The Misperception of the Direction of a Feature Added on a Moving Line is Eliminated with Static Cues

It has been reported earlier that a feature (a gap or a dot) added on an oblique line which is translating vertically behind a horizontal rectangular aperture (we call this the basic display), appears to slide along that line while the perceived direction of the line is horizontal. Castet and Wuerger observed that, when the aperture is a visible circular outline, the perceived ‘sliding’ of the dot is significantly reduced compared to the conditions where the aperture is invisible. They suggested that the misperceived direction of the dot in the invisible-aperture condition might be due to a lack of static cues. To investigate this hypothesis further we ran a similar experiment in which the aperture was a rectangle. Subjects had to report with an adjustable arrow the direction of the dot added on the line. Results showed that the sliding effect completely disappeared when the rectangular aperture was visible (white outline) compared to the condition where the aperture was invisible. We hypothesised that better performance is obtained when the outlined aperture is rectangular rather than circular because of additional vertical cues. Therefore, in a second experiment using the basic display mentioned above, we tested how a single static dot affects the ‘sliding’ effect. The static dot was either vertically aligned or not, with the dot placed on the moving line. Results showed that the sliding effect was significantly reduced but not eliminated when the static dot was aligned vertically with the moving dot, compared to the conditions where the static dot was not aligned vertically with the moving dot (or where there was no static dot at all). This suggests that static features providing alignment information are optimal for eliminating the misperception of the direction of a moving feature.