SDMT: Symbol→Digit
Encode symbols by lookup table
About this trainer
The SDMT shows you a key that pairs nine abstract symbols with the digits 1 to 9. A row of symbols then appears, and your job is to type the matching digit for each one, as many as you can before the timer runs out. It is a pure speed task: the rule is trivial, but you have to apply it again and again without slowing down.
What it develops
It mainly trains processing speed, the rate at which you take in a stimulus, look up the right response and act on it. Because you constantly glance back at the key, it also leans on visual scanning, sustained attention and a bit of short-term memory once you start holding pairs in your head.
History
The symbol-digit format grew out of the digit-symbol substitution task used in intelligence batteries since the early 20th century, including the Wechsler scales. Aaron Smith reworked it in the 1960s so the person says or writes the digit rather than drawing the symbol, which made it faster and language-light. After its 1973 publication the SDMT spread widely in neuropsychology and, from the 1990s, became a standard measure of cognitive speed in multiple sclerosis research.
Who created it — and when
It was created by the American neuropsychologist Aaron Smith, who piloted it through the 1960s and published it formally in 1973 through Western Psychological Services in Los Angeles. The underlying substitution idea is older and not his invention, but this particular symbol-to-digit version is his.
How to train
Burn the key into memory early so you can stop looking back; the first few pairs you memorise give the biggest speed gain. Keep a steady left-to-right rhythm instead of racing and stalling, and resist the urge to double-check answers you already know. If you misremember a pair, re-anchor it deliberately rather than guessing repeatedly.
How long to practise
Short bouts work best: a few timed rounds of one to two minutes, a handful of days a week. The task is mentally taxing, so quality drops fast once you fatigue; stop when your speed plateaus within a session rather than grinding on.
Evidence base
Evidence is strongest for exactly what you would expect, which is that you get faster at symbol-digit substitution itself, and the clinical version reliably tracks processing-speed changes in conditions like multiple sclerosis. What it does not cleanly show is broad transfer: practising it does not reliably make unrelated thinking, memory or general intelligence better, and researchers stress the test is not process-pure, so improvement can come from memorising the key as much as from faster processing. Treat the score as a measure of this skill, not proof of a sharper mind overall.
Recommendations
Memorise the key in the first ten seconds, then keep a smooth unbroken pace; the people who pause to verify lose more time than the few errors would have cost.
FAQ
Is this the same as a memory test?
Not really. Memory helps once you learn the key, but the core demand is speed of matching, so it sits closer to attention and processing speed than to memory.
Why does my score jump around so much between rounds?
Short timed speed tasks are naturally noisy, and how well you remember the key on that run matters a lot. Look at your average over several rounds rather than any single score.
Will training this make me smarter in everyday life?
It makes you faster at this kind of substitution and similar speeded tasks. There is little solid evidence it lifts general intelligence or unrelated skills, so enjoy the sharpening here without expecting broad spillover.
Variants
There is an oral version, where you say the digit instead of typing, used clinically to remove handwriting speed from the result, and a written version where you fill the digits in by hand. The closely related Digit Symbol Substitution Test reverses the demand by having you draw the symbol, and computerised editions add randomised keys and rolling difficulty.