Optokinetic nystagmus in the rabbit and its modulation by bilateral microinjection of carbachol in the cerebellar flocculus

1. In the alert, pigmented rabbit, eye movements were recorded during optokinetic nystagmus (OKN) and during optokinetic afternystagmus (OKAN). These responses were elicited by steps in surround-velocity ranging from 5-110°/s during binocular as well as monocular viewing. 2. In the baseline condition, OKN showed an approximately linear build-up of eye velocity to a steady-state, followed by a linear decay of eye velocity during OKAN after the lights were turned off. Build-up during binocular viewing was characterized by a constant, maximum eye-acceleration (about 1°/s²) for stimulus velocities up to 60°/s. OKAN, instead, was characterized by a fixed duration (about 10 s) for stimulus velocities up to 20°/s. Steady-state eye velocity saturated at about 50°/s. 3. Monocular stimulation in the preferred (nasal) direction elicited a build-up that was on average twice as slow as during binocular stimulation. Steady-state velocity during monocular stimulation saturated at about 20°/s. OKAN was of equal duration as during binocular stimulation. In the non-preferred direction, a very irregular nystagmus was elicited without velocity build-up. The stronger response to binocular stimulation, compared to the responses under monocular viewing condition in either nasal and temporal direction suggests potentiation of the signals of either eye during binocular viewing. 4. OKN and OKAN were re-assessed after intra-floccular microinjection of the nonselective cholinergic agonist carbachol. In the binocular viewing condition, eye-acceleration during build-up was strongly enhanced from 1°/s² before to 2.5°/s² after injection. The saturation level of steady-state eye velocity was also increased, from 50°/s before to more than 60°/s after carbachol. The duration of OKAN, however, was shortened from 10 s before to 6 s after injection. The response to monocular stimulation in the preferred direction revealed similar changes. 5. The flocculus appears to be involved in the control of the dynamics of OKN in the rabbit. Cholinergic mechanisms affect the floccular control of the rate at which slow-phase velocity can be built up and the rate of decay of eye velocity during OKAN. Cholinergic stimulation of the flocculus enhances the dynamics of OKN, while velocity storage is shortened.