A cross-coupling model of vertical vergence adaptation
Vertical disparity vergence aligns the two eyes in response to vertical misalignment (disparity) of the two ocular images. An adaptive response to vertical disparity vergence is demonstrated by the continuation of vertical vergence when one eye is occluded. The adaptive response is quantified by vertical phoria, the eye alignment error during monocular viewing. Vertical phoria can be differentially adapted to vertical disparities of opposite sign located at two positions along the horizontal or vertical head-referenced axes. Vertical phoria aftereffects vary in amplitude as the eyes move from one adapted direction of gaze to another along the adaptation axis. A cross-coupling model was developed to account for the spatial variations of vertical phoria aftereffects. The model is constrained according to both single cell recordings of eye position sensitive neurons, and eye position measurements during and following adaptation. The vertical phoria is computed by scaling the activities of eye position sensitive neurons and converting the scaled activities into a vertical vergence signal. The three components of the model are: neural activities associated with conjugate eye position, cross-coupling weights to scale the activities, and vertical vergence transducers to convert the weighted activities to vertical vergence. The model provides a biologically plausible mechanism for vertical vergence adaptation.
Available from: sciencedirect.com
- "With proper weighting, the model could reproduce the stimulus gradient eVect as well as all of the patterns of adaptation that have been reported for both nonconcomitant and concomitant adaptation. This includes a case of nonmonotonic adaptation wherein the vertical phoria was trained to change from a right hyperphoria at the extreme upper and lower eye positions to a left hyperphoria in the center (McCandless et al., 1996), a result that we have not been able to model with simple gain changes (as with Orbit, for example). The model proposed that the reason adaptation increases when the stimulus gradient is low is that more eye-position-sensitive neurons are uniquely active for a particular vertical vergence response. "
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ABSTRACT: Precise binocular alignment of the visual axes is of utmost importance for good vision. The fact that so few of us ever experience diplopia is evidence of how well the oculomotor system performs this function in the face of changes due to development, disease and injury. The capacity of the oculomotor system to adapt to visual stimuli that mimic alignment deficits has been extensively explored in laboratory experiments. While the present paper reviews many of those studies, the primary focus is on issues involved in maintaining good vertical and torsional alignment in everyday viewing situations where the parsing of muscle forces may vary for the same horizontal and vertical eye positions due to changes in horizontal vergence and head posture.
Vision Research 11/2006; 46(21):3537-48. DOI:10.1016/j.visres.2006.06.005 · 1.82 Impact Factor
Available from: berkeley.edu
- "The limited spread of aftereffects with lateral gaze in the exaggerated condition is surprising given our prior work on non-concomitant vertical phoria adaptation. Those studies found that non-concomitant vertical phoria aftereffects spread uniformly to untrained gaze directions and distances (Maxwell & Schor, 1994; McCandless, Schor, & Maxwell, 1996; Schor & Mc- Candless, 1997). The cyclophoria aftereffects at untrained locations might become more uniform with longer periods of training, or if the disparity stimuli were presented in several horizontal gaze directions, i.e. with a more natural pattern of disparities over a longer period of time. "
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ABSTRACT: Binocular alignment of foveal images is facilitated by cross-couplings of vergence eye movements with distance and direction of gaze. These couplings reduce horizontal, vertical and cyclodisparities at the fovea without using feedback from retinal image disparity. Horizontal vergence is coupled with accommodation. Vertical vergence that aligns tertiary targets in asymmetric convergence is thought to be coupled with convergence and horizontal gaze. Cyclovergence aligns the horizontal retinal meridians during gaze elevation in symmetrical convergence and is coupled with convergence and vertical gaze. The latter vergence-dependent changes of cyclovergence have been described in terms of the orientation of Listing's plane and have been referred to as the binocular extension of Listing's law. Can these couplings be modified? Plasticity has been demonstrated previously for two of the three dimensions of vergence (horizontal and vertical). The current study demonstrates that convergence-dependent changes of the orientation of Listing's plane can be adapted to either exaggerate or to reduce the cyclovergence that normally facilitates alignment of the horizontal meridians of the retinas with one another during gaze elevation in symmetrical convergence. The adaptability of cyclovergence demonstrates a neural mechanism that, in conjunction with the passive forces determined by biomechanical properties of the orbit, could play an active role in implementing Listing's extended law and provide a means for calibrating binocular eye alignment in three dimensions.
Vision Research 02/2001; 41(25-26):3353-69. DOI:10.1016/S0042-6989(01)00149-3 · 1.82 Impact Factor
Available from: Johb L Semmlow
- "For the open-loop control component (open-loop in the sense that visual feedback was not involved), Horng et al.  adopted a slightly modified version of Zee et al.'s model , and for the closed-loop component he used the model developed by Krishnan et al. . The purpose of most previous models was to represent the overall structure of the vergence control system in order to study basic control issues such as dynamics and stability , , , , , or to evaluate neural control theories through simulation –, , . The model developed here has a different and more limited purpose: to transform dynamic vergence behavior into equivalent motor command signals, and to represent those signals quantitatively by a small number of parameters. "
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ABSTRACT: A new method to analyze the dynamics of vergence eye movements was developed based on a reconstruction of the presumed motor command signal. A model was used to construct equivalent motor command signals and transform an associated vergence transient response into an equivalent set of motor commands. This model represented only the motor components of the vergence system and consisted of signal generators representing the neural burst and tonic cells and a plant representing the ocular musculature and dynamics of the orbit. Through highly accurate simulations, dynamic vergence responses could be reduced to a set of five model parameters, each relating to a specific feature of the internal motor command. This dynamic analysis tool was applied to the analysis of inter-movement variability in vergence step responses. Model parameters obtained from a large number of response simulations showed that the width of the command pulse was tightly controlled while its amplitude, rising slope, and falling slope were less tightly regulated. Variation in the latter three parameters accounted for the most of the movement-to-movement variability seen in vergence step responses. Unlike version movements, pulse width did not increase with increased stimulus amplitude, although the other command signal parameters were substantially influenced by stimulus amplitude.
IEEE Transactions on Biomedical Engineering 11/1999; 46(10):1191-8. DOI:10.1109/10.790495 · 2.35 Impact Factor
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