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George A. Alvarez, Patrick Cavanagh; Independent attention resources for the left and right visual hemifields. Journal of Vision 2004;4(8):29. doi: 10.1167/4.8.29.
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© ARVO (1962-2015); The Authors (2016-present)
In two tracking experiments, we find that twice as many targets can be successfully tracked when they are divided between the left and right hemifields as when they are all presented within the same hemifield. Method: In the first experiment, observers tracked one of two bars that formed an X shape that rotated around its center. Two rotating Xs were always presented, either both in the same visual field or one in each field. Observers tracked a bar in one X or one bar in both Xs. The speed of rotation was adjusted for each observer so that tracking a single target was taxing but successful on most trials (about 90% accuracy). Tracking a single target at this level of accuracy requires continuous attention and the resources required to track a second target ought to degrade performance substantially if both targets draw on the same, limited attention resources. Of principal interest is whether the cost of tracking a second target is equal when the two targets are in the same or different visual fields. Results: Accuracy dropped from 89% to 63% (50% chance) when the number of targets increased from one to two within in the same hemifield (t(7) = 6.45, p < .001), but there was no significant difference in accuracy when the two targets were tracked in separate, left and right, hemifields (94% vs. 90% for 1 and 2 targets, respectively, t(7) = 0.90, p = .40). The observed accuracy levels are consistent with the level of performance expected if only 1 target can be tracked within a hemifield but two targets can be tracked in opposite hemifields without cost. This pattern of results was replicated in a second experiment in which observers performed a multiple object-tracking task (tracking a subset of several identical moving targets). Conclusion: These findings strongly suggest complete independence in the capacity to select targets in the left and right visual hemifields, placing broad constraints on the anatomy and mechanisms of attentional selection.
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