SHIFTING ATTENTION BETWEEN GLOBAL FEATURES AND SMALL DETAILS - AN EVENT-RELATED POTENTIAL STUDY

In two experiments, large letters H or Z composed of small letters (al so H or Z) were presented. Subjects had to make a two-choice motor res ponse (e.g. H - left key, Z - right key). A cue presented 500 ms befor e the letter indicated which level (global or local) was relevant. In Experiment I, a third letter (T) sometimes appeared either at the cued or the non-cued level; in the former case, subjects had to shift thei r attention and to respond to stimulus features located at the non-cue d level. The interference effect (RT delay in response to incongruent stimuli as compared to congruent ones) was larger when the local, rath er than global, level was cued. A slow anterior negativity preceding g lobally-cued stimuli and shorter N1 and P2 ERP component latencies to these stimuli indicated better preparation for processing of global, a s compared to local, stimulus features. The shift from local to global focus yielded a larger increase of RT, error rate, and of the P600 la tency than the global-to-local shift. The P600 latency changes were pa rallel to those of RT. In Experiment II, the attentional shift was pro voked by stimulus color: red-colored letters meant that the cue was in valid, and thus, subjects had to respond to the non-cued level. Neithe r the interference nor the attentional shift demonstrated any asymmetr y between the global and local levels. ERPs also did not differ substa ntially after local and global cues. In the condition demanding a shif t of focus (invalid cue, incongruent letter), a positive deflection of the lateralized readiness potential indicated the activation of the w rong response channel. The large RT increment in this condition was no t accompanied by an increase of the P600 latency. Two possible mechani sms of attentional shift may be proposed, the first related to percept ual processes (e.g. an additional visual search), and the second, to t he competition between two response intentions. (C) 1997 Elsevier Scie nce B.V.