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Lindsey B. Joyce, Chad Stockert, Mark Nawrot; Eye movements, not head translations, determine of perceived depth sign in motion parallax. Journal of Vision 2006;6(6):374. doi: https://doi.org/10.1167/6.6.374.
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The unambiguous perception of depth from motion parallax (MP) relies on a pursuit-like eye movement signal, regardless of head translation (Nawrot, 2003; Naji & Freeman, 2004). That is, stimulus motion in the same direction as the eye movement is perceived nearer than a translating fixation point that drives the eye movement (and remains stationary on the retina). To determine whether this relationship is lawfully obeyed regardless of head translation, we studied depth from MP in several conditions where head, monitor, window-on-monitor translations were co-varied and either elicited or prevented a pursuit signal. Within the stimulus window was a Rogers & Graham (1979) type random-dot MP stimulus having local horizontal dot translations which were varied to alter the phase and amount of depth. In all conditions, stimulus motion in the same direction as the eye movement was perceived nearer than the fixation point. Head translation was neither required for, nor deterministic of, perceived depth sign in MP. Instead, head translations are important in that they elicit compensatory eye movements that are a combination of translation vestibulo-ocular response and pursuit-like visually-driven eye movement. For instance, with identical head translation and local MP stimulus dot movement, direction of stimulus window movement determined both the pursuit signal direction and the perceived depth. A stationary stimulus window and stimulus window that translates identically with the observer's head translation generate opposite depth percepts. Indeed, an intermediate between these two extremes generates ambiguous depth because the TVOR provides the necessary compensation and the pursuit signal is absent.
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