The effects of stress on nystagmus (INS): acuity vs. time to see

Purpose People with congenital nystagmus (INS) report that stress has a deleterious impact their vision, yet recent research suggests that stress does not affect visual acuity (VA). This study investigated what underlies the perception that stress causes poorer vision in INS. Methods VA was measured using a two alternate forced choice (2AFC) staircase method. Subjects were asked to identify whether the gap in a Landolt C of decreasing size was either left or right. Eye movements and skin conductance (a measure of arousal level) were simultaneously recorded. Stress was induced with the threat of a brief shock from a Transcutaneous Electric Nerve Stimulation (TENS) machine at double the subject’s previously determined threshold, although this was never actually done. Subjects carried out the visual task four time: twice under relaxed conditions, and twice under stressed conditions (i.e. shock linked to performance and shock at random intervals). Results The intensity of eye movements was significantly increased during periods of stress (paired t test, p<0.05), as was skin conductance (p<0.05). Consistent with other studies, VA was not affected (p=0.73), but the time taken for subjects to respond to the stimulus was significantly longer during periods of stress (p<0.05). Conclusion The results suggest that aspects of “vision” other than VA can better reflect the problems experienced by people with INS when under stress. We confirm that VA is unaffected; subjects do take longer to identify the target. We conclude that clinical or experimental measures of VA alone are insufficient to determine the visual problems faced by people with nystagmus. Additional tests such as measurement of the time taken to identify a target are recommended.