Contrast Sensitivity as an Independent Measure of Foveal Function

During recent years there has been a burgeoning interest in contrast sensitivity measurement as a parameter of foveal function. Typically sine wave gratings are presented on a TV monitor or oscilloscope and the just visible contrast between dark and light bars is established as a threshold value. The threshold varies with fineness of the grating so it is useful to measure a series of thresholds with gratings of different 'spatial frequency'. Fine gratings have many bars per unit visual angle and are described as having high spatial frequency while coarse gratings of low spatial frequency have few bars per unit visual angle. The human visual system is most sensitive for gratings around 4 cycles/degree and is less sensitive (requires higher contrast to achieve threshold) at lower and higher spatial frequencies. Snellen visual acuity is tested with maximum contrast black on white figures having sharp edges (square wave function) and the stimulus is made progressively finer (analogous to higher spatial frequencies for a repetitive stimulus like a grating) until the just visible fineness threshold is reached. Thus Snellen acuity constitutes but a single point on the curve relating contrast sensitivity to spatial frequency and in this sense the contrast sensitivity curve is an expanded measure of foveal function as compared with the Snellen threshold.