Abstract
Purpose: :
Although a full-field high contrast stimulus rotating around a subject elicits a strong response of optokinetic nystagmus (OKN), normal individuals can suppress this nearly completely by actively fixating a stationary point in the otherwise moving environment. This requirement for active fixation raised the possibility that if attention were simultaneously directed elsewhere in the visual field, then OKN suppression might be impaired. In a recent study (Williams et al, 2006) we demonstrated that if subjects were required to detect the first appearance of a feature of the OKN stimulus (a red T in a row of red Cs and blue Ts), that OKN suppression declined in an age-dependent manner. It was unclear, however, whether this was due to directing attention towards the OKN stimulus or to having to spread it over a wide spatial area. The present study addresses this question.
Methods: :
15 healthy subjects aged sat in the centre of a full-field stimulus 1.5 m in diameter moving at 24°/s and composed of columns of red Cs and blue Ts, with the eye-level row also containing 3 red Ts. Red laser spots were projected on the curtain under computer control at 0° and 7.5° left or right. After baseline "stare" OKN testing, all other tasks required subjects to fixate a 0° spot. 3 tasks required detection of a blink of either the central or peripheral laser spot and 3 required detection of a red T passing the same spots. OKN gain was measured for all tasks and compared with Friedman’s ANOVA and also regressed against subjects’ ages. Results : Mean gains on the 3 blink detection tasks were 0.06, 0.05 and 0.06; on the corresponding T detection tasks they were 0.14, 0.15 and 0.16. ANOVA showed a significant main effect of task on gain (p<.001); pairwise post-hoc tests found significant gain differences (p<.05) when detecting blinks vs. detecting red Ts. Regressions against age were all non-significant.
Conclusions: :
For the 6 detection tasks, stimulus conditions were identical across the pairs of tasks but attention was directed either towards the moving curtain or the static laser spot at each location. Gain was significantly higher when the OKN stimulus was made more salient via top-down allocation of attention. The lack of an age effect may result from 7.5° targets not sufficiently stretching the attentional spotlight to stress even elderly subjects’ resources.
Keywords: nystagmus • eye movements: conjugate • eye movements