Accuracy of evoked potential refractometry using bar gratings.

We recorded cortical potentials evoked by phase-alternating square-wave gratings. Significant effects of varying the refractive power of the eye are obtained only at 5 cycles/degree (cy/deg) and above. The extrapolation to zero amplitude of the evoked response as a function of spatial frequency predicts the psychophysical visual acuity even for the myopic eye. As the sensitivity of meridional refraction using sufficiently fine bar gratings is better than 0.5 D it provides a suitable means for the measurement of astigmatism.