Controlling spatial attention without central attentional resources: Evidence from event-related potentials

The present study examined whether the control of spatial attention requires central attentional resources using a modified Psychological Refractory Period paradigm. We varied across experiments whether Task 1 was a two- or four-choice speeded task and whether it was auditory or visual. Task 2 (unspeeded response) was to identify a visual letter in a specific target colour, while ignoring letters in other colours. We measured the N2pc effect (reflecting lateralized attentional allocation) elicited by Task 2 as a function of the stimulus–onset asynchrony (SOA) between Task 1 and Task 2. The question was whether spatial attention could shift to the Task 2 stimulus at short SOAs, while central attention was still allocated to Task 1. For the two-choice Task 1, Task 2 elicited a strong N2pc effect (indicating capture) even at short SOAs, regardless of whether Task 1 was auditory (Experiment 1) or visual (Experiment 2). But for the four-choice Task 1, the N2pc effect elicited by Task 2 was attenuated strongly at short SOAs, both for the visual Task 1 (Experiment 3) and the auditory Task 1 (Experiment 4). N2pc attenuation was also observed in Experiment 5, which mapped four auditory stimuli onto two responses for Task 1. This finding suggests that the attenuation is due to the difficulty of stimulus classification on Task 1, not the number of responses. Experiment 6 showed that the attenuation of N2pc effect on Task 2 was due to central operations on Task 1, not the mere presence of the Task 1 stimulus. We propose that controlling spatial attention without central resources is possible, but the quality of attentional settings degrades if concurrent tasks impose a sufficiently great load on working memory (especially the load related to stimulus classifications).