Abstract: A diagnostic test for glaucoma, which can detect the early stages of this disease, uses a pattern electroretinogram of a subject to assess any reduction of the density of M.sub.y ganglion cells in a part of the subject's retina. The visual field of the eye of the subject is divided into a number of zones, and a respective visual stimulus signal is applied to each zone. The visual stimulus signals are generated in accordance with data obtained from determinations of the critical spatial frequency (Nyquist frequency) of persons with normal vision, obtained from observations of the "aliasing" phenomenon. Typical visual stimulus signals are grating patterns, with the contrast of the signal applied to each zone modulated with a different temporal frequency in the range of from 10 Hz to 45 Hz. Any differences between the electroretinogram response component for an individual zone and the expected response component for that zone indicates a change in the M.sub.