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Megan L. Frankl, Mark Nawrot; Extra-retinal signals in motion parallax: Support from eye movement asymmetries in strabismus. Journal of Vision 2005;5(8):991. doi: 10.1167/5.8.991.
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The asymmetric horizontal pursuit and optokinetic response (OKR) eye movements found in some strabismic observers (Tychsen & Lisberger, 1986; Demer & von Noorden, 1988; Westall et al, 1998; Levi & Schor, 1984) provide an excellent opportunity to explore the role of an extra-retinal eye movement signal in the perception of depth from motion parallax (MP). We have found that elevated MP thresholds in strabismus (Thompson & Nawrot, 1999) are linked to a pursuit anomaly (Nawrot & Frankl, 2004). Specifically, MP thresholds are elevated when the observer is making abnormal, low-gain nasal-temporal (NT) pursuit eye movements. Conversely, MP thresholds are normal when the observer is making the opposing, normal-gain temporal-nasal (TN) pursuit eye movements. Since strabismic observers often have asymmetric OKR, in addition to asymmetric pursuit, we explored the role of extra-retinal information though a novel MP display that relies on opposing pursuit and OKR eye movement signals. In this display a depth-sign-ambiguous motion parallax stimulus (Rogers & Graham, 1979) is presented with a large, horizontally translating, high contrast, square-wave grating background. This translating background is well known for eliciting OKR. However, translation of this background also disambiguates perceived depth in the MP display, presumably by eliciting both an OKR signal and a countermanding pursuit signal, with the latter disambiguating perceived depth from MP. We found that in strabismic observers asymmetric OKR and pursuit are linked to asymmetric MP thresholds with this stimulus. MP thresholds were lower (more normal) when the background translated in a direction of low OKR gain, opposite the direction of normal pursuit. We interpret this result to mean that a normal pursuit signal is required to disambiguate the perceived depth order in an otherwise ambiguous MP stimulus, further supporting the role of an extra-retinal eye movement signal in MP.
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