The lazy shadow:: a monocular counterpart to the Pulfrich stereo phenomenon

Background: The Pulfrich phenomenon is a dynamic stereo dysmetropsia attributed to an asymmetry of neural conduction between the eyes. The phenomenon may arise spontaneously with ocular and neurological disease and may be induced in normal subjects by placing a light-attenuating filter before one eye. By analogy, it is predicted that a localised variation of retinal illumination within one and the same eye should affect the perception of moving targets. Methods: A rotating, nesting square display was generated by computer graphics. The inner square was painted bright white, the outer dim grey. Luminances, rates of rotation and angular sizes were varied. Results: On rotation, the outer, dimmer square appeared to lag behind the inner, brighter one, as a "lazy shadow''. The lag was measured quantitatively in normal observers by applying a compensatory lead to the lagging square. The magnitude of lag was found to depend on luminance, spin rate and visual angle. Lags exceeding 10 were observed under optimum conditions. Conclusions: The experimental results confirm the existence of a monocular counterpart to the binocular Pulfrich phenomenon. Distortions of moving images are likely to occur spontaneously with monocular, localised visual field defects.