In the animation below, you can see a Christmas tree full of baubles. The baubles are arranged in two more-or-less vertical columns, but not quite: Some are shifted a bit to the left, some a bit to the right. Or are they? In actuality, of course, the baubles are arranged in two perfectly straight columns. The apparent displacement is caused by the motion of the stripes on the baubles.

This illusion is a demonstration of motion-induced displacement: the phenomenon that the perceived position of an object is affected by its motion (or motion in the object’s environment, as I’ve shown before). This effect is particularly strong if an object’s position is ill-defined, for example because it has fuzzy edges like our baubles here.

The first to show this (that I know of) were Leonard Matin and his colleagues. They showed that two line-segments that rotated around a central dot appeared to be shifted in the direction of their movement. This is similar to what happens to the perceived position of the ‘baubles’ in the animation shown above: When the texture of the bauble is moving to the right, the position of the bauble appears to shift to the right as well. The fact that the bauble itself is not moving (just its texture) is a nuance that is lost on our visual system: Essentially any kind of nearby motion will ‘grab’ the object and perceptually drag it along.

The obvious question is whether motion-induced displacement serves a purpose. Is there any reason why we perceive moving objects as being a little bit ahead of their actual position? This question is topic of debate, but, with the risk of over-simplifying things, I submit that the answer is quite simply that we predict. After all, imagine that you want to catch a ball that is thrown at you. Clearly, you will want to know where the ball will be (rather than is), so that you can prepare a movement to catch it.

Happy holidays!

References

Matin, L., Boff, K. R., & Pola, J. (1976). Vernier offset produced by rotary target motion. Perception & Psychophysics, 20(2), 138–142.

Whitney, D. (2002). The influence of visual motion on perceived position. Trends in Cognitive Sciences, 6(5), 211-261 [full text]