Variations in human cortical response to patterns and image quality.

Selective excitation of the human visual cortex by patterned stimuli was examined in a randomly selected sample of normal adtdts and adults with severe monocular visual loss resulting from childhood strabismic amblyopia. Six of the eight normal eyes showed selective but variable configuration cortical evoked responses (VER, passband 0.2 to 50 Hz.) to patterned stimulation when lenses were used to vary the retinal focus. The remainder of the normal sample also showed response peaks where resolution was maximum, but these were in the signal passband 50 to 500 Hz., previously called the fast occipital potential (FOP). We applied normalizing signals to correct for individual response variations. Statistical proofs indicated that this procedure reduced variability about mean results. Two of the three amblyopic eyes produced no response peak, VER or FOP, at the refractive value for best resolution. A small VER peak was found in one amblyopic subject who had eccentric fixation. Subtraction of mean nonpattern from pattern-evoked signals at optimal refraction disclosed a cortical signal which may be a relatively "pure pattern" response. As a clinical aid, the VER and its high frequency correlate, the FOP, appear to provide an objective test of the resolving power of the visual system with accuracy equal to or exceeding 0.5 D. sphere.