Thus, how widely spaced the digits are is a factor (you can more easily handle seven or so digits if they’re close together than you could four or five if they were widely spaced), and this can be affected by such things not only as the randomness of the software itself, but also the resolution of your monitor (higher resolutions make the whole image smaller and thus more likely to fit within your macula if you focus on the center), distance from head to screen (further distance means smaller image in the retina), etc.

Obviously, people with macular degeneration (partial or complete blindness in just the central, sharper portion of vision) would be handicapped in this test for reasons that have little or nothing to do with brain acuity. Macular degeneration often progresses with age which can of course appear to correspond with the test yet wouldn’t truly indicate age of the BRAIN — but it can also happen as a result of glaucoma, diabetic retinopathy, etc.

On the other end of the scale, people with tunnel vision (the reverse of macular degeneration), if using a low resolution monitor setting or are close to the monitor, might not even see digits close to the horizontal edges, let alone corners.