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Anand C. Joshi, Vallabh E. Das; Medial Rectus Motoneuron Activity in Non-Human Primates with Exotropia. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4689.
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© ARVO (1962-2015); The Authors (2016-present)
Previous studies from our laboratory have shown that monkeys reared under conditions of alternating monocular occlusion during their first few months of life show large horizontal strabismus, A-patterns and Dissociated Vertical Deviation (DVD). During eye movements, the A-patterns and DVD manifest as an inappropriate cross-axis eye movement in the non-fixating eye only. The objective of the current study was to record neuronal activity of horizontal (medial rectus) motoneurons in the oculomotor nucleus and examine their responses in relation to the state of horizontal misalignment and the inappropriate horizontal cross-axis movement observed during vertical tracking.
Burst tonic activity from twenty medial rectus motoneurons was recorded from two monkeys (S1, S2) with exotropia as they performed horizontal and vertical smooth pursuit (0.2Hz, ±10deg) under monocular viewing conditions. A simple first order model (FR=B + K*Epos + R*Evel) was used to characterize the neuronal firing rate(FR) during horizontal eye movements in each tracking condition, i.e. horizontal tracking under monocular right or left eye viewing and horizontal component observed in the non-fixating eye during monocular right or left eye viewing vertical tracking (horizontal cross-axis condition).
Neuronal activity was modulated in correlation with horizontal eye position and eye movements due to horizontal tracking, changes in static horizontal misalignment or the inappropriate horizontal component in the non-fixating eye during vertical tracking. Statistical comparison of model coefficients in the right eye and left eye viewing horizontal tracking conditions yielded no significant difference in the bias term and the position coefficient (p>0.05) but a significant difference in the velocity coefficient (p=0.016). Statistical comparison of the model coefficients in the horizontal tracking Vs the horizontal cross-axis condition yielded no significant difference in the bias term and the position coefficient (p>0.05) but a significant difference in the velocity coefficient (p<0.01).
Our data suggests that, in animals with sensory-induced strabismus, central innervation to the extraocular muscles provides the majority of the driving force for setting the state of horizontal misalignment as well as the inappropriate cross-axis eye movement that leads to change in misalignment with gaze position. However statistical differences in model velocity coefficients suggest that certain mechanical factors such as pulley dynamics or muscle-length adaptation may also play a role in determining strabismus components.
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