Studies of human intelligence provide strong evidence for the neural efficiency hypothesis, which suggests more efficient brain functioning (i.e., less or more focused activation) in more intelligent individuals. Recent studies have specified the scope of the neural efficiency hypothesis by suggesting that the relationship between brain activation and intelligence only holds true for problems of moderate difficulty and can be altered through training and is only found in frontal brain regions. We investigated the moderating roles of task difficulty and training on the neural efficiency phenomenon in the context of working memory (WM) training.

In two studies of 54 participants (study 1) and 29 participants (study 2), cortical activation was assessed by means of electroencephalography (EEG), or more precisely by means of event-related desynchronization (ERD) in the upper alpha band. ERD was assessed during the performance of WM tasks in a pre-test – training – post-test design, comparing groups of lower and higher intelligence.

We found supportive evidence for the neural efficiency hypothesis only in moderately difficult WM tasks in frontal brain regions, even in the absence of performance differences. There was no effect of intelligence on the simple or highly demanding, adaptive WM tasks. In the latter task, however, an intelligence-related difference emerged at the behavioral level, but training did not modulate the relationship between intelligence and brain activation.

These results corroborate the moderating role of task difficulty in the neural efficiency hypothesis in the context of WM demands and suggest that training does not impact the neural efficiency phenomenon in the context of WM demands.