Spatial-bisection acuity in infantile nystagmus

► Bisection acuity was compared in infantile nystagmus (IN) and normal observers. ► Poorer acuity occurs in IN for targets parallel and orthogonal to the nystagmus. ► A sensory deficit contributes to the reduced bisection acuity in idiopathic IN. This study measured spatial bisection acuity for horizontally and vertically separated line targets in five observers with infantile nystagmus syndrome (INS) and no obvious associated sensory abnormalities, and in two normal observers during comparable horizontal retinal image motion. For small spatial separations between the line targets, bisection acuity for both horizontally and vertically separated lines is worse in the observers with IN than normal observers. In four of the five observers with IN, bisection acuity for small target separations is poorer for horizontally compared to vertically separated lines. Because the motion smear generated by the retinal image motion during IN would be expected to influence horizontally separated targets, the degradation of bisection acuity for both vertical and horizontally separated lines indicates that a sensory neural deficit contributes to impaired visual functioning in observers with idiopathic IN.