Comparisons among Snellen, psychophysical, and evoked potential visual acuity determinations.

Acuity limits were determined for gratings of progressively finer spatial frequency directly from the visually evoked potential. The evoked potential was retrieved in real time, while spatial frequency was electronically changed or swept. The results of the swept evoked potential technique are compared to acuity thresholds determined psychophysically with similar gratings, and with Snellen acuity determinations over the range 6/60 (20/200) to 6/3.6 (20/12), obtained by defocusing with positive spherical lenses. One line of Snellen acuity is easily discriminated; the absolute Snellen level can often be identified to within two lines with 95% certainty. Correlations between visually evoked response (VER) grating and Snellen optotype acuity levels are poorer than correlations between VER- and psychophysically-determined grating acuity limits, perhaps because of variability inherent in the Snellen task itself. The swept evoked potential affords a rapid, objective, and potentially useful measure of acuity in young or minimally cooperative subjects.