Digit Symbol Substitution Test (DSST)

Background

The Digit-Symbol Substitution Task (DSST) is a widely-used psychological assessment tool of processing speed. The basic task is is to quickly 'translate' digits into abstract symbols (or vice versa) using a translation key. The DSST is an integral part of most intelligence batteries and is particularly useful as a sensitive marker for brain dysfunction.

First versions of the test go back to the beginning of the 20th century. Arthur Otis, for example, included the DSST as a subtest in the Otis Group Intelligence Scale in 1918. To this day, the Gold standard of the DSST is the 'coding' subtest of the Wechsler Adult Intelligence Scale – Fourth Edition (WAIS-IV), a paper and pencil test. Participants receive a paper with rows of digits and have to draw the corresponding symbols under each digit (based on a provided translation key) within 2 minutes.

Computerized adaptations of the DSST are offered by several agencies, such as through the Pearson's Q-interactive platform, the National Institute of Health (NIH Toolbox) and Cogstate who offers a specialized DSST-Med version that uses medicine-related pairings instead of abstract symbols. The computerized DSST tasks are typically run on touchscreen devices such as ipads. Instead of presenting all the items at once - as in the paper and pencil administration- computerized adaptations often present the individual items in a 'scrolling' fashion: The interface only shows 9 boxes at a time. The first four boxes contain already made substitutions, and the last four boxes contain the substitution tasks to be made in the near future. The current target substitution box is the center one and is highlighted in some fashion. Once the participant decided on a response, the box 'scrolls' to the left and the next box, that moves into the center gets highlighted. New items move in on the right while old items leave the screen on the left, so that there are always 9 boxes.

The latest Millisecond computerized implementation of the DSST provides the task in such a 'scrolling' fashion. It can be run with keyboard input or mouse/touch input, and it can either ask participants to match digits to symbols (called Digit-Symbol Substitution by Millisecond) or match symbols to digits (called Symbol-Digit Substitution by Millisecond).

Task Procedure:

Participants work on 5 practice problems before the actual 2 minute testing period starts. In both keyboard and mouse/touch versions of the Millisecond DSST task, the target box is highlighted in purple and sits in the center. Once a response has been made, the next box moves into the target position and gets highlighted. A new box 'scrolls' in at the right end, whereas the 'oldest' box leaves the screen on the left side.

During practice, participants receive error feedback and need to correct their responses before moving on. During test trials, no feedback is provided and previous responses cannot be corrected. The test ends after 2 minutes or if no further matches are available.

What it Measures

The DSST is predominantly a measure of processing speed.

Psychological domains

• Processing Speed: Speed with which the brain can make simple matching operations
• Working Memory: Ability to keep symbol-digit pairs in memory to speed up the task
• Visuomotor Coordination: Ability to quickly scan the translation key information and make motor responses

Main Performance Metrics

• Correct Count: Number of correct matches made in the given timeframe
• Speed: Average time (in seconds) used to make a correct match

Psychiatric Conditions

DSST performance tends to be impacted in the following patient groups:

• Alzheimer’s Disease (AD)
• Schizophrenia
• Major Depressive Disorder (MDD
• Attention Deficit Hyperactivity Disorder (ADHD)