August Colenbrander, MD:
Early studies of human contrast sensitivity were built on studies for the design of lens systems.
For such studies, including studies of the optics of the eye and the correction of refractive error, sine-waves are often preferred because they have advantages for calculations.
However, when attention is broadened to the entire visual system and eventually to visually-guided behavior, we must consider not only the optics of the eye, but also the characteristics of the photoreceptors and the effects of neural processing, which starts in the inner retina, and continues in the visual cortex and in higher visual centers.
Repetitive patterns, such as gratings, are common in man-made objects, but not in natural scenes. Fuzzy edges, as in sine-wave gratings, are not common in natural scenes either. It is highly unlikely, therefore, that the neural system would have evolved to detect sine-waves.
An important function of the retina is edge detection and edge enhancement; thus the detection of sharp edges, as in letters on Pelli-Robson and Mars cards, appears to be closer to the detection and interpretation of natural forms than is the detection of sine-wave gratings. It thus may be a better predictor of the ability to perform Activities of Daily Living (ADL).
John Robson, MA, PhD, ScD:
Measurement of the contrast threshold for sine wave gratings of many different spatial frequencies allows a subject’s complete “contrast sensitivity function” to be explored directly. However, there is little evidence that this time consuming measurement provides a clinically more useful assessment of the subject’s visual capabilities than can be obtained more expeditiously and precisely by conventional measurement of acuity together with measurement of contrast sensitivity for a test pattern whose visibility is dependent upon the peak value of the subject’s contrast sensitivity function.
It would in principle be possible to measure a subject’s peak contrast sensitivity using a sine-wave grating of appropriate spatial frequency, but grating test patterns do not lend themselves to incorporation into the robust and rapid psycho-physical procedures that are important for practical testing in a clinical setting. Speed and robustness can most easily be obtained by using a “many-alternative forced choice” psycho-physical method where the subject is required to identify which of many alternative test targets is being presented.
Such a procedure cannot in practice be satisfactorily implemented using grating patterns. However, a procedure with 10 or more alternatives is conveniently implemented using letters (of appropriate angular size) with which most subjects are extremely familiar and can easily name. The importance of the many-alternative identification procedure was recognized by Hermann Snellen when he designed the first acuity charts and it is the adoption of this procedure with 10 or more different letters in contrast sensitivity charts such as the Pelli-Robson and Mars cards which primarily accounts for their reliability.
Ian Bailey, OD, MS, FBCO, FAAO
There is a special sweetness about the mathematical approach of analyzing the light distribution within a visual scene in terms of “Fourier” sinusoidal components, and then using the observer’s contrast sensitivity function (CSF) to predict the visual percept- that is, how the scene will be seen. However, this theory often fails to predict the visual capabilities or difficulties of observers with damage to the neural components of the visual system. Macular degeneration is an obvious example.
The major practical problems that patients have as a result of impaired contrast sensitivity are related to mobility and orientation. Being able to detect objects and navigate efficiently largely depends of the visual detection of borders or edges. Contrast sensitivity tests should determine thresholds for edge detection and this means that test targets for detection or recognition should have sharp visual edges. The widely used Pelli-Robson test and the Mars test do this by having their test targets as letters or numbers of relatively large angular size.
With such targets, the contrast threshold for detection is similar to that for recognition. If you can see that it is there, it might take time but you probably recognize the shape.
Testing contrast sensitivity with sinusoidal targets of low spatial frequencies may have some value in predicting how well the observer might be able to see some things like undulations in the surface of a walk-path, using CS tests that have sharp edges has more broad relevance to understanding and predicting functional abilities at real world tasks, especially orientation and mobility.