The reverse motion illusion in random dot motion displays and implications for understanding development

Across two independent developmental labs, we have been puzzled by the observation that a small proportion of our child and adult participants consistently report perceiving motion in the direction opposite to that presented in random dot motion displays, sometimes even when the motion is at 100% coherence. In this review, we first draw together existing reports of misperceptions of motion direction in random dot displays across observers in a small percentage of trials, before reporting evidence of consistent reverse motion perception in a minority of observers, including previously unreported observations from our own studies of visual development. We consider possible explanations for this reverse motion illusion, including motion induction, motion energy, correspondence noise and spatial undersampling. However, more work is required to understand the individual differences relating to this percept. We suggest that errors in perceived motion direction are likely to be more widespread than can be currently gleaned from the literature and explain why systematic study is needed, especially in children. Finally, we list some remaining open questions and call for collaborative efforts to document this phenomenon and stimulate future investigation.

Faces behind bars: illusory eye movements induced by gratings

I describe a novel illusion in which perceived eye movements are induced by moving a vertical grating across a single image of a forward-looking face. By varying properties of the grating, a wide range of illusory eye movements can be generated including nystagmus, a ‘swirling’ motion of the eyes, and vertical scanning/blinking. I suggest that the phenomenon is closely related to the footsteps illusion, but reveals the role that object shape and grating spatial frequency together play in determining the direction of illusory motion that observers perceive. I also discuss the relationship between the current illusion, the footsteps illusion, and Moire pattern animations.

Hybrid motion illusions as examples of perceptual conflict

Shapiro and Hedjar (2019) proposed a shift in the definition of illusion, from ‘differences between perception and reality’ to ‘conflicts between possible constructions of reality’. This paper builds on this idea by presenting a series of motion hybrid images that juxtapose fine scale contrast (high spatial frequency content) with coarse scale contrast-generated motion (low spatial frequency content). As is the case for static hybrid images, under normal viewing conditions the fine scale contrast determines the perception of motion hybrid images; however, if the motion hybrid image is blurred or viewed from a distance, the perception is determined by the coarse scale contrast. The fine scale contrast therefore masks the perception of motion (and sometimes depth) produced by the coarser scale contrast. Since the unblurred movies contain both fine and coarse scale contrast information, but the blurred movies contain only coarse scale contrast information, cells in the brain that respond to low spatial frequencies should respond equally to both blurred and unblurred movies. Since people undoubtedly differ in the optics of their eyes and most likely in the neural processes that resolve conflict across scales, the paper suggests that motion hybrid images illustrate trade-offs between spatial scales that are important for understanding individual differences in perceptions of the natural world.

The Confetti illusion

The Munker illusion, which produces two apparently different hues from a base color, can be extended to multiple apparent colors. By having foreground stripes in more than two colors, it is possible to create multiple foreground colors that lead to multiple apparent hues in the base objects. All of these apparently different colors are due to color assimilation rather than to differences in luminance, as in the Munker-White illusion. This technique works well for three colors and can be extended to four and even six colors.