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P. M. Grove, H. Ono, H. Kaneko; The bifixation field as a function of viewing distance. Journal of Vision 2001;1(3):232. https://doi.org/10.1167/1.3.232.
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Based on his theory of equal innervation, Hering (1977/1868) reported the area over which he could bifixate a target was smaller at near viewing distances than far viewing distances and predicted that in extreme horizontal gaze positions, the templeward directed eye lags behind the nasally directed eye. We tested these predictions using binocular fusion as the criterion for bifixation and measured the angular extent of the bifixation field over a frontoparallel plane at three different distances, noting the relative positions of the eyes when fusion broke. Methods: Observers sat with their head fixed in a chin and forehead rest. Dichoptic stimuli consisted of a binocular dot flanked top and bottom by Nonius lines. Right and left eye images, presented at one of three separations, were free fused to simulate one of three viewing distances (57, 28.5, 10 cm). A trial began with the fused stimulus directly in front of the observer. Observers moved the stimulus out along one of 12 radii, at 30-degree intervals around the clock, until fusion broke. On other trials, observers moved the diplopic stimulus towards the center until the Nonius lines were aligned. Twelve trials (6 in each direction) were completed for each condition. Results: The area over which the two eyes can move together to maintain binocular fusion is significantly smaller at near viewing distances than far distances. When fusion broke the nasally directed eye lagged behind the templeward directed eye. Conclusions: We confirm Hering's first prediction that the bifixation field is smaller at near viewing distances than far. Contrary to his second prediction, however, we found that the nasally directed eye lags behind the templeward directed eye.
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