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Kyriaki Mikellidou, Peter Thompson, David Burr; Two mechanisms subserve the oblique effect. Journal of Vision 2014;14(10):1416. doi: https://doi.org/10.1167/14.10.1416.
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Introduction: The origins of the well-known oblique effect (poor discrimination for oblique orientations) are still unclear. Some studies point to asymmetries in orientation tuning in primary visual cortex (Furmanski & Engel, 2000), while others suggest it is related to gravitational vertical (Day & Wade, 1969). We disentangled these two alternatives by measuring precision of orientation discrimination at different angles of head-tilt, both while seated normally and lying horizontally to eliminate gravity. Methods: Participants indicated which of two sequentially presented gratings appeared more clockwise, for 12 different base orientations ranging from 0° (vertical) to ±90°. Psychometric functions were fitted with gaussian error functions, whose standard deviation gave an estimate of threshold. In different sessions, subjects either sat upright on a chair or lay horizontally on their back. In the upright condition the task was performed at three different head tilts (0°, 15°, 30°). In the horizontal condition, both the head and the body were either orientated at 0° or 30° or dissociated with the head positioned at 30° and the body at 0°. Results: In the upright position, thresholds were always lower at gravitational 0° or 90°, irrespective of head tilt, with no significant differences between the three head positions. On the other hand, in the absence of gravity, orientation discrimination depended primarily on retinal coordinates, varying with the tilt of the head. Conclusion: Our results reveal two distinct mechanisms causing the oblique effect: the strongest is gravity-based, independent of head position. However, when gravity is unavailable, a retinotopic oblique-effect emerged, probably reflecting asymmetries in orientation tuned mechanisms in early visual cortex. The results reconcile previous apparently conflicting evidence, and are in accordance with recent evidence on variable primate vestibular signals of different brain areas to changes in eye, head and body positions (Chen et al., 2013).
Meeting abstract presented at VSS 2014
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