Visual contour masking

Some perceptual effects generated by rotating graftings

Rotation of a grating at around 2π rad s−1 in continuous illumination results in the appearance of a band lying almost perpendicular to the lines. Two possible bases for the occurrence of this effect were examined. Firstly, the band might represent a moiré fringe due to the overlap of the present grating with one persisting in the visual system from a previous position. Secondly, those parts of the lines farthest from the bisecting diameter will have the greatest linear displacement during rotation; thus, the extremities of each line might fail to be resolved, while those parts nearer the centre are adequately resolved, resulting in the appearance of a band. Three experiments were conducted to test predictions from the two hypotheses. The first experiment involved manipulation of the spatial frequency of the square-wave gratings and the size of the discs bearing them. In the second the line spacing was varied while the line width was kept constant. In the third experiment the angular velocities required to produce bands of different widths were determined. The results did not support either hypothesis unequivocally, and the possibility that both are involved in the effect cannot be discounted.

Observation of the rotating gratings under stroboscopic illumination produced either moiré fringes or complex doily-like patterns, depending upon the angular velocity and strobe frequency. These ‘overlap’ effects were used to determine the duration of visual persistence in a fourth experiment. At certain combinations of high angular velocities and strobe frequencies, apparent rotation effects are visible simultaneously in opposite directions.