Alpha-band activity reflects reduction of mental effort in a comparison task: a source space analysis

Comparison processes contribute to many core phenomena of social cognition research. Whenever humans judge a given target, they rely on comparisons with a pertinent standard. We propose that comparison processes may be so ubiquitous because they reduce mental effort. To investigate this possibility, we used dense-array Electroencephalogram (EEG) recordings together with a minimum norm source projection approach. As the dependent variable, we examined changes in parietal alpha (8-12 Hz) amplitude during a judgment task. Spectral changes in the alpha frequency range have been reliably related to attentional load, cognitive arousal, or mental effort. Two groups of participants (n=22) were procedurally primed to solve a series of target judgments in a more comparative (experimental group) versus more absolute (control group) manner. While the participants performed the critical judgment tasks, we recorded changes in alpha amplitude. Continuous EEG was transformed into a spherical source space using the minimum norm (L2) estimate and spectral changes were subsequently calculated in the source domain. Statistical parametric mapping in combination with permutation statistics was employed to map regions showing significant group differences. Results demonstrate that comparative processing was associated with smaller changes in alpha amplitude than absolute processing. This difference was most pronounced at parietal source locations, where alpha reduction was at a maximum. Temporal analysis suggested that this effect was present particularly during task preparation and execution. We conclude that comparative information processing may reduce mental effort in judgment tasks.