Abstract
It is still a matter of debate whether observers can attend simultaneously to more than one location. Using essentially the same paradigm as was used by Bichot, Cave & Pashler (Perception & Psychophysics, 1999) to show that attention can be “split” among 2 separate locations, we demonstrate that their previous findings only reflect intermediate stages of (incomplete) attentional selection.
Our subjects were asked to discriminate the shapes (circle or square) of 2 oddly colored targets within an array of 8 stimuli. After a certain SOA, 8 letters were flashed at the previous stimulus locations, followed by a mask. For a given SOA, the performance of subjects at reporting letters in each location was taken to reflect the distribution of spatial attention. In particular, by considering the proportion of trials in which none or both of the target letters were reported, we were able to infer the respective amount of attention allocated to each target without knowing, on a trial by trial basis which location (if any) was receiving the most attentional resources.
Our results show that for SOAs around 100–150 ms, attention can be equally split between the two targets, a conclusion compatible with previous reports. However, with longer SOAs, attention ultimately settles at the location of one single stimulus.
This is a natural prediction of a computational model of attention (Hamker, Soc. Neurosci. Abstr, 2001) in which the planning of saccadic eye movements guides attentional selection. The results can be accounted for by a model of decision making in which the current output of a “refined” saliency map continuously feeds areas in the fronto-parietal network, which select the unique location of an eye movement by a competition over time. Activity from these areas is fed back continuously to extrastriate visual areas. Thus, the SOA determines the state of this competition at the time the letters were flashed and ultimately the distribution of attention at different locations.
This work was supported by DFG HA2630/2-1 and in part by the ERC Program of the NSF (EEC-9402726).