In simple terms
The Digit Symbol Substitution Test (DSST) is a brief timed coding task (typically 90–120 seconds) in which people use a key to pair digits with symbols and produce as many correct matches as possible.
The score on this task is the number correct within the time limit.
This task involves:
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fast visual scanning,
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sustained and selective attention,
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working memory/associative learning (as pairs become familiar),
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and fine motor/response speed
From dementia and depression clinics to drug studies and online trials, the DSST is a good index of processing speed and related executive demands that also predicts everyday functioning.
Research history
The Digit Symbol Substitution Test goes back more than 100 years. In those days as a so called paper & pencil test. For example, the famous early American psychologist Thordike already used a version of this task and since then has been used in numerous tasks and intelligence tests.
For an academic overview of the task, I recommend Judith Jaeger’s 2018 review paper. In her paper, Figure 4 is a nice overview of effects, such as the positive effect of caffeine on DSST performance in healthy volunteers and a negative effect of caffeine deprivation.
Versions
There are quite a few digit substitution tests around. The current experiment is based on the study by Williamson and colleagues (2023).
Task design
In this task, participants perform the 120 second version using symbols. At the top of the screen, the "key" is shown, that is, a lookup table of what symbols the digits 1 to 9 are matched to.
In the middle, numbers are show to be matched with a further display of symbols at the bottom of the screen. It is easiest just to try the task yourself below to see how it works.
| Note that the first 3 trials are actually not scored or timed, because people need some training. Participants are not aware of this as a separate training block. |
Scores
The score on the task is the simply the amount of correctly matched symbols. Thus, a score of 50 means that a participants matched 50 symbols in the 2 minute period of the task.
Based on their figures, in their study 2 (Willianson et al., 2022, Figures 2 and 3), here are some statistics:
In their Study 1 (Figure 2) with a wide age range (from 18 to 80 years old), the median performance was 48 points, ranging from 25 points up to 82 points. In their Study 2 (Figure 3), unimpaired older adults had a median of 44 points, ranging from 25 to 63 points. Older participants with Mild Alzheimer’s disease had a median scorre of 17 points, ranging from 2 to 47 points.
Run the demo
Data output file
| You do not need this information, unless you want to understand the output data file. You can ignore this if you just want to find out your own score. This is only necessary if you want to carry out the experiment with multiple participants. |
| In PsyToolkit, the data output file is simply a textfile. The save line of the PsyToolkit experiment script determines what is being saved in the data output file. Typically, for each experimental trial, you would have exactly one line in your text file, and each number/word on that line gives you the information you need for your data analysis, such as the condition, response speed, and whether an error was made. |
Meaning of the columns in the output datafile. You need this information for your data analysis.
The first two line of the datafile is actually no participant data, but show the order of symbols in the key (ORDER_KEY) and the order of the symbols in the bottom panel from which people select (ORDER_BOT). If you want, you could somehow consider this in your analysis but most people won’t.
| Colum | Meaning |
|---|---|
1 |
Type of data (for first to lines, ORDER_KEY and ORDER_BOT, and after that DATA) |
2 |
For DATA lines, this is the name of the block (training or real) |
3 |
training trial, starts with 3, 2, 1, and in real data block this is 0. |
4 |
How many repeats of people selecting wrong symbol. If 0, people made no mistake before, if 1, this is the first repeat, etc (see notes below) |
5 |
The current digit in the yellow box that needs to be matched |
6 |
The reaction time |
7 |
Correct or incorrect (1=correct, 2=wrong, 3=timeout) |
8 |
The average RT so far in the experiment |
9 |
The total count of correct responses so far. |
10 |
Total number of errors so far |
Note about repeats. On each trial, people respond to a digit in the yellow box. Even when people click the wrong symbol at bottom (followed by the red cross error feedback), we go ahead with a new trial. Participants would not experience this a new trial, but in the PsyToolkit code and data file, this is a new trial. Thus, on trials when people make no mistake, there will be 0, but if this number is 1, it means this is the second time people tap on a symbol for selection.
How to analyze data from experiments
If you use the analyse section, you can simply get the total score (how many digits matched altogether) by setting the following:
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Dependend variable: 9
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Only include last n lines (set to 1)
Check out the source code
Download
| If you have a PsyToolkit account, you can upload the zipfile directly to your PsyToolkit account. |
If you want to upload the zipfile into your PsyToolkit account, make sure the file is not automatically uncompressed (some browsers, especially Mac Safari, by default uncompress zip files). Read here how to easily deal with this.
Further reading
Jaeger, J. (2018). Digit symbol substitution test: the case for sensitivity over specificity in neuropsychological testing. Journal of clinical psychopharmacology, 38(5), 513-519. Read for free
Williamson, M., Maruff, P., Schembri, A., Cummins, H., Bird, L., Rosenich, E., & Lim, Y. Y. (2022). Validation of a digit symbol substitution test for use in supervised and unsupervised assessment in mild Alzheimer’s disease. Journal of Clinical and Experimental Neuropsychology, 44(10), 768-779. Link to publisher