Measuring working memory span with WAIS-IV: Digit sequence is the superior span test

The Digit Span test has been part of the Wechsler tests from the first version. In the WAIS-IV the Digit Span Sequencing subtest (DSS) was introduced and in the forthcoming WAIS-5 working memory span will also be measured in the visual modality. The present study analyzes WAIS-IV Digit Span, Letter- Number Span (LNS) and WMS-III Spatial Span (SS) performance in a mixed clinical sample, expecting to find that Digit Span Forwards (DSF) lacks sensitivity to the Working Memory impairment evident in D-KEFS Trail Making Test-4 (TMT-4) scores ≤1 SD below normative means in the sample. The results showed DSF score above normative means, Digit Span Backwards (DSB) around mean, while SS and LNS was slightly impaired and DSS impaired at the same level as TMT- 4. A double dissociation was observed in DSF and SS performance between subjects with Language- and Non-verbal learning disorders. Most subjects scored in the average range on the LNS-span and high kurtosis reduced sensitivity. Taking LNS and TMT-4 as criterion measures of WM, regression analyses showed no unique contribution of DSF to the variance in these tests. The study supports prior critiques regarding the composite Digit Span measure and demonstrates that the DSS are more effective than the current version of LNS in identifying reduced Working Memory capacity.