The visibility and fading of thin lines visualized by their controlled movement across the retina

1. The entoptic shadows of the retinal blood vessels were visualized by temporal modulation of their contrast.2. For perception of fine detail, the shadows must move successively from one photoreceptor to the next. To see coarser detail the contrast need only be temporally modulated.3. The contrast threshold of the pattern of shadows rises steadily as they are viewed until the shadows can no longer be seen even with the highest contrast available.4. This elevation of contrast threshold is partially binocularly transferred, suggesting that the perceptual fading has a central origin.5. The fading of the shadows is specific for their orientation, direction of movement and their width.6. Image movements like those produced by fixational eye movements have been simulated. The shadows still fade; the results can be explained in terms of spatial adaptation (Blakemore & Campbell, 1969) of spatial frequency and orientation channels.