Electrophysiological correlates of early attentional feature selection and distractor filtering.

Using electrophysiology, the attentional functions of target selection and distractor filtering were investigated during visual search. Observers searched for multiple tilted line segments amidst vertical distractors. In different conditions, observers were either looking for a specific line orientation ("feature-based" selection), or for any tilted line ("salience-based"). The search array could contain both left- and rightward tilted lines simultaneously (requiring spatial filtering) or only one line type (no filtering). The amplitude of the P1 event-related potential component was reduced during feature-based selection, compared to salience-based selection. The N1 showed a similar effect, at least when filtering was required. Amplitudes were also somewhat reduced when competing nontarget stimuli required filtering. Interactions between selection and filtering became stronger on the N2a and P3. When both feature-based selection and filtering were required, N2a amplitude was highest, and P3 amplitude was lowest. The results support an early locus of feature-based attentional selection in multi-item search.