Hemispheric lateralization of posterior alpha reduces distracter interference during face matching

• Face matching performance in an attended hemifield is impaired by unattended face distracters.
• Average alpha power is smaller contralateral to targets, and larger contralateral to distracters.
• Only alpha-power changes in the left hemisphere predict individual matching performance.
• Up/down regulation of left hemisphere alpha for distracters/targets resp. boosts performance.
• The dominance of left alpha in face matching shows that the role of alpha interacts with task demands.

Previous M/EEG studies on visuospatial attention have shown that attending to one hemifield, while ignoring the other, leads to a decrease in occipito-parietal alpha power contralateral to the target and a concurrent ipsilateral alpha increase. Here, we tested whether this alpha modulation facilitates the processing of attended faces in the presence of distracters. Face processing was tested in a match-to-sample task, in which participants matched a target face in a cued hemifield to a previously seen sample face. The target faces in the cued hemifield were presented together with a distracter face (intact or scrambled faces) in the other hemifield. The behavioral data indicated a larger impairment of matching performance when the distracter was another face, rather than a scrambled face. We hypothesized that enhanced alpha power contralateral to the distracter would enhance target matching by decreasing the interference from similar distracters. We found this effect, but only in the left hemisphere. Moreover, we found that with targets contralateral from the left hemisphere, a down-regulation of relative alpha power in the left hemisphere also correlated with increased target matching performance. Hence, the left hemisphere could protect the target from distracter interference either by decreasing alpha power in response to a contralateral target, or increasing alpha power to a contralateral distracter. Remarkably, alpha power in the right hemisphere was not predictive for matching performance. These findings support the hypothesis that alpha modulations contribute to the suppression of task-irrelevant information, but suggest a dominant role herein of the left hemisphere during face matching in the presence of distracters.