Chad Orzel has challenged the ScienceBloggers to come up with the greatest experiments in their respective fields. While Greta and I are reluctant to say this is the greatest experiment ever (there are so many great experiments!), we both independently came up with the same one: Roger Shepard and Jacqueline Metzler’s 1971 experiment on mental rotation. It’s certainly our favorite, and it’s difficult to overstate its importance.
The design of the experiment is simple and brilliant; yet it was not easy to execute at the time. Today researchers studying vision almost always use computers to display stimuli. In 1970, when the experiment was designed, psychologists didn’t have easy access to computers capable of displaying Shepard and Metzler’s complex, three-dimensional images. They wanted to show participants two objects in two different orientations, then measure how long it took for them to determine if the objects were the same. Here are a few of the 800 different pairs of images they created for the study:
The objects were created using a computer program, but the computer’s output was a set of coordinates, which then had to be plotted out by hand on graph paper. These images were transferred to cards, which were displayed to viewers using a tachistoscope, a viewing box with a shutter that allowed a precise measurement of when an image was revealed. Participants pulled one handle to indicate the objects were the same, and a different handle when they were different. Though half the objects were the same and half were different, Shepard and Metzler were only interested in the results when objects were the same.
The objects were either rotated in the picture plane so that the two-dimensional image did not change, or around the vertical axis (imagine the object spinning around a central pole), in which case the two-dimensional image was completely different. In both cases, the results were similar:
Though there are many possible explanations for the cognitive process that led to this result, the simplest is the one provided by the viewers themselves: to determine whether the objects were the same, they rotated one of the objects in their head and determined whether it matched the other object. This mental rotation occurred at a constant rate, so reaction times were longer when the rotation required to make the comparison was larger. The data matches this explanation perfectly. If the rate of rotation is constant, then after subtracting out the time required to make the comparison and operate the machine, then participants should take twice as long to make a 40-degree rotation as a 20-degree rotation, and three times as long for a 60-degree rotation. When you plot the results on a graph, the data points should line up in a diagonal line with a constant slope. And they do, all the way to the longest, 180-degree rotation. It didn’t matter whether the objects were rotated in the picture plane or the seemingly more difficult vertical axis — the results followed the same linear function.
So why is this experiment so important? In the early 1970s, the Sapir-Whorf hypothesis was taken quite literally — the idea that we can’t think about anything without language, that language itself shapes our thought. The Shepard and Metzler experiment, along with hundreds of similar studies that followed, demonstrated quite clearly that there are many aspects of cognition that don’t involve language. Just try comparing the any of the two objects above using only a linguistic explanation — your reaction times most certainly will not be related to how far the objects are rotated: instead, you’ll have to say things like “the first object has three cubes in a row, followed by a right angle, then two more cubes…” then offer a similar description of the second object, and finally, compare these descriptions to see if they match. By this time, your mentally rotating friends will have completed dozens of trials of the same task.
In addition, the mental rotation paradigm has been used to study a number of other phenomena — we’ve discussed how it relates to focal dystonia, for example. It has become one of the most powerful tools for studying the vision system, inspiring thousands of scholars.
Do you have a different opinion about the most important cognitive science experiment? Please let us know in the comments.
Shepard, R.N., & Metzler, J. (1971). Mental rotation of three-dimensional objects. Science 171, 701-703.