Optokinetic nystagmus: modeling the velocity storage mechanism.

To account for the oscillatory behavior of the optokinetic after-nystagmus (OKAN), a nonlinear model of the optokinetic system is proposed here that includes 2 first-order storage elements interconnected in a negative feedback loop. The adequacy of the model is tested by comparing its predictions with experimental data available in the literature. In addition, the question of the contribution of the storage element responsible for secondary OKAN (OKAN II) to the dynamic properties of the optokinetic nystagmus (OKN) is addressed. The results show that the model is compatible with all modifications of the OKAN time course observed under various experimental situations. By comparing computer simulations and experimental data, it is inferred that (1) the dynamic properties of the optokinetic system during OKN and during OKAN are different; (2) the switching in velocity storage dynamics is not determined by the light-dark transition, but is induced whenever nystagmic slow phase velocity (SPV) is not sustained by an appropriate retinal slip error signal; (3) although no signs of adaptation are seen during OKN, the storage element responsible for OKAN II becomes charged during optokinetic stimulation; and (4) the time constants of the integrators are affected by the parameters of the preceding optokinetic stimulation.