Take a look at the short movie I’ve linked below (Click on the picture to play. QuickTime required). The movie shows a virtual gripping device (two red balls) lifting rectangular objects and placing them on a conveyor belt. Do you notice anything unusual happening as the objects are being moved?
This is a recording of the image seen by a volunteer using a virtual-reality headset and two force-feedback devices to simulate moving objects by picking them up with a thumb and index finger. The force-feedback devices offer resistance so that the volunteer feels as if he is lifting real objects:
So, did you spot the anomaly in the movie? I’ll spare you the suspense and tell you that the fourth block actually changed shape during the course of the motion. What’s really interesting about this is that volunteer viewer never noticed — even though he was “carrying” it at the time. You might have noticed it, though, especially since I hinted to you that a change was going to occur. Incredibly, nearly all the time, the volunteers in this experiment did not notice a change.
This is an amazing example of a phenomenon we’ve discussed several times before on Cognitive Daily: change blindness. In the traditional change blindness paradigm, which is amazing enough, an object must be briefly removed from view for the change not to be noticed. As this demonstration shows, when the change occurs right before our eyes, it’s much easier to spot. So why were the volunteers in the virtual reality experiment so bad at spotting the changes? The research team that conducted the experiment, led by Jochen Triesch, has some ideas. But first, let’s take a look at what they actually did in their study.
Fifty-nine college students agreed to don virtual reality helmets and attach their hands to the force-feedback device while they did one of three tasks:
- “Pick up the bricks in front to back order and place them on the closer conveyor belt.”
- “Pick up the tall bricks first and put them on the closer conveyor belt. Then, pick up the small bricks and also put them on the closer conveyor belt.”
- “Pick up the tall bricks first and put them on the closer conveyor belt. Then, pick up the small bricks and put them on the distant conveyor belt.”
In each task, ten percent of the time one of the objects they are asked to move changes shape while it’s being carried to the conveyer belt, as you saw in the movie above.
Notice that in the first task, the size of the object doesn’t matter at all. In task 2, size only matters when the objects are being picked up. In task 3, size matters both when the objects are being picked up and when they are put on the belts. The students weren’t told that the objects could change, but they were told that the computer system was just upgraded, so they should let the experimenter know right away if they spotted anything unusual during the experiment. Each student moved 100 blocks as specified by their task group. Did they notice the objects changing shape? Here are the results:
The vast majority of students in Task 1 and most students in Task 2 didn’t ever stop the experimenter to say a shape had changed. Even in Task 3, when participants had to pay attention to the shape both when picking up the objects and when setting them down (and the object was now a different shape), fewer than half of the shape changes were noticed.
Even after they finally noticed a change, participants still often didn’t notice when other objects changed, as this graph illustrates:
So what’s going on — why do viewers miss so many of the changes? One possibility is that they were simply looking away from the objects at the time the shape change occurred. But Triesch’s team actually monitored the eye movements of the students as they performed the task; they could use this information to determine where they were looking when the shape changed. In Task 1 and 2, it turns out, even when participants were looking directly at the objects, they still failed to notice the change more than fifty percent of the time! In Task 3, accuracy improved across the board, but many shape changes were still missed when viewers looked right at the objects!
The authors argue that the visual system computes only just enough information to complete the task at hand at any given moment. Everything else is automatically ignored. Of course, we’re generally not conscious that this is what our visual systems are doing, which is why the phenomenon of change blindness is so surprising.
Triesch, J., Ballard, D.H., Hayhoe, M.M., & Sullivan, B.T. (2003). What you see is what you need. Journal of Vision, 3(1), 86-94. DOI: 10.1167/3.1.9