| Abstract: | The process by which we localize an object is a multi-dimensioned, synergistic integration of retinal and motoric signals. Knowing the convergence angle of the eyes (based on phoria), one can predict perceived target location by utilizing Hering's Law. When one eye is occluded, that eye returns to a resting state in which phoria is reduced. This reduced phoria results in a corresponding change in perceived visual direction (the monocular egocentric direction (MED) illusion). In addition, pilot data indicated a bias in lateral heterophoria when the head was moved from a neutral to a laterally rotated position. The purpose of this study was to examine the relationship between monocular occlusion, head rotation, phoria, and perceived target location. In Experiment 1, phoria was measured in both the occluded and non-occluded eye. The results indicated that monocular occlusion resulted in a decreased phoria in the occluded eye and that 60 degrees (d) head rotation resulted in phoria in the active eye being either reduced or increased depending on the direction of head rotation. The effect of head rotation was only noted at the 60 d condition. This suggests, that monocular occlusion and head rotation interact in such a manner as to amplify or attenuate perceived target location. In Experiment 2, perceived target location was inferred from limb positioning data. This data indicated that monocular occlusion resulted in the perceived target shifting in the direction of the occluded eye and appearing farther than the actual location, and that 60 d right head rotation resulted in the perceived target location being perceived more leftward than perceived by monocular occlusion alone. This suggests that monocular occlusion and head rotation result in additive effects on perceived target location. Results of both experiments support the predictions of Hering's Law and indicate monocular occlusion and/or head rotation interact to influence both phoria and perceived target location. |
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