TEST

Digit Span, Forward and Backward

Immediate verbal memory and working memory in a few minutes

A short task that yields clinical information out of all proportion to its length. Comparing forward and backward does not measure the same thing twice: it estimates the cognitive cost of moving from holding to manipulating.

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WHAT THE TEST IS

A classic that endures for what it separates

Digit span is one of the classic measures of immediate serial recall, but its present value lies in a distinction rather than in tradition. Modern psychometric work shows that the forward and backward conditions share variance but are not equivalent, and should not be read as though they measured the same process (Richardson, 2007).

Forward digits report on the capacity to hold verbal material that is no longer present. Backward digits add mental reorganisation, attentional control and manipulation. Comparing the two gives the overall level and, above all, the cost of manipulation — the logic that makes the test sensitive across neurodevelopmental disorders, acquired brain injury, neurological disease and psychiatric presentations (Gregoire & Van der Linden, 1997).

Its brevity should not invite a simplistic reading: performance is shaped by age, education, clinical state, fatigue, sustained attention, affective symptoms and sensory impairment.

HOW IT IS ADMINISTERED

Listen to the string and repeat it; then reverse it

For the patient this is a fully auditory task. Sequences are delivered by standardised voice at one digit per second with uniform intonation, which removes examiner-to-examiner variability. They start at two digits and grow until two trials fail at the same level. The clinician records the response in either of two ways: verbatim, which allows qualitative error analysis (omissions, intrusions, partial reversals, perseverations), or as a dichotomous correct/incorrect when clinic time is tight.

WHAT THE TEST MEASURES

Indicators and their interpretation

Forward digit span

Longest string repeated correctly in forward order on at least one trial. Measures short-term verbal maintenance, auditory attention and phonological store efficiency.

High: sound capacity to hold brief verbal information without complex manipulation and, in context, effective immediate attention.
Low: a ceiling on maintenance capacity, lapses of immediate attention, interference from anxiety or fatigue, or contamination by hearing difficulties.

Forward hits

Total strings correct in the forward condition. Captures performance across the whole difficulty curve and is usually more sensitive than span alone.

High: efficient, stable maintenance across different levels of load.
Low: fragile or inconsistent performance, even where the span reached looks acceptable.

Backward digit span

Longest string reproduced in reverse order. Adds verbal working memory, sequencing and executive control on top of simple maintenance.

High: sound mental reorganisation and control over the verbal information being held.
Low: a higher executive cost, difficulty reordering verbal material, or inefficiency in updating and monitoring the response.

Backward hits

Total strings correct in the backward condition. Measures the cumulative effectiveness of verbal working memory under increasing load.

High: the person not only reaches a reasonable length but does so robustly.
Low: executive difficulty, slowing or fluctuation under manipulation demands; especially informative where the backward span is not low.

Backward/forward ratio

Backward hits divided by forward hits. Quantifies the cost added by active manipulation. In classic normative studies it sits between 0.60 and 0.77 and declines moderately with age.

High: backward performance falls little relative to forward: the additional executive cost is low.
Low: a high executive cost, with possible involvement of working memory and attentional control. An auxiliary index: it does not replace analysis of the whole profile.

Reliable Digit Span (RDS)

The sum of the longest length passed on two trials, forward and backward. It is the most widely used embedded performance validity indicator in the literature.

High: consistency; they lower the suspicion of suboptimal effort.
Low: very low values (typically ≤ 7 in adults) are associated with suboptimal effort or feigning, with cut-offs varying by population, age and education (Schroeder et al., 2012).

REFERENCES

Bibliography

Richardson, J. T. E. (2007). Measures of short-term memory: A historical review. Cortex, 43(5), 635–650.
Schroeder, R. W., Twumasi-Ankrah, P., Baade, L. E., & Marshall, P. S. (2012). Reliable Digit Span: A systematic review and cross-validation study. Assessment, 19(1), 21–30.
Baddeley, A. D. (2012). Working memory: Theories, models, and controversies. Annual Review of Psychology, 63, 1–29.
Baddeley, A. D., & Hitch, G. J. (2019). The phonological loop as a buffer store: An update. Cortex, 112, 91–106.
de Paula, J. J., Malloy-Diniz, L. F., & Romano-Silva, M. A. (2016). Reliability of working memory assessment in neurocognitive disorders: A study of the Digit Span and Corsi Block-Tapping tasks. Brazilian Journal of Psychiatry, 38(3), 262–263.
Alloway, T. P., Gathercole, S. E., Kirkwood, H., & Elliott, J. (2009). The cognitive and behavioral characteristics of children with low working memory. Child Development, 80(2), 606–621.