Amphibian optokinetic after nystagmus: properties and comparative analysis in various species

A model for optokinetic eye movements in turtles that incorporates properties of retinal-slip neurons

The turtle's optokinetic response is described by a simple model that incorporates visual-response properties of neurons in the pretectum and accessory optic system. Using data from neuronal and eye-movement recordings that have been previously published, the model was realized using algebraic-block simulation software. It was found that the optokinetic response, modelled as a simple negative feedback system, was similar to that measured from a behaving animal. Because the responses of retinal-slip detecting neurons corresponded to the nonlinear, closed-loop optokinetic response, it was concluded that the visual signals encoded in these neurons could provide sufficient sensory information to drive the optokinetic reflex. Furthermore, it appears that the low gain of optokinetic eye movements in turtles, which have a negligible velocity storage time constant, may allow stable oculomotor output in spite of neuronal delays in the reflex pathway. This model illustrates how visual neurons in the pretectum and accessory optic system can contribute to visually guided eye movements.