Interestingly, several studies have found that attention can be divided between different locations, rather than spread over a large field. For example, reaction time studies showed that observers can devote attention to several non-contiguous locations (Awh & Pashler,
2000; Kramer & Hahn,
1995). Evidence was also found for attention being paid to distinct regions from brain activity measurements (McMains & Somers,
2004; Müller, Malinowski, Gruber, & Hillyard,
2003). We should recognize, however, that these results do not exclude the possibility of attention spreading over the region around the cued locations, as our results do not rule out the possibility of the division of attention. Indeed, hints of attention spread can be seen in data from some reports on the division of attention. Awk and Pashler (
2000), for example, demonstrated psychophysically that visual processing was facilitated in the middle of the cued regions, although the effect was smaller than that at either of the cued locations. Similarly, in an fMRI experiment, McMains and Somers (
2004) showed the attention effect at the intervening regions, which was smaller than that at the locations of the attended stimuli. That is, studies that suggest division of attention often show effect of attention spread. It is likely that attention is a flexible system that can be divided into separate regions in some cases and can be spread over contiguous region in other cases (Awh & Pashler,
2000). If it is an easier strategy to pay attention to a contiguous region in the visual field (the midlocation placement strategy; McCormick, Klein, & Johnston,
1998), observers may adopt this. This may be the case when strong attention is required to perform a task such as tracking an object in fast motion, as in the present experiments.