Point-light displays are an amazing demonstration of how the visual system creates order out of what initially seems to be a random pattern. Take a look at this short movie (QuickTime required). Just looking at the first frame, it might be difficult to tell what’s being displayed, but after watching for just a second, it all becomes quite clear:
Just these 13 dots, when placed in motion, instantly convey a very clear picture. We can even determine the gender of the person walking or recognize friends, just from displays like this. But now take a look at this movie:
It might take a bit longer, but soon we get the idea.
So we can even recognize “unnatural” movies like an upside-down version of the first movie. Now how about this:
This too is an “upside-down” movie, but this time the “walker” is actually an acrobat walking on his hands. Again, for most people, it’s trivial to tell the difference between the upside-down walker and the man walking on his hands. But these videos give rise to an interesting question: what do we recognize first from these displays: the shape of the figure or the type of motion we’re seeing?
Thomas Shipley showed movies like these to 23 psychology students, but he made the task more difficult by obscuring the movies with additional distractor dots moving in time with the actual dots representing the walker’s joints. There were two levels of distraction: an equal number of distractor dots and real dots (1:1) or twice as many distractors (2:1). In some displays, there was no walking figure at all, just distractors. The students were asked whether they could see a moving figure in each video. Here are the results:
In both cases, the viewers were significantly less accurate in identifying the upside-down pictures compared to right-side-up pictures. Even though the upside-down walking-on-hands picture looked in some ways like the upright walking-on-feet picture, it was much more difficult for the viewers to recognize. Shipley argues that this demonstrates that viewers recognize the dynamics of a display first — the physical relationship between the motion of the figure and the world. If that motion is unrealistic, it’s more difficult to recognize.
So why is the realistic figure walking on his hands recognized later than the upside-down figure walking on his feet? Perhaps simply because people are less familiar with what “realistic” hand-walking looks like.
Shipley points out that this experiment still leaves many aspects of motion perception unanswered. For example, do we recognize more complex figures in motion (such as two people dancing) in the same way? At some point, must we not group together different aspects of a scene in order to understand what’s going on?
Whatever the answers to these questions, there’s no question that point-light displays are lots of fun. You can see dozens of examples of these displays on Shipley’s web page.
Shipley, T.F. (2003). The effect of object and event orientation on perception of biological motion. Psychological Science, 14(4), 377-380. DOI: 10.1111/1467-9280.24471