Diverted by dazzle: perceived movement direction is biased by target pattern orientation

Abstract

'Motion dazzle' is the hypothesis that predators may misjudge the speed or direction of moving prey which have high-contrast patterning, such as stripes. However, there is currently little experimental evidence that such patterns cause visual illusions. Here, observers binocularly tracked a Gabor target, moving with a linear trajectory randomly chosen within 188 of the horizontal. This target then became occluded, and observers were asked to judge where they thought it would later cross a vertical line to the side. We found that internal motion of the stripes within the Gabor biased judgements as expected: Gabors with upwards internal stripe motion relative to the overall direction of motion were perceived to be crossing above Gabors with downwards internal stripe movement. However, surprisingly, we found a much stronger effect of the rigid pattern orientation. Patches with oblique stripes pointing upwards relative to the direction of motion were perceived to cross above patches with downward-pointing stripes. This effect occurred only at high speeds, suggesting that it may reflect an orientation-dependent effect in which spatial signals are used in direction judgements. These findings have implications for our understanding of motion dazzle mechanisms and how human motion and form processing interact.

Apparent shift in long-range motion trajectory by local pattern orientation

“…Secondly, the fact that Hughes et al . 42 employed continuously-moving stimuli may have weakened the effects relative to those in the present study. It is widely known that continuous (or “short-range”) motion is detected by early motion energy detectors whereas long-range motion (produced by our stimuli with discrete-jumps and blank ISIs) is processed by higher-order motion detectors 43 , 44 .…”

“…It is sensible that orientation information should also assist higher-order motion processing if it can contribute to more accurate estimates of motion direction. The orientation-induced shift in perceived motion direction is increased by shorter motion trajectories as well as by faster motion (<15 deg/sec) 42 . Note that we found a subtle proportional increment of the maximum shift when apparent motion became faster up to about 15 deg/sec (Experiment 1) and speed will be another factor of the directional shift or even it might be a by-product of speed modulated by orientation information 54 .…”

“…Recently, Hughes et al . 42 had observers localize apparent position as extrapolated from the motion trajectory of grating stimuli and revealed that directional shifts in motion trajectory depend on the grating orientation. The directional shift found in our study (~6 deg) is much larger than that reported in the Hughes et al .…”

Grab my tail: evolution of dazzle stripes and colourful tails in lizards

“…fish (Kjernsmo & Merilaita, ); mammals (Powell, ); lizards (Cooper & Vitt, ); insects (Prudic et al ., )), such markings can protect the prey from predation. Another type of striking prey coloration, which have been termed motion dazzle patterns, includes stripes, bands and zigzags and are thought to protect moving prey by hampering the attacking predator's perception of speed or trajectory (Stevens et al ., ; Scott‐Samuel et al ., ; Murali & Kodandaramaiah, ; Hughes et al ., ), an idea initially proposed by Thayer ().…”

Camouflage in a dynamic world

“…False cues of direction or speed can introduce error via so-called dazzle coloration [39,40,[44][45][46][47]. Here, high contrast, repetitive patterns induce contradictory or biased motion cues [48], influencing estimations of speed [46,49] and/or direction [44]. Interference with optic flow cues may be why biting flies are deterred from landing on zebras [50].…”