Grating Acuity Test at Low Contrast Levels
Grating tests have been used to measure contrast sensitivity since the 60's. These computer controlled grating tests have not become widely used in clinical medicine because they are expensive and require an experienced technician to use them. However, studies with them have taught us some important principles in measurement of contrast sensitivity in cases of low vision. The most important finding is that contrast sensitivity values in nearly all cases of low vision are different when measured with gratings of different sizes: the larger the grating, the higher the contrast sensitivity value. This is particularly common in cases of central scotoma that 'eats up' some of the stimulus and thus the effective stimulus is smaller than the physical stimulus.
A. Contrast sensitivity as a function of stimulus size in normally sighted subjects: the larger the grating, the higher the contrast sensitivity values at low spatial frequencies. B. Contrast sensitivity curves in case of macular degeneration, L = the normal left eye, R = the right eye with dry macular degeneration. Contrast sensitivity measured with 10 degree stimulus is nearly as good as in the normal left eye, whereas when measured with 5 degree stimulus it is one fifth of the maximum value of the 10 degree curve and when measured with 2.5 degree stimulus the maximum value is only one twentieth of the 10 degree maximum. C. Contrast sensitivity curves of a person with optic atrophy in both eyes. With the 2.5 degree stimulus contrast sensitivity is barely measurable, yet with the 20 degree stimulus the values at low spatial frequencies are normal.
Small grating stimuli would often give a misleading picture of visual function at low contrast. Therefore it is wise to make one measurement with a large grating stimulus to learn about the subject's ability to see low contrast information. On the other hand, it is interesting to evaluate the function of the fixation area by using a smaller stimulus. This is possible by covering the grating stimulus with a grey folder that leaves either one fourth or one tenth of the stimulus visible.
Low Contrast Gratings measure contrast sensitivity at 3 contrast levels, 100%, 10% and 2.5% using 3 grating frequencies: 0.5cpcm, 2cpcm and 8cpcm. The grey folder covers part of the grating and leaves visible either one forth or one tenth of the stimulus. With this simple technique it is possible to find out the effect of stimulus size on the threshold values. When the grating is turned into a new orientation it is covered with a grey surface.