Try to name the ink colour of these words as quickly as you can: RED, YELLOW, BLUE. Most people slow down and some make a mistake, since the word you read keeps competing with the colour you are asked to name. The effect has a name, the Stroop effect, and it is the most famous demonstration of how hard it is to switch off an automatic process.
The experiment
John Ridley Stroop described it in 1935 in his doctoral research. He had participants read colour words printed in black ink, name colour patches, and name the ink colour of colour words printed in a different colour. The last task was clearly slower, by several seconds across a page of items. Reading, which you learned years ago, has become so automatic that you cannot prevent it from happening when you see a word. Naming a colour is less practised, so reading wins the race and you have to override it.
In the laboratory it is measured as the difference in reaction time between incongruent trials (the word RED in blue ink) and congruent trials (RED in red ink). A difference of about 50 to 150 milliseconds per item is common, and it is quite reliable from person to person.
What it tells us
- Automatic versus controlled processing. Some processes, such as reading, are run without your intent. Others, like naming a colour while ignoring the word, require effort.
- Executive control. The task needs you to hold a goal ("name the colour"), suppress an unwanted response and monitor errors. That is why Stroop-like tasks are used to study the frontal lobes, ageing, ADHD and the effect of fatigue.
- Interference is symmetric only in part. The colour hardly disturbs reading the word, but the word strongly disturbs naming the colour. The more automatic process wins.
Some variants use other conflicts: the Simon task, where the position of a stimulus competes with the correct response; the flanker task, where surrounding arrows point the wrong way; and the emotional Stroop, where threat words slow down naming in anxious people. All of them measure how well your attention filters out what it should ignore.
Selective attention in daily life
The same ability is needed to read in a noisy cafe, to find a face in a crowd or to drive while a passenger talks. Attention works like a spotlight combined with a filter: what falls in the beam is processed in depth, the rest only superficially. Wide beams are quick but shallow, narrow beams are slow but accurate. The question is how well you can place the beam and keep it there.
Visual search and Schulte tables
In a visual search task you look for a target among distractors. If the target differs in one clear feature, such as colour, it seems to "pop out" and the time hardly depends on the number of distractors. If the target is defined by a combination of features, you need to check the items one by one and the time rises with the number of items. This is the basis of Anne Treisman's feature integration theory from 1980.
A Schulte table is a grid of numbers in random positions that you have to find in order, from 1 upwards. The German psychiatrist Walter Schulte used such tables to test the attention and endurance of patients, and Russian and Soviet sport psychology adopted them to train athletes. A fast time means that you can keep a plan, scan the grid with efficient eye movements and avoid re-checking squares. A common tip is to look at the middle of the grid and use your peripheral vision instead of moving your eyes to every number. Our Number Order game uses the same principle with different grid sizes.
Go/No-Go and impulse control
In a go/no-go task, you respond to some signals and withhold on others. It tests response inhibition, the ability to stop an action that is already prepared. Errors of commission (pressing when you should not) become more frequent as the go signals are more common, since the response becomes a habit. In our game you can feel this: after a long run of green, the red sneaks up on you.
Training attention
As with working memory, claims of wide transfer should be treated with care. Practice improves the specific task, and strategies carry over: reduce the field you attend to, keep your eyes still and respond only after you have identified the target. What does have a big effect on performance is the state you are in. Sleep, breaks and not multitasking will do more for your focus than any game.