The neural processing fate of singleton target and nontarget stimuli.

The neural processing fate of target and nontarget singleton stimuli was investigated in a series of visual search tasks. The first experiment showed that the ERPs elicited by nontargets defined in the same feature dimension as targets were identical to those of targets until a relatively late divergence in the P3 time range. The second experiment showed that increased stimulus duration allowed slightly faster attentional selection: The ERPs of targets and nontargets now diverged earlier at the N2pc component, although nontargets still elicited a reliable N2pc, which was indicative of the processing of features of these stimuli. It furthermore seemed that task difficulty did not modulate the observed differences between target and nontarget processing. The third experiment investigated the impact of stimulus-response mapping as well as target probability. The former did not modulate the observed differences, and while the latter modulated absolute ERP amplitude, it again did not change the overall pattern of results. No evidence was found in these experiments for differential processing of targets and nontargets defined in the same feature dimension in the time range of the P2 component or before. In a final experiment, targets were compared with nontargets defined in the same or another feature dimension, and for the latter nontargets a clearly much earlier locus of divergent processing was observed, starting at the P2. The N2pc to these nontargets was also strongly suppressed. The relatively late locus of attentional selection between targets and nontargets defined in the same feature dimension suggested that early attentional processes cannot yet fully distinguish between specific within-dimension features.