Abstract
Purpose: :
Most smooth pursuit studies use uniform backgrounds that do not represent realistic day-to-day conditions. When structured backgrounds are utilized, smooth pursuit gains tend to stay unchanged and only decrease modestly (10%) at higher speeds in normals. A simulated scotoma in normals has been shown to result in a slightly greater gain reduction (20%). In monkeys with a small foveal ablation, gains for pursuing a target against a structured background were dramatically reduced compared to a uniform background. We investigated the effect of a structured background on smooth pursuit in humans with real scotoma.
Methods: :
Normal controls (NC) and persons with central scotoma (CS) were asked to visually track a less than 1 degree yellow dot moving along either a horizontal or a vertical trajectory against a uniform grey background and a binary random checkerboard background. The target waveform was a 10-degree periodic sinusoid with frequencies ranging from 0.10 to 0.45 Hz. The stimulus was presented on a 120Hz CRT monitor 60cm from the subject. An EyeLink II eye tracker, running at 250Hz, was used to record eye movements. Standard procedure was used to extract the smooth component from raw eye position data. The composite and smooth gains were obtained.
Results: :
On a uniform background, NC maintained a near unity smooth pursuit gain up to target frequency 0.4 Hz while CS had slightly lower gains at lower frequencies, but suffered a 20% or more gain reduction at 0.4 Hz. On a structured background, NC had a 5% gain reduction in horizontal pursuit and a 30% reduction in the vertical pursuit. In contrast, CS suffered a much larger gain reduction (up to 60%) in horizontal pursuit but a similar gain reduction (30%) in vertical pursuit. Composite gains remained near unity for NC and CS on uniform and structured backgrounds at lower frequencies, and only modestly decreased (up to 20%) on higher frequencies in the vertical direction.
Conclusions: :
The fact that a central scotoma had a much greater impact on smooth pursuit against a structured background than against a uniform background may account for some of the debilitating functional deficiencies seen in patients with central scotoma in real visual environments. Our findings also emphasize the importance of an intact fovea in suppressing peripheral motion signals induced by smooth eye movements.
Keywords: eye movements: saccades and pursuits • macula/fovea • low vision