Contrast thresholds vs border enhancement: sensitivity to retinal defocus

The two visual responses, contrast threshold and border enhancement, are both affected by retinal image quality. Thus, with increasing defocus of the retinal image contrast thresholds with sinusoidal line gratings increase, and the enhanced region near a border becomes wider. These two visual responses were compared to determine their suitability as gauges of retinal image quality. Border enhancement was found to be more sensitive to deterioration of the retinal image in the form of defocus than contrast thresholds throughout the greater portion of the grating frequency spectrum. However, the contrast threshold increases in sensitivity toward cutoff frequency, and beyond a certain high frequency it surpasses border enhancement in that respect. At the same time, for these optimum frequencies, the contrast range is exhausted within a relatively small defocus range. Therefore, a limitation is imposed on the usefulness of the contrast threshold as an index of retinal image quality.