Abstract
Purpose: :
Perceptual fading is a filling–in related phenomenon in which a static target in the visual field fades from view after a certain delay, or fading time (FT). Perceptual fading is thought to be reset by eye movements, however, perceptual fading during active eye movements has not been systematically investigated. We have compared fading time of targets during active saccadic and pursuit eye movements in comparison to equivalent movement of targets across the retina during steady fixation. Perceptual fading in patients with involuntary eye oscillations caused by nystagmus were also compared.
Methods: :
Eleven normal volunteers and four with nystagmus performed a perceptual fading task consisting of following an oscillating fixation cross of either sine wave (pursuit task) or square wave waveform (saccadic task) while viewing a static peripheral spot of low contrast (1.5x threshold). This was compared to FT during a static fixation cross and the peripheral spot moving. Also, FT during both fixation cross and peripheral target moving synchronously was compared to a static cross and target. Targets at horizontal and vertical eccentricities were compared and eye movements recorded.
Results: :
During pursuit, FT was significantly longer for the static cross + moving target condition compared to the moving cross + static target (p=0.04) and moving cross + moving target (p=0.001). In contrast, during the saccadic trial, FT was similar for static cross + moving target, and, moving cross + static target tasks, but both were lower than the moving cross + moving target task. Of the five subjects with nystagmus, two showed similar filling–in patterns to the control group, and two had difficulty filling–in horizontally placed targets.
Conclusions: :
We describe the filling–in of moving targets when stimuli are applied at low contrasts. For equivalent target movements on the retina in normals, FTs are less when an active eye movements are made compared to when the target moves relative to the eye. This was only during pursuit but not the saccadic task. Interestingly, patients with involuntary eye movements could also fill–in targets although responses are more variable compared to normals.
Keywords: eye movements • nystagmus • perception