TEST
Symbol Digit Modalities Test (SDMT)
The most sensitive marker of processing speed
A brief symbol-digit matching test (Smith, 1982) considered the most sensitive measure of cognitive slowing in multiple sclerosis and included in the BRB, MACFIMS, and BICAMS batteries. The digital version adds response time per item, variability, fatigue, learning curve, and incidental memory.
9
SYMBOL-DIGIT PAIRS
120 s
DURATION
2
INCIDENTAL MEMORY TRIALS
ms
RT PER ITEM
WHAT THE TEST IS
A pure measure of processing speed
Commercialized by Smith (1982), the SDMT asks the person being assessed to match a series of geometric symbols with digits from 1 to 9 as quickly as possible, using a reference key visible at all times. Its speed (≈5 minutes) and sensitivity have made it the leading test for detecting early cognitive slowing.
Performance integrates psychomotor speed, sustained attention, visual search, working memory, and coding efficiency (Lezak, 2004; Strauss et al., 2006; Ryan et al., 2020). In MS, Benedict et al. (2017) validate it as the main cognitive outcome. The digital version adds metrics that are inaccessible on paper: response time per item, variability, intra-task fatigue, and incidental learning of the associations.
HOW IT IS ADMINISTERED
Associate symbols with digits for 120 seconds
A fixed key with 9 symbol-digit pairs is displayed at the top of the screen. Below it, a row of symbols is highlighted one by one with a blue frame. The person being assessed enters the corresponding digit using a physical keyboard (PC) or direct tap (tablet), for 120 seconds from the first response.
At the end of the main phase, and without prior warning, an incidental memory task is administered: the 9 symbols are presented in two random orders and the person is asked to recall the associated digit. This assessment of implicit learning complements the speed measure without lengthening administration (Joy et al., 2003).
WHAT THE TEST MEASURES
Indicators and their interpretation
The digital version makes it possible to reconstruct the full process: speed, accuracy, attentional stability, fatigue, and incidental learning.
Number of correct responses
Symbols correctly matched in 120 s. Combines processing speed, sustained attention, and visual accuracy.
High correct responses: cognitive speed and attentional control.
Low correct responses: slowing, distractibility, or difficulty internalizing the key.
Number of errors
Responses that do not match the key. Reflects attentional accuracy and possible impulsivity.
High errors: sustained attention failures, responses without verification.
Low errors: careful and focused execution.
Mean response time
Mean interval between correct responses. A direct measure of psychomotor and cognitive speed.
High time: cognitive slowing, fatigue, impairment.
Low time: fast and efficient processing.
RT variability
Standard deviation of response times. Indicates attentional consistency during the test.
High variability: attentional lapses, lack of concentration.
Low variability: sustained focus and stable performance.
Learning and fatigue rate
Learning: improvement in RT in the first 10 items. Fatigue: difference between the final and initial quartile.
Flat learning: difficulty internalizing the key.
Positive fatigue: progressive slowing at the end.
Incidental memory
Correct responses in the two surprise post-task trials (max. 18) and consistency between trials (0-9). Measures implicit learning of associations.
High scores: effective incidental encoding.
Low scores: fragile learning, limited attentional resources.
REFERENCES
Bibliography
- Smith, A. (1982). Symbol Digit Modalities Test: Manual. Los Angeles: Western Psychological Services.
- Benedict, R. H., DeLuca, J., Phillips, G., LaRocca, N., Hudson, L. D., & Rudick, R. (2017). Validity of the Symbol Digit Modalities Test as a cognition performance outcome measure for multiple sclerosis. Multiple Sclerosis Journal, 23(5), 721–733.
- Ryan, J. J., et al. (2020). SDMT: A review of normative data and psychometric properties. (Review).
- Salthouse, T. A. (1996). The processing-speed theory of adult age differences. Psychological Review, 103(3), 403–428.
- Joy, S., Fein, D., & Kaplan, E. (2003). Decoding digit symbol: Speed, memory and visual scanning. Assessment.
- Peña-Casanova, J., et al. (2009). NEURONORMA Project: Normative data for SDMT. Archives of Clinical Neuropsychology, 24(4), 321–341.
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The gold standard for detecting early cognitive slowing, now with response time per item, variability, and incidental memory.
