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Larry Abel, Elizabeth Bowman, Isla Williams; Baseline OKN and OKN suppression during divided attention tasks are differently affected by multiple sclerosis. Invest. Ophthalmol. Vis. Sci. 2013;54(15):176. doi: https://doi.org/.
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To compare normal subjects’ and multiple sclerosis (MS) patients’ ability to generate optokinetic nystagmus (OKN) and to suppress it when suppression serves as an index of the ability to divide visual attention. We previously examined the ability of normal subjects to suppress OKN while also being asked to identify either a stationary (e.g., blinking light) or dynamic (e.g., a feature of the OKN curtain) element in the visual stimulus (Williams et al, 2006; Rubinstein & Abel, 2011).
Eye movements were recorded while subjects (13 controls, aged 44.7+/-21yrs) and 7 MS patients (aged 44.3+/-14.3 yrs) sat surrounded by a 24 deg/s full-field OKN stimulus curtain consisting of rows of red Cs and blue Ts, with 3 red Ts present in the eye level row. Green laser targets served for fixation and for some tasks as reference spots at +/-10 deg. Target detection was indicated via a button press. Tasks were baseline OKN and 5 others requiring OKN suppression: detection of blinks in central laser spot, detection of blinks in peripheral laser spot and the passing of a red T by either a peripheral laser spot or by the central one. As in Williams et al (2006), they also indicated when the red T was first visible in the periphery. OKN gains were calculated.
Baseline gain was significantly lower in MS patients (ctrl 0.845, MS 0.621,t-test, p<0.033). Age did not correlate with baseline gain. Two-way repeated measures ANOVA across tasks and groups found a significant effect of task (F=11.27, p<0.0001) but not of group and no interaction. As in earlier studies, dynamic task gains were higher than static task gains. The same pattern was found when gains for the 2 static and 3 dynamic tasks were averaged. For controls, age significantly correlated with mean dynamic (r2 = 0.434, p =0.014) but not mean static gain.
While groups are small, we found that MS impairs patients’ generation of stare OKN but it does not appear to affect their ability to suppress OKN when simultaneously attending to either a static or dynamic stimulus feature. As seen in our earlier studies, attending to the dynamic elements of the scene causes significantly more OKN breakthrough and this breakthrough increases with age. It is likely that attending to the moving background, not spatially spreading attention, caused the age-related increase in breakthrough we reported earlier (Williams et al 2006).
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