The effects of task-induced stress and mental workload on visual performance and eye movement behaviour in nystagmus and controls

© Dr. Marzieh Salehi FadardiBackground and aims: During daily activities, individuals may experience stress when a visual task is combined with an irrelevant mental task (e.g. conversing). Clinical examinations of patients with infantile nystagmus syndrome (INS) may elicit subjective reports of worsened nystagmus under internal states such as stress. Although the negative effects of stress on visual function have been widely studied in healthy subjects, few studies are available for patients with INS. Previous findings have demonstrated that INS is influenced by task conditions (i.e. visual demand and internal states such as stress); however, these studies limited their methodologies to only the null or central gaze position. Gaze position, solely, can affect visual function in INS and can affect INS parameters such as foveation time. By acknowledging that gaze position can affect INS, one gaze position (e.g. central) may not be enough to measure the changes in INS arising from task variations. Here, we hypothesised that a task-induced change from baseline in INS parameters at the null position would differ from task-induced changes at the gaze position away from the null. Patients with INS may also complain of being slow to see. Target acquisition time and the following visual processing time can affect the total period taken to react to a visual object. Surgical improvement in foveation has resulted in a reduction in both target acquisition time and visual recognition time. A few studies have reported that stress exacerbates nystagmus by increasing its intensity and reducing foveation time. Since tenotomy has been reported to decrease target acquisition time in INS, there was a possibility that changes in INS parameters under internal states such as stress affected target acquisition time. Thus, our second hypothesis stated that the effects of stress on visual timing would differ between control and INS subjects. The objective was to measure the extent to which saccades, target acquisition and visual processing times are affected by cognitively induced stress in INS subjects in comparison to the control group. Methods: To test our first hypothesis, we conducted a visual acuity experiment. We varied task conditions by manipulating levels of mental load and visual demand, thus inducing varying stress levels. Participants with idiopathic INS were required to determine the direction of Tumbling-E targets, which varied in size and contrast, using a staircase procedure across ±25° gaze positions, with 5° steps from the centre. To test our second hypothesis, we conducted a saccade experiment, which required INS and control subjects to respond to the direction of horizontal E targets presented randomly at the post-saccade position, set at ±25° with 5° step, away from the centre. Visual task performance was measured by subjects’ reaction time and their response accuracy to the direction of the post-saccade target. Each of the above tasks was performed under two conditions: alone (low mental load) and with mental arithmetic and time restriction (induced high mental load). Perceived workload across these task conditions was assessed via changes in heart rate as well as other physiological measurements such as skin conductance level, and subjective ratings. The subjects’ eye movements and visual task performance were recorded across varying task conditions. Results: The induction of the mental arithmetic task and time restriction increased mental workload. For both visual acuity and saccade experiments, the metrics of task performance worsened under high mental load. For the visual acuity experiment, there was a significant interaction between mental load and gaze position for foveation time and task performance. Interaction effects were interpreted as follows; during high visual demand, the change in foveation time due to mental load was greater at the null than away from it. Similarly, the increase in last optotype size from the low to high mental load condition was more pronounced at the null than away from it. During both low and high mental load, saccade latency, target acquisition time, and visual recognition time were longer in the INS group in comparison to controls. For both control and INS subjects, saccade latency and target acquisition time increased with high mental load. The LATER model (Linear Approach to Threshold with Ergodic Rate), implemented to analyse saccade latency in the control group, highlighted that the rate of rise in saccade decision signal decreased during high mental load. Visual recognition time increased with high mental load only in the INS group. The effects of mental load on the gain of prosaccades and target acquisition time varied between INS and control subjects. Discussion: We suggested that the extent to which foveation duration contributes to visual function varies across both task conditions and gaze positions. We agreed with the notion that an extended foveation time does not guarantee an improvement in visual task performance in INS. Previous studies have highlighted that INS subjects are late in detecting a new target, which can result in a delay in target acquisition. Consistent with the literature, our study concluded that difficulty in the accurate programming of eye movements used to acquire a new target can also delay target acquisition time. Our results suggest that a further increase in target acquisition time with stress worsens the phenomenon of being slow to see in INS. We concluded that an increase in saccade latency with high mental load is due to a reduction in the capacity of information processing resources in both individuals with and without INS. However, visual recognition time in INS is multifactorial and can show additional increases under stress (e.g. driving) when compared to normal subjects. We have suggested that the efficacy of visual processing further decreases with effort to see in INS. Conclusion: Possible main and interaction effects of task condition and gaze position on foveation time can lead to an overestimation of visual function in INS. Accuracy and speed of the subjects’ response to visual targets should be measured with a mental load resembling real world conditions. The results can be useful in relation to work safety and clinical assessment of INS, especially in terms of treatment outcomes