台大心理系

The mechanism by which we search for target items amongst task-irrelevant non-targets or distracters is one of the most extensively studied areas of cognitive neuroscience. One important influence on these mechanisms is the perceptual characteristics of the stimuli. In this study, we investigated how neural activity is modulated as we search for targets, using electroencephalography (EEG), and in particular whether this is sensitive to targets and non-targets (T-N) similarity. Participants searched for a pre-defined target letter (X or N) among five non-targets in a visual search task. The T-N similarity was manipulated with three levels: high, middle, and low. We tested for the influences of T-N similarity on an event-related potential (ERP, e.g., N2pc) and lateralised power and phase-locking value (PLV) in the alpha band (9-13 Hz). We conducted a model-based analysis to test for the relationship between the N2pc and the RTs on a trial-by-trial basis and also tested the relationship between the N2pc and lateralised alpha power and also with PLV using the Jackknife approach. The behavioural data showed increasing T-N similarity would decrease the accuracy and increase the RTs. Next, we observed the N2pc amplitude was reduced and occurred later when targets and distractors were highly similar to one another, relative to the low and middle similarities. Moreover, we observed close relationship between N2pc and behaviour. N2pc amplitude was inversely correlated with the RTs across all similarities – the bigger the N2pc, the faster the RT on that trial. Whilst no significant effect on power estimate was observed, we found a significant lateralisation of alpha PLV preceding the N2pc for high-similarity trials. However, no such effect was observed for the middle- and low-similarity trials. Finally, we found the relationship between lateralised alpha PLV and N2pc for high similarity – on high similarity trials, the stronger the PLV, the bigger the N2pc. In consistent with recent evidence (Kuo, Nobre, Scerif, & Astle, 2016), we have isolated two electrophysiological effects that are sensitive to competition during visual search. We provided evidence showing that efficient search requires effective gating, which is reflected by alpha lateralisation. After that target representations can be accessed and selected for making search decisions. Both steps are influenced by the competitive interaction between targets and non-targets.