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 an d accessory optic system. Using data from neuronal and eye-movement re cordings that have been previously published, the model was realized u sing algebraic-block simulation software. It was found that the optoki netic response, modelled as a simple negative feedback system, was sim ilar 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 en coded in these neurons could provide sufficient sensory information to drive the optokinetic reflex. Furthermore, it appears that the low ga in of optokinetic eye movements in turtles, which have a negligible ve locity storage time constant, may allow stable oculomotor output in sp ite of neuronal delays in the reflex pathway. This model illustrates h ow visual neurons in the pretectum and accessory optic system can cont ribute to visually guided eye movements.