EFFECT OF RETINAL IMAGE MOTION ON VISUAL-ACUITY AND CONTOUR INTERACTION IN CONGENITAL NYSTAGMUS

This study determined how contour interaction (the degradation of visu al acuity by the presence of nearby contours) is affected by the inces sant retinal image motion that occurs in observers with congenital nys tagmus (CN). Visual acuity was measured for single, high-contrast, bla ck Landolt Cs, presented without and with flanking bars (contour-to-C separation = 1, 2, 5 or 10 multiples of the gap width of the C). Stimu li were presented against either a white or a black surround. For comp arison, acuity was also determined in normal observers, with and witho ut motion of the stimulus to simulate the retinal image motion in jerk CN. The results show that the peak magnitude of contour interaction ( the maximal degradation in acuity attributable to contour interaction) is significantly larger in the observers with CN than in normals. Whe n acuity targets are presented against a black surround, contour inter action also occurs over a wider spatial extent in the observers with C N. Imposed image motion increases the extent of contour interaction in normal observers, but not sufficiently to account fully for the resul ts of the observers with CN. We suggest that the additional contour in teraction found in observers with CN may be attributable to the presen ce of amblyopia. For a small contour-to-C separation, contour interact ion is significantly greater when stimuli are presented against a blac k rather than a white surround. Consequently, single-letter acuity may be appreciably underestimated clinically when an adjustable window is used to isolate letters on a projected acuity chart.