Modernizing the Digit Symbol Substitution Test (DSST): A case for digital cognitive testing

The Digit Symbol Substitution Test (DSST) is one of five cognitive assessments in BrainCheck’s standard battery of tests, applied for the assessment of executive function. Development of the DSST began over a hundred years ago, while its usefulness for clinical purposes was more recently established during the 1940s. The DSST is prevalent throughout clinical neuropsychology, primarily due to its simplicity and accessibility. It is a commonly used test that eliminates the impact of culture, language, or education on test outcomes.  The diagnostic benefits were examined in a recent study of remote testing reliability (“Study Finds BrainCheck is Reliable When Taken Alone and Across Different Devices”).

 

How the assessment works

 

Originally a written executive function test using a single sheet of paper, this cognitive assessment required the test-taker to correspond symbols to numerals based on a key/legend atop the page (this has been adapted by our developers and neuroscience team for digital mobile surfaces). The individual would copy the symbol into spaces under a row of numbers; digit-symbol pairs could be simple punctuation marks, diacritic marks, or other simple symbols (for example,1/$, 2/& … 7/%, 8/*, 9/@). The number of symbols correctly matched within an allotted time (such as a range of 1.5 minutes to 2 minutes) comprises a score, and allows an examiner to apply norms or refer to research for comparison. 

World War II field physicians used the test to screen for brain damage in soldiers and after becoming incorporated in the Wechsler-Bellevue Intelligence Scale (WBIS) adoption grew, with the most recent iteration found in the Wechsler Adult Intelligence Scale-IV (WAIS). 

Digital platform cognition tests such as the DSST, whether administered in a clinical setting or remotely, offer value for longitudinal monitoring of patients. BrainCheck is used by many physicians to track memory decline in the elderly and to establish a baseline for early detection of dementia.  An advantage of the DSST within the digital platform is its ready accessibility for patients along with normative scoring, allowing a clinician to quickly see how a patient has performed relative to healthy individuals in his or her age group. 

 

How the DSST is used to determine cognitive function

 

The Digit Symbol Substitution Test is clinically useful because of its sensitivity to cognitive impairment from a wide range of causes, from the onset of dementia to traumatic brain injury or the effect of drugs on cognition. While further testing of the patient is called for in determining the nature of brain disease or dementia, the widespread clinical use of the DSST has produced a strong set of data that may be referenced as an aid.

Cognitive health (mental operations such as motor speed, attention, reading, manual dexterity) is indicated by higher scores on the digital-symbol substitution test. Associative learning (recall of the symbols without reference to the key/legend) is also assessed. An active effort by the test-taker to shorten performance time using working memory also exhibits executive functions (planning and strategy). While Complex attention is part of the performance outcome, the relative slowness of an elderly test-taker must be taken into account. This is one shortcoming of the pencil and paper test, compared with a digital version: copying speed involves manual writing of the symbols, but dragging or tapping the digital version lowers the disparities within the test caused by senescence in motor skills (Lezak MD. Neuropsychological Assessment.)

One study measured the effect of “active training” on performance, given that both elderly and young individuals improved during the same test day, and between days: they learned the pairs, and memorization affected the efficacy of the test. Thus, a randomized approach possible with a greater symbol set, using a digital platform can remove the risks of a pre-printed test which patients learn and recall in later visits. 

In another study, the authors used DSST to scrutinize electroencephalography (EEG) variables and cognitive performance, revealing that test performance was associated with the frontal lobes of the brain, supporting the theory that executive functioning plays a part in DSST performance.

The widespread use of the DSST within so many clinical environments has permitted surprising comparisons.  New attention to clinic-based, long-term monitoring of cognitive change in patients, and the sensitivity of the DSST to cognitive change, suggests significant clinical value may be obtained by understanding the meaningfulness of changing results on the digital symbol test. 

 

Conclusion

 

The Digit Symbol Substitution Test is effective and sensitive for detecting cognitive deficits, while not specific to the exact cognitive domains that are impaired in a wide range of populations.  It can be clinically meaningful when used as part of a defined battery, processing speed, executive functioning, and working memory. Neurology specialists and other clinicians find that even a redundant assessment or measurement helps formulate conclusions regarding cognitive dysfunctions. When your objective is to identify impairment (generally) or to track potential declines within a patient and not specifically diagnose or find cause, a brief and simple assessment like the digital version of DSST benefits any practice.

Scores on the DSST can correlate with the ability to perform everyday tasks, or indicate recovery from a loss of function. The test may also provide a pragmatic tool to detect relevant effects from treatment and medication. For physician decision-making, the DSST and the battery of tests within the BrainCheck neurocognitive assessment platform can serve and augment deeper evaluations at the individual patient level, with automatically recorded and tracked results. 

Contact us to learn more about how clinicians are using the BrainCheck platform to test Executive function in their patients and monitor patient scores easily and consistently.

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