Transfer of training from one working memory task to another: behavioural and neural evidence

Abstract

N-back working memory (WM) tasks necessitate the maintenance and updating of dynamic rehearsal sets during performance. The delayed matching-to-sample (dMTS) task is another WM task, which in turn involves the encoding, maintenance, and retrieval of stimulus representations in sequential order. Because both n-back and dMTS engage WM function, we hypothesized that compared to a control task not taxing WM, training on the n-back task would be associated with better performance on dMTS by virtue of training a shared mental capacity. We tested this hypothesis by randomly assigning subjects (N = 43) to train on either the n-back (including 2-back and 3-back levels) or an active control task. Following training, dMTS was administered in the fMRI scanner. The n-back group performed marginally better than the active control group on dMTS. In addition, although the n-back group improved more on the less difficult 2-back level than the more difficult 3-back level across training sessions, it was improvement on the 3-back level that accounted for 21% of the variance in dMTS performance. For the control group, improvement in training across sessions was unrelated to dMTS performance. At the neural level, greater activation in the left inferior frontal gyrus, right posterior parietal cortex, and the cerebellum distinguished the n-back group from the control group in the maintenance phase of dMTS. Degree of improvement on the 3-back level across training sessions was correlated with activation in right lateral prefrontal and motor cortices in the maintenance phase of dMTS. Our results suggest that although n-back training is more likely to improve performance in easier blocks, it is improvement in more difficult blocks that is predictive of performance on a target task drawing on WM. In addition, the extent to which training on a task can transfer to another task is likely due to the engagement of shared cognitive capacities and underlying neural substrates-in this case WM.

Keywords: cognitive training; delayed matching-to-sample; n-back; prefrontal cortex; working memory.

FIGURE 1

Trial structure of dMTS. dMTS, delayed Matching-to-Sample Task. Participants completed 40 trials of identical structure. During encoding participants memorized the stimulus. During maintenance participants maintained the stimulus in working memory. During retrieval participants pressed the button corresponding to the stimulus (left, right) that matched the stimulus presented during encoding. The location (left, right) of the matching stimulus was counterbalanced across trials. The ITI varied randomly between 4,000 and 6,000 ms. ITI, inter-trial interval. Arrow indicates direction of trial.

FIGURE 2

The effects of session and level on n-back performance during training.

FIGURE 3

The relationship between degree of training-related improvement in (A) n-back and (B) 4-choice RT and dMTS performance. Whereas the degree of training-related improvement in n-back predicted dMTS performance, degree of improvement in 4-choice RT did not (see text).

FIGURE 4

Neural differences between experimental and control conditions during the maintenance phase of dMTS. There was greater activation in left IFG (A), right PPC (B), and cerebellum (C) in the experimental than control group during maintenance. SPMs rendered into standard stereotactic space and superimposed on to transverse (A), coronal (B), and saggital (C) MRI in standard space. Bar represents the corresponding T-score. dMTS, delayed Matching-to-Sample Task, IFG, inferior frontal gyrus, PPC, posterior parietal cortex.

FIGURE 5

Relationship between training-related improvement in 3-back and brain activation in the maintenance phase of dMTS. Activation in right lateral PFC and motor cortex covaried with degree of training-related improvement in 3-back. SPM rendered into standard stereotactic space and superimposed on saggital MRI in standard space. Bar represents the corresponding T-score. dMTS, delayed Matching-to-Sample Task.

FIGURE 6

Relationship between training-related improvement in 2-back and brain activation in the maintenance phase of dMTS. Activation in a distributed network including the superior parietal lobe and middle frontal gyrus (left, right, medial) covaried with degree of training-related improvement in 2-back. SPM rendered into standard stereotactic space and superimposed on transverse MRI in standard space. Bar represents the corresponding T-score. dMTS, delayed Matching-to-Sample Task.