Saccades under Mental Load in Infantile Nystagmus Syndrome and Controls

This study compares saccades and visual task performance in patients with infantile nystagmus syndrome (INS) with that in normally sighted individuals under mental load. The results highlighted that to more completely evaluate INS therapies recognition time should also be measured with mental load, resembling real-world conditions.

Patients with INS may complain of "being slow to see." Stress is reported to worsen nystagmus and to prolong visual recognition time. We hypothesized that the effects of mental load on timing indices of visual recognition, for example, saccade latency, target acquisition time, target viewing time, and subjects' reaction time, differ between the INS and control groups.

Eye movements were recorded when participants (INS group, n = 15; controls, n = 25) reported the direction of tumbling-E targets presented randomly across ±25°. The task was repeated with both mental arithmetic and time restriction to impose high mental load, confirmed through subjective ratings and heart rate measurement.

Mental load increased saccade latency (mean, 32.69 milliseconds; 95% confidence interval [CI], 21.17 to 44.20 milliseconds; P < .001) and target acquisition time (57.00 milliseconds; 95% CI, 34 to 81 milliseconds; P < .001). Patients with INS showed longer saccade latency (39.79 milliseconds; 95% CI, 23.98 to 55.62 milliseconds; P < .001) and target acquisition time (134.00 milliseconds; 95% CI, 96 to 172 milliseconds; P < .001) compared with controls. The interaction between task and group was significant for saccade gain (0.11; 95% CI, 0.02 to 0.19; P = .015), target acquisition time (37.93 milliseconds; 95% CI, 36.91 to 38.96 milliseconds; P = .011), and subjects' reaction time (95.37 milliseconds; 95% CI, 65.91 to 124.84 milliseconds; P = .043). There was an inverse correlation between the changes in subjects' response errors and target viewing time with mental load only for controls (r = -0.484, P = .014). Total foveation exposure time and target viewing time remained unchanged.

Mental load worsens "being slow to see" in INS because of delayed target acquisition and possibly because efficiency of visual processing decreases more in patients with INS compared with controls. To investigate outcomes of INS therapies, visual recognition time should be also measured with mental load.