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G. L. McCormack, T. Lowe, L. Deng, T. Nguyen; Convergence Control in Darkness. Invest. Ophthalmol. Vis. Sci. 2007;48(13):887.
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It has been proposed that the spatiotopic control of vergence may be driven by a feedback control process in which perceived or remembered distance is compared to sensed convergence angle (1). This study tested the feedback control hypothesis by comparing step vergence responses in darkness to feedforward stimuli and to feedback stimuli.
Informed consent was obtained from fifteen naive and normally binocular subjects prior to experimentation. In dark surrounds subjects first fixated a haploscopic luminant target (a playing card) at 4M, then at a variable middle distance which they were asked to remember, and finally at 20cm. The target was then extinguished and subjects were asked to maintain their gaze on the 20cm target distance in the dark. Following a delay of 2 or 8 seconds from darkness onset, an auditory cue prompted subjects to look at the middle target distance. Convergence was measured by infrared oculography. The feedforward stimulus was assumed to be the spatial difference between the remembered middle and 20cm target distances. The feedback stimulus was assumed to be the difference between the remembered middle target distance and the measured convergence angle at the instant of the auditory cue. A linear mixed model was used to evaluate the relationship between vergence step response and feedforward and feedback stimuli.
Cued vergence step responses in the dark were predominantly divergent, but were uncorrelated with the magnitude of the feedforward stimulus (slope=-0.026, p=0.81). Vergence step responses were positively correlated with the feedback stimulus (slope=0.47, p<0.0001), but with a significant divergence bias (-7.8 prism diopters, p<0.0001). The divergence bias is attributed to a far perceptual bias in our haploscopic stimulus system.
A spatiotopic feedback control process drives voluntary convergence in the dark. 1. Schor et al. Ophthal & Physio Opt 12:307-318, 1992.
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