The role of motion streaks in the perception of the kinetic Zollner illusion

Abstract

In classic geometric illusions such as the Zollner illusion, vertical lines superimposed on oriented background lines appear tilted in the direction opposite to the background. In kinetic forms of this illusion, an object moving over oriented background lines appears to follow a titled path, again in the direction opposite to the background. Existing literature does not proffer a complete explanation of the effect. Here, it is suggested that motion streaks underpin the illusion; that the effect is a consequence of interactions between detectors tuned to the orientation of background lines and those sensing the motion streaks that arise from fast object motion. This account was examined in the present study by measuring motion-tilt induction under different conditions in which the strength or salience of motion streaks was attenuated: by varying object speed (Experiment 1), contrast (Experiment 2), and trajectory/length by changing the element life-time within the stimulus (Experiment 3). It was predicted that, as motion streaks become less available, background lines would less affect the perceived direction of motion. Consistent with this prediction, the results indicated that, with a reduction in object speed below that required to generate motion streaks (< 1.12°/s), Weber contrast (< 0.125) and motion streak length (two frames) reduced or extinguished the motion-tilt-induction effect. The findings of the present study are consistent with previous reports and computational models that directly combine form and motion information to provide an effective determinant of motion direction.

A Short Line Segment Squirms Along a Zigzag Line

“…That the perceived direction of motion of a dot is contrasted with the orientation of background lines when it crosses the tilted lines has been demonstrated (Cesaro & Agostini, 1998; Khuu, 2012; Swanston, 1984). The motion streak in a contrasted orientation is thus considered to be a component of motion direction perception (Geisler, 1999).…”

Interactions between motion and form processing in the human visual system

“…In addition to their effect on motion thresholds, motion-streaks also alter the appearance of supra-threshold motion. Several papers report changes in the apparent direction of moving elements when they are superimposed on a static background of tilted lines (see Swanston, 1984; Khuu, 2012). A possible mechanism for this direction effect involves mutual inhibition between orientation-selective cells, some of which are activated by the tilted background while others are activated by the motion-streak.…”

An investigation into perceived motion paths by means of the Slalom Illusion

“…A hybrid variant, with a static line segment on one end and a disappearing moving dot on the other end, did produce the effect. Khuu (2012;Khuu & Kim, 2013) replicated a variant of the Zollner illusion in a kinetic display, whereby a small group of dots moved vertically across a parallel set of tilted lines. The direction of motion appeared tilted away from the orientation of the lines.…”

“…It is proposed that the encoding of motion direction is biased towards perpendicularity around the points of intersection with the tilted lines (Cesàro & Agostini, 1998). The main argument for this is the dependence of the magnitude of the illusion on the angle of the lines, with no illusion occurring in the case of vertical lines, as well as the prevalence of a perpendicularity bias in other types of stimulus displays (Fineman & Melingonis, 1977;Wenderoth & Johnson, 1983;Khuu, 2012;Khuu & Kim, 2013). The exact origin of this bias in the context of the slalom illusion is unclear.…”

“…Finally, in the Tilt Illusion ( Figure 5.2) (Gibson & Radner, 1937;Clifford, 2014), no direct intersection is present, but the orientation of the middle grating is still biased towards perpendicularity with the tilted surrounding grating. Kinetic versions of these static geometric illusions, where the straight line is replaced by a straight dot trajectory, exist in the case of Poggendorff (Fineman & Melingonis, 1977;Wenderoth & Johnson, 1983) and the Zollner (Khuu, 2012;Khuu & Kim, 2013) illusions. Note that in trajectory motion displays, the aperture problem would also not be relevant, because a dot stimulus is necessarily end-stopped and does not suffer from motion direction ambiguity in the same manner as a line does.…”