Mind Gym: one classic experiment a day

01 Digit span (forward)

What you do: digits appear one at a time, about one a second. When they stop, type them back in the same order. You start with three; each correct answer adds one more. Two misses end the round. Your score is the longest list you got right.

The research: repeating back a string of digits is one of the oldest tasks in psychology. Joseph Jacobs, writing in 1887, is usually credited with the first digit-span experiments, which he called “prehension”. In 1956 George Miller pulled together many studies of immediate memory in a famous paper titled “The magical number seven, plus or minus two”, and showed that people stretch the limit by grouping items into chunks. Later, Nelson Cowan (2001) argued that when chunking and rehearsal are prevented the real limit is closer to about four chunks.

Jacobs (1887), Mind 12, 75–79. Miller (1956), Psychological Review 63, 81–97. Cowan (2001), Behavioral and Brain Sciences 24, 87–114.

02 Stroop colour-word

What you do: a colour word appears, such as RED or BLUE, printed in coloured ink. Press the button for the ink, not the word. Half the words match their ink and half do not. Your score is your typical (median) time on correct answers; the page also shows how much slower you were when word and ink disagreed.

The research: J. R. Stroop reported in 1935 that naming the ink colours of 100 conflicting colour words took 74% longer than naming the colours of 100 plain squares (110.3 against 63.3 seconds on average), while conflicting ink barely slowed the reading of the words. Reading is so practised that the word arrives whether you want it or not. It is one of the most reproduced effects in psychology; Colin MacLeod’s 1991 review covered more than half a century of it.

A note on colour vision: this exercise uses red, green, blue and yellow. If you have a colour-vision difference, red and green may be hard to tell apart, and the score will not mean much. Skip it with no loss.

Stroop (1935), Journal of Experimental Psychology 18, 643–662. MacLeod (1991), Psychological Bulletin 109, 163–203.

03 Simple reaction time

What you do: watch the circle. After a random wait of between one and a half and four seconds it lights up; press the space bar or tap it as fast as you can. Pressing before the light, or faster than 100 milliseconds, counts as a guess and that trial is repeated. Ten trials; your score is the median.

The research: in 1868 the Dutch physiologist F. C. Donders compared simple reactions (one signal, one response) with choice reactions (several signals, each with its own response). Choice reactions were slower, and Donders reasoned that subtracting one time from the other measured how long the extra mental step took. This “subtraction method” is usually treated as the starting point of measuring the mind by timing it.

About the number: screens, keyboards, touchscreens and browsers each add their own delay, often tens of milliseconds and different on every device. Compare your reaction times only with your own earlier results on the same device.

Donders (1868), translated by W. G. Koster as “On the speed of mental processes”, Acta Psychologica 30 (1969), 412–431.

04 Visual search

What you do: find a cyan circle in a field of shapes, and answer “it’s there” or “not there” as fast as you accurately can. Sometimes the other shapes are all violet circles, so only the colour differs. Sometimes they are a mix of violet circles and cyan squares, so you have to find the one item with the right combination of colour and shape. Fields have 8 or 24 shapes.

The research: Anne Treisman and Garry Gelade (1980) found that a target differing from its neighbours by a single feature, such as colour, “pops out”: search time hardly changes as more distractors are added. A target defined by a conjunction of two features is slower to find, and the time grows roughly in step with the number of items. Their feature-integration theory proposed that attention is needed to bind features into objects. Later work has softened the sharp line between the two kinds of search, but you can often see the basic pattern in your own results.

Treisman & Gelade (1980), Cognitive Psychology 12, 97–136.

05 2-back

What you do: letters appear one at a time, one every two and a half seconds. Press the space bar or the Match button whenever the letter is the same as the one two letters before. Twenty-two letters; your score is the percentage of the twenty scorable letters you handled correctly (pressing on a match, holding back otherwise) — the first two cannot match, so they are not scored.

The research: Wayne Kirchner introduced this kind of task in 1958, using a display of lights that changed rapidly, and found that older participants’ performance fell off sooner than younger participants’ as the task got harder. The “n-back” has since become one of the standard working-memory tasks in psychology and brain-imaging research.

On “brain training”: n-back practice became popular as a way to raise intelligence. A 2016 meta-analysis by Monica Melby-Lervåg, Thomas Redick and Charles Hulme, covering 87 publications, found reliable short-term gains on other working-memory tasks but no convincing evidence of gains in abilities beyond them, such as reasoning, reading or arithmetic. The authors concluded that the effects are short-term and specific and do not generalise to “real-world” cognitive skills. Practice makes you better at the 2-back. That is all it promises here.

Kirchner (1958), Journal of Experimental Psychology 55, 352–358. Melby-Lervåg, Redick & Hulme (2016), Perspectives on Psychological Science 11, 512–534.

06 Change detection

What you do: a set of coloured squares appears for half a second, disappears for one second, then comes back. Either nothing has changed or one square has a new colour. Answer “same” or “changed”. Half the displays have 3 squares and half have 6. Your score is the percentage you got right.

The research: Steven Luck and Edward Vogel (1997) used this design to measure visual working memory. They reported that people could hold only about four colours or orientations at once, yet could hold both the colour and orientation of four objects just as well, which suggested the memory stores whole objects rather than loose features. Most people find six squares noticeably harder than three.

Luck & Vogel (1997), Nature 390, 279–281.