Refers to the perception of three dimensions imparted to a two-dimensional projection of a moving three-dimensional object.

The original experiments, which demonstrated this effect used the projected shadow of a wire-frame box.

It is normally referred to as an illusion, because the wire-frame shadow is often perceived to change its rotation-direction as it is being observed, even though it continues to be rotated in a single direction.




. . . . . . .
It is my convention to refer to this as a perception.

As stated, this is referred to, traditionally, as an illusion. It is more helpful (imo) to think of this effect as the natural by-product of our brain producing a three-dimensional perception of the world around it. In other words, this effect is simply our brain interpreting (or pre-filtering) its sensory inputs in a way which is consistent with all other previously-made observations of its milieu.




. . . . .
Related Videos

There are a variety of ways this effect can manifest in normal every-day experiences. Three are presented below, the most well known of which, is the mask effect (second video).




Above, you may observe the kinetic depth effect when the image-frame zooms toward, or away from still photographs. Here, it is likely that making input stimuli consistent with previous ubiquitous experiences of a three dimensional universe, is primarily responsible for the effect. Though consistency with three dimensions is the primary cause of the effect in the above video it probably isn't the only cause. Another ubiquitous perception about our world has been the human form. Filtering afferent signaling, in order to make it consistent with those previous perceptions, may be playing a minor supporting role in the above video. The following video really brings the point home.





A possible contributing cause of the above effect may be top-down modifications of input stimuli to make them consistent with past experiences of a universe that has three-dimensions. As stated in the video, however, the top-down filtering of input stimuli also work to make those stimuli consistent with deeply embedded previous experiences regarding human facial features (possibly the primary mechanism at work here?).




. . . . .
Conventional Wisdom?

Much of the above discussion regarding the role of the human form in the perception of the kinetic depth effect is simply my parroting of explanations that have been provided by early pioneers on the subject. In some circles, these explanations have come to be treated as conventional wisdom, and, as such, they are seldom questioned or tested.

Does the mask effect really have, as its primary cause, our deeply impressed expectations about the human face? One way to test this assertion might be to try to replicate the mask illusion, but without the human facial features that are so often credited for its effectiveness.





In the above example, the human form has been completely eliminated from the concave drawing, yet the cube still pops out and appears convex when set in motion.

About the only thing that is more ubiquitous and consistently uniform than the appearance of the human face in our lives, is the three dimensional, volumetric, nature of every aspect of our universe.

I'm a firm believer in the idea that nothing should ever be considered conclusive at this level of understanding. That said, we can at least accept that this demonstration provides strong support for the notion that it is the volumetric nature of reality, and not so much the human form, that is responsible for the illusion.