Human visual acuity assessment through linear extrapolation to threshold of bar grating VERs.

Visual evoked responses (VERs) were obtained from seven normal eyes, using high contrast square-wave gratings of varying bar size on an 8 degrees or 24 degrees stimulus field. VER amplitude was plotted as a function of bar size. These plots, called square wave transfer functions (SWTFs), have one or more peaks when generated by a 24 degree field. In some individuals, the small bar size extreme of the function can be fitted well with a straight line for purposes of extrapolation to the visual acuity bar size. However, interindividual differences in the shape of the 24 degree field SWTF invalidates any a priori assumption of what bar sizes may be included in the extrapolation. Results from three subjects, using an 8 degree field, suggest that a more uniform single-peaked triangular SWTF with a peak near 5 min arc may be assumed for extrapolation purposes. The interindividual SWTF differences obtained with the 24 degree field were not random, the subjects falling into one of two groups on the basis of SWTF shape. This SWTF shape dichotomy may be correlated with the presence or absence of strabismus.