, Corliss and Bailey1, this only yes or no responses to the presence of stimuli of variable method is the simplest and most direct method for intensity were required. The threshold value is defined where estimating thresholds. The method of adjustment is 50% of the stimuli are detected, and in this figure the stimulus criterion dependent (yes/no responses). The subject intensity threshold is 23.5 (Reproduced with permission from adjusts the contrast level themselves until he or she Webvision) 5. can just see the stimulus (or is just unable to see the stimulus). The examiner begins each measurement b. Criterion-free methods by setting the initial value of the stimulus well above In forced choice paradigms subjects are forced to or below the anticipated threshold. The measurement select between two or more intervals/choices, one of procedure is repeated several times, and the mean is which contains the stimulus or target. A two-alternative calculated. Elliot in Benjamin7 reports that contrast forced choice (2AFC) paradigm forces the subject to threshold measurements from unseen to seen are choose between two alternatives, a four-alternative higher when compared to those from seen to unseen. forced choice (4AFC), between four alternatives, et This is due to the effects of retinal ganglion cell cetera. The percentage correct for the various stimuli adaptation, where the cells that respond to the grating intensities is used to construct a psychometric graph. or stimulus fatigue and reduce their response. Thus, In a 2AFC method there is already a 50% chance this method is prone to two errors; one of habitation of a correct response and therefore the threshold is (when subjects develop a habit of responding

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