Oculomotor data : Increase of phoria in near and far vision

1. GRAPHYCAL ANALYSIS 2,5 2 1,5 1 0,5 0 -0,5 Accommodation stimuli (D) -2 -1 0 1 2 2,5 3 4 5 Vergence response (AM) phoria pretest phoria posttest break pretest blur pretest break posttest blur posttest recovery pretest recovery posttest recovery pretest recovery posttest phoria line pretest phoria line posttest ZCSBV pretest ZCSBV posttest input threshold latency phasic cross coupling funnel limiter tonic range limiter plant output input controller: adaptation cross coupling tonic output transient + sustained component component Fig. 1: Schor model (1992) Fig. 2: Hung model (1992) MINISTÈRE DE LA DÉFENSE Do dynamic models of accommodation / vergence relationship apply for hyperstereoscopic stimuli? Pascaline Neveu°, Anne-Emmanuelle Priot°, Philippe Fuchs1, Corinne Roumes° °Institut de Recherche Biomédicale des Armées - Antenne Brétigny sur Orge - IMASSA, BP 73, 91223 Brétigny-sur-Orge cedex, France 1Mines ParisTech, 60 Boulevard Saint-Michel, 75272 PARIS cedex 06, France Contact : pneveu@imassa.fr CONTEXT EXPERIMENT and RESULTS Accommodation and vergence states vary according to the distance of the fixated element. These two visuo-motor mecanims interact in order to maximize their efficiency. Several models were proposed in order to describe these interactions between the two components. For nearly 30 years, models have been driven by both blur and disparity of the visual stimuli. Thus, the interaction between the two systems has been modelled by including cross-links representing accommodative convergence and vergence accommodation (Eadie and Carlin, 1995). In the current study, we will focus on the two most famous of these models developped by Hung and collaborators and Schor and collaborators. Such quantitative models were empirically built to fit experimental data of accommodation and vergence considered in open-loop or in their normal close-loop interactions. The aim of the current study was to test whether these models could predict oculomotor behavior in non usual viewing conditions, in particular when the cross-coupling is affected. We used a telestereoscope which alters the coupling by inducing an over demand in convergence, while accommodation remains almost unchanged. Disparities are increased leading to hyperstereoscopic vision. Apparatus: The telestereoscope optically increased the interocular distance and disrupted the link between the vergence required to fuse the fixation item and the accommodation required to focus on it. • Subjects: • 15 adults (19-44 years old) with normal vision and a minimal acuity of 20/20 in the each eye Stimulus: • Convergence via the telestereoscope was increased 3 times for an average interpupillary distance (64mm) • Fixation : letters of 10/20 of visual acuity • Regular fixation transfer from distance (4.5m) to near (0.4m) at a frequency of 0.1Hz • Protocol: • Paradigm: pretest, exposure (10min), posttest • Clinical assessment: Phoria, calculated AC/A and Gradient +1.00, fusional and relative vergences Fig 3: Telestereoscope Two pairs of mirrors angled at 45° and parallel in pairs. The internal and external mirrors are separated by a distance of 64mm. When a subject looks through the optics, the effective inter-ocular separation is increased by 128mm. Schor first proposed a model for the control of the vergence motor system (1979, lower part of fig. 1). He then included adjustment due to accommodative interactions (1986, cross-link) and extended to the accommodative component the dual mechanism phasic (fast, left part of the fig. 1) and tonic (slow, right part of the fig. 1) initially specified for vergence. In 1992, a refinement of the model attempted to address the adaptation capability of the accommodation/vergence mechanisms but remained far below the effective adaptation processing as the system parameters remain constant (Eadie et al., 2000). The best of Schor model appears in dynamic conditions when the distance of fixation changes (Eadie et al., 1995) as the relative latencies of accommodation and vergence are precisely considered (second box of each process in fig. 1). Right from the first version of his model (1980), Hung considered accommodation and vergence adjustments when fixating as well as their cross-links. The elementary latencies of both mechanisms were disregarded. Both transient and sustained components of the system were gathered in a joint process (larger boxes of fig. 2) defined as a controller. Specificity of this model is a dynamic adaptive loop driven by the controller output (red path in fig. 2) that governs the time constant of the controller (1992). Therefore, it accurately accounts for static behaviour of the oculomotor system at various levels of adaptation. Fig 4: Clinical testing Testing was performed before and after exposure to telestereoscope. In posttest examination, one eye was occluded in order to keep the oculomotor state constant. • Oculomotor data: • Increase of phoria in near and far vision • Increase of AC/A (calculated and Gradient) • Shift of the ZCSBV toward convergence • Decline of the recovery capacities in divergence VS improvement in convergence p < 0,05 DISCUSSION The modification of the vergence capacities (shift of the ZCSBV toward convergence associated to the decline of the recovery capacities in divergence VS improvement in convergence) can be explained by the increase of phoria (depending on the tonic vergence/Schor or sustained controller/Hung) and the AC/A predicted by the two models (all components highlighted in green in figure 1 and 2). The trend of the oculomotor alterations was qualitatively in agreement with those models which predict the bilateral impact of blur and disparity processing in natural conditions. Only steady states of the oculomotor balance was addressed in the current protocol. The dynamic models will be further questioned to determine how far each of their components can quantitatively account for adaptation when facing changes in stereoscopic conditions. References: Eadie, A. S., et al. (2000) Modelling adaptation effects in vergence and accommodation after exposure to a simulated virtual reality stimulus. Ophthalmic and Physiological Optics, 20, 242-251. Eadie, A. S. & Carlin, P. J. (1995) Evolution of control system models of ocular accommodation, vergence and their interaction. Medical and Biological Engineering and Computing, 33, 517-524. Hung, G. K. & Semmlow, J. L. (1980) Static behavior of accommodation and vergence: computer simulation of an interactive dual-feedback system. IEEE Transactions on Biomedical Engineering, 27, 439-447. Hung, G. K. (1992) Adaptation model of accommodation and vergence. Ophthalmic and Physiological Optics, 12, 319-326. Schor, C. M. (1979) The relationship between fusional vergence eye movements and fixation disparity. Vision Research, 19, 1359-1367. Schor, C. M. & Kotulak, J. C. (1986) Dynamic interactions between accommodation and convergence are velocity sensitive. Vision Research, 26, 927-942. Presented at the ECVP 2009, Aug. 24-28, Regensburg, Germany