The primary aim of the present report was to determine whether the n-back task is a valid clinical neuropsychological measure of working memory. To answer this question, we examined the convergent validity between n-back accuracy and digit span backward, an established working memory measure. Owing to the task's time-pressured design, we also examined the relationship between n-back accuracy and measures of information processing speed. Correlational analyses revealed that n-back accuracy (at each of the 0-, 1-, 2-, and 3-back loads) did not significantly correlate with digit span backward. A series of correlational analyses between n-back accuracy at each level, TMT A, Stroop word reading, and Stroop color naming found a significant correlation between 2-back accuracy and TMT A only.
An additional study aim was to compare n-back performance and performance on standard measures of working memory and processing speed in control and PD groups to examine the n-back's ability to detect between-groups differences. We found that the two groups did not significantly differ with respect to digit span forward or backward, TMT A, Stroop word reading, or Stroop color naming. In contrast, the PD group showed significantly poorer accuracy on the n-back as well as a trend toward slower RTs, suggesting the n-back may be able to detect subtle group differences in performance.
One potential explanation for the surprising lack of relationship between n
-back accuracy and digit span backward may be that the n
-back is a visually presented, as opposed to aurally presented, working memory task. This may prime participants to use a mental imagery strategy rather than a verbally mediated strategy. One way to examine this hypothesis would be to include a measure of visual working memory, such as a spatial span task or self-ordered pointing task (Petrides & Milner, 1982
) to see if significant correlations emerge between the n
-back and these tasks.
The present results are somewhat consistent with n
-back findings from Parmenter, Shucard, Benedict, and Shucard (2006)
. These authors conducted a principal components analysis of performance on the n
-back, Paced Auditory Serial Addition Task (PASAT), and other measures of executive function in a combined sample of patients with multiple sclerosis and healthy controls. They found that n
-back accuracy and RTs, PASAT accuracy, and TMT A and B all loaded on a common factor, which they conceptualized as speeded information processing. Digit span forward and backward loaded together on a nonspeeded working memory factor. This is similar to our finding that n
-back performance showed a stronger relationship with a test of speeded information processing (TMT A) than with a nonspeeded test of working memory (digit span backward).
Our findings are consistent with past reports of a weak association between n
-back performance and performance on other working memory tasks, including reading span, complex span tasks, and mathematics-based tasks (Kane, Conway, Miura, & Colflesh, 2007
; Oberauer, 2005
; Roberts and Gibson, 2002
). On the basis of these findings, Kane and colleagues (2007)
concluded that “the n
-back has too long been used by cognitive neuroscientists without serious efforts to assess its construct validity, and now we may have to reappraise past findings (p. 621).” Indeed, the results of the current study do not support the validity of the n
-back as a pure measure of working memory, at least in our limited combined sample of controls and PD patients.
The present study has several limitations, including a small sample size (thus it may be underpowered), a predominantly male sample, and a highly educated PD group consisting of candidates for DBS surgery, which may not represent the typical PD patient. The PD group was also tested while on dopaminergic medication, which may have improved task performance given prior findings that dopamine modulates working memory (Costa et al., 2003
In conclusion, our study did not demonstrate convergent validity between the n-back and a known working memory measure, digit span backward. Instead, our results suggest that n-back accuracy may rely more on information processing speed or motor speed than on working memory in a PD sample, as evidenced by a correlational relationship with TMT A (albeit at the 2-back load only). Our study argues against using the n-back as a measure of working memory in a PD population; however, our results suggest that n-back accuracy scores may be useful in detecting subtle differences in cognitive functioning between control and PD groups. Until further validation studies are conducted that clearly elucidate the constructs underlying n-back performance, we recommend clinicians continue to assess working memory in PD through established tests that do not rely upon speeded processing, such as digit span and letter–number sequencing.