Modulations in alpha oscillations (∼10 Hz) are typically studied in the context of anticipating upcoming stimuli. Alpha power decreases in sensory regions processing upcoming targets compared to regions associated with distracting input, hereby promoting processing of relevant information while suppressing irrelevant information. In the current EEG study using healthy human volunteers we examined whether modulations in alpha power also occur after the onset of a bi-laterally presented target and distractor. Spatial attention was manipulated through spatial-cues and feature- based attention through adjusting the color-similarity of distractors to the target. Consistent with previous studies, we found that informative spatial cues induced a relative decrease of pre-target alpha power at occipital electrodes contralateral to the expected target location. Interestingly, this pattern re-emerged relatively late (300-750 ms) after stimulus onset, suggesting that lateralized alpha reflects not only preparatory attention, but also ongoing attentive stimulus processing.
Uninformative cues (i.e., conveying no information about the spatial location of the target) resulted in an interaction between spatial and feature-based attention in post-target alpha lateralization. When the target was paired with a low-similarity distractor, post-target alpha was lateralized (500-900 ms). Crucially, the lateralization was absent when target selection was ambiguous because the distractor was highly similar to the target. Instead, during this condition mid-frontal theta was increased, indicative of conflict processing. Behaviorally, the degree of alpha lateralization was negatively correlated with the RT distraction cost induced by target-distractor similarity. These results suggest a pivotal role for post-stimulus alpha lateralization in protecting sensory processing of target information.
Significance Statement: A substantial amount of research has been dedicated to elucidating the role of alpha oscillations in preparation for upcoming targets and distractors. Considerably less research has focused on alpha activity after presentation of those stimuli. Using a novel paradigm, in which spatial attention was manipulated using informative and uninformative spatial cues and feature-based attention using distractors that where varied parametrically in color-similarity to the target, we show that post-stimulus alpha hemispheric lateralization protects target processing after spatial and feature-based target selection. Modulations in alpha power can therefore be regarded as a general mechanism for direction of attention, not only for pro-active but also for reactive attention.
The authors declare no conflict of interest.
This work was supported by the National Science Foundation (BCS-1230377-0) to JG and a Veni Grant from the Netherlands Organization for Scientific Research (NWO) to AM.