Perception of moving lines: interactions between local perpendicular signals and 2D motion signals

An oblique line translating vertically behind a horizontal rectangular aperture is perceived as moving in the horizontal direction, i.e., in the line-ending's direction. When a feature is added on the line, and thus provides a vertical unambiguous motion signal, the line's perceived direction is still horizontal. In parallel, the feature appears to slide obliquely along the line (Wallach, 1935). We first show that this finding which we call "the sliding effect" is robust and easy to replicate for different orientations of the rectangular aperture (up to about 20 deg from vertical). This effect also occurs with an invisible circular aperture. In this case, using an adjustment task, observers have great difficulty extracting the actual direction of the feature (a gap or a dot on the line). Instead, a systematic bias towards the direction of "sliding" is observed. This misperception of the feature's velocity is markedly reduced or even suppressed when the circular aperture is outlined or when a visible circle is drawn around this invisible aperture. The line's perceived direction is always roughly orthogonal to the line's orientation regardless of the presence of a visible circular outline. This latter result is important because it shows that the feature on the line does not disambiguate the perpendicular signals extracted along the contour, even in conditions where this feature's motion is almost correctly perceived. Altogether, these results suggest that line-endings are not used by the visual system in the same way as a feature on the line when it comes to determining the line's perceived direction.