Orientation discrimination and variability of torsional eye position in congenital nystagmus

Eye movements elevate crowding in idiopathic infantile nystagmus syndrome

Idiopathic infantile nystagmus syndrome is a disorder characterised by involuntary eye movements, which leads to decreased acuity and visual function. One such function is visual crowding – a process whereby objects that are easily recognised in isolation become impaired by nearby flankers. Crowding typically occurs in the peripheral visual field, although elevations in foveal vision have been reported in congenital nystagmus, similar to those found with amblyopia. Here, we examine whether elevated foveal crowding with nystagmus is driven by similar mechanisms to those of amblyopia – long-term neural changes associated with a sensory deficit – or by the momentary displacement of the stimulus through nystagmus eye movements. A Landolt-C orientation identification task was used to measure threshold gap sizes with and without either horizontally or vertically placed Landolt-C flankers. We assume that a sensory deficit should give equivalent crowding in these two dimensions, whereas an origin in eye movements should give stronger crowding with horizontal flankers given the predominantly horizontal eye movements of nystagmus. We observe elevations in nystagmic crowding that are above crowding in typical vision but below that of amblyopia. Consistent with an origin in eye movements, elevations were stronger with horizontal than vertical flankers in nystagmus, but not in typical or amblyopic vision. We further demonstrate the same horizontal elongation in typical vision with stimulus movement that simulates nystagmus. Consequently, we propose that the origin of nystagmic crowding lies in the eye movements, either through image smear of the target and flanker elements or through relocation of the stimulus into the peripheral retina.

Spatial-bisection acuity in infantile nystagmus

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.

Direction-of-motion discrimination is facilitated by visible motion smear

Recent evidence indicates that motion smear can provide useful information for the detection and discrimination of motion. Further, it has been shown that the perception of motion smear depends critically on the density of dots in a random-dot (RD) stimulus. Therefore, in the present experiments, the contribution of perceived motion smear to direction-of-motion discrimination was evaluated using RD targets of different densities. Thresholds for direction-of-motion discrimination and the extent of perceived motion smear were determined for RD stimuli with densities of 1, 2, and 10 dots/deg(2), presented for 200 msec at a velocity of 4, 8, or 12 deg/sec. To evaluate the contribution of information about orientation from motion smear, thresholds for orientation discrimination were measured using parallel lines with the same length as the extent of perceived smear. Despite the opportunity for increased summation as RD density increases, our results indicate that direction-of-motion discrimination worsens. Because perception of motion smear is reduced with an increase in RD density, our results are consistent with a facilitation of direction-of-motion discrimination by visible motion smear.

Infantile-onset nystagmus

PURPOSE OF REVIEW

Recent studies on the various forms of infantile-onset nystagmus have advanced our understanding of these disorders. The previously described waveforms of infantile nystagmus syndrome (congenital nystagmus) may be identified in infants less than 7 months of age, including the more mature forms; the visual status of these patients may be directly correlated with their mean foveation times.

RECENT FINDINGS

Refractive errors in patients with infantile nystagmus syndrome do not follow the expected trend toward emmetropization during infancy and early childhood. A study on the torsional component of nystagmus present in most patients with infantile nystagmus syndrome found it to be generated centrally and not by peripheral or mechanical dynamics. Two psychophysical studies on patients with infantile nystagmus syndrome revealed significant differences compared with control subjects. Two different animal models imply that lack of normal visual motion during a critical period of development in infancy leads to infantile nystagmus syndrome. An electron microscopic study of the enthesis site-where the extraocular muscle tendon inserts onto the sclera-in normal individuals versus patients with infantile nystagmus syndrome revealed significant abnormalities in the latter. Significant demographic and socioeconomic differences were reported between patients with infantile nystagmus syndrome and those with spasmus nutans.

SUMMARY

New evidence supports a role for sympathomimetic modulation of infantile nystagmus syndrome. Novel surgical treatments for infantile nystagmus syndrome are also discussed.