Sustained division of spatial attention to multiple locations within one hemifield.

Attending to a location in space significantly improves stimulus perception at that location. Everyday experience requires the deployment of attention to multiple objects at different locations. Recent empirical evidence suggests that the "beam" of attention can be divided between non-contiguous areas of the visual field. Whether this is only possible when stimuli are presented in different hemifields and harder, if not impossible, when stimuli are in the same hemifield is an ongoing debate. Here we use an electrophysiological measure of sustained attentional resource allocation (the steady-state visual evoked potential, SSVEP) to address this question. In combination with behavioural data we demonstrate that splitting the attentional "beam" is in principle possible within one hemifield. However, results showed that task performance was in general lower for same-hemifield presentation as opposed to our previous study with different-hemifield presentation [M.M. Müller, P. Malinowski, T. Gruber, S.A. Hillyard, Sustained division of the attentional spotlight, Nature 424 (2003) 309-312]. SSVEP amplitude showed a mixed pattern of results for stimuli presented in the upper versus lower quadrant of the left visual hemifield under conditions of attending to two separated locations. Results are discussed in the light of the bilateral distribution advantage hypothesis and differences in stimulus salience between the upper and lower visual field.