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Alistair P. Mapp, Mykola Khokhotva, Hiroshi Ono; Hitting the target: Relatively easy, yet absolutely impossible?. Journal of Vision 2004;4(8):23. doi: https://doi.org/10.1167/4.8.23.
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It is generally agreed upon that absolute direction judgments require information about eye position, whereas relative direction judgments do not. The source of this eye position information, particularly during monocular viewing, is a matter of debate. It may be either binocular eye position or the position of the viewing eye only that is crucial. We performed two experiments, using more ecologically valid stimulus situations than the traditional LED in the dark type experiment, to determine the source of the eye position information. In Experiment 1 observers threw darts at targets that were fixated either monocularly or binocularly. In Experiment 2 observers fired a laser gun at targets while fixating either the rear or the front gunsight monoculalry or the target either monocularly or binocularly. In both experiments we measured the observers' phoria (a measure of binocular eye position) and the accuracy of their absolute direction judgments. In Experiment 2 we also measured the accuracy of the observers' relative direction judgments. We found that (a) relative direction judgments were accurate and independent of phoria and (b) monocular absolute direction judgments were inaccurate, and the magnitude of the inaccuracy was predictable from the magnitude of phoria. These results confirm that relative direction judgments do not require information about eye position. Moreover, they show that binocular eye position information is crucial when judging the absolute direction of both monocular and binocular targets.
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