The Stroop Effect: Score one for object-based attention

One of the amazing things about the Stroop Effect is how much good research is being done based on this simple phenomenon, over 70 years later. One of the neatest recent experiments was created by Peter Wühr and Florian Waszak. I think I've created a simple animation that replicates their results. Click on the image below to bring up a short animated GIF. You'll see an image flash quickly, followed by a blank screen. As quickly as possible after the image flashes, say the color of the rectangle in front. Ignore any words printed on the rectangles; you just want to name the color of the rectangle in front. In case you don't get the idea right away (the images flash pretty quickly), the animation will repeat itself and you'll have a chance to try again.

So, which part was easier, Part 1 or Part 2? You can give your answer in the poll, below the fold.

The basic Stroop Effect involves trying to name the color a word is printed in instead of reading a word. If the word itself is a color, then when the color the word is printed in is different than the color spelled out by the letters, we're slower name the color (we've made a quick demo of the effect here).

More recently, researchers have found that even if the task is to name the color of a shape, people are still slower when words naming different colors are printed nearby (for example, the word "red" next to a blue square). Amazingly, as long as they are scaled up in size to compensate for poorer peripheral vision, it doesn't matter how far away the words are printed -- the Stroop Effect still occurs.

What we're dealing with, then, is the issue of space- versus object-based attention. If the space between objects doesn't remove the Stroop Effect, then perhaps locating the distracting words on different objects will. That's what the animation above was designed to demonstrate: in Part 1, the distracting words were printed on the object you were attempting to describe; in Part 2, they were the same distance away, but printed on a different object. If object-based attention affects the Stroop Effect, then Part 2 should have been easier than Part 1.

Wühr and Waszak measured reaction times in a similar task much more precisely. A computer timed the precise moment when participants named the color of the rectangle, and the process was repeated over 288 trials. In their experiment, half the time the word matched the color of the rectangle, and half the time it did not. Here are the results:

The blue line shows the difference in reaction times when the word did not match the rectangle's color. As you can see, when the word was on the relevant object -- the one participants were asked to name -- reaction times were slower than when the word was on the irrelevant object. This was true even though the words were the same distance away from the fixation point at the center of the screen. The yellow line shows reaction times when the color and the word matched. As you can see, the reactions are much faster. When the word was printed on the irrelevant object, there was still a Stroop Effect: the non-matching words still had a slower reaction time, but the difference between the two reaction times was significantly smaller.

Two additional experiments, where the participants were asked to identify the color of the rectangle in back, and when distractor words were printed on the background instead of one of the rectangles, confirmed the result.

Wühr and Waszak conclude that object-based attention plays an important role in the Stroop task, possibly more important than that of space-based attention.

So, did our little demonstration work for you? Could you detect the effects of object-based attention without the benefit of measuring reaction time? Let us know about your experiences with this task in the comments.

Wühr, P., & Waszak, F. (2003). Object-based attentional selection can modulate the Stroop Effect. Memory & Cognition, 31(6), 983-994.