N.M. Gerrits, T.J.H. Ruigrok and C.1. De Zeeuw (Eds.) Progress in Brain Research, Vol 124 © 2000 Elsevier Science BV. All rights reserved. CHAPTER 18 On the nature of gain changes of the optokinetic reflex M.A. Frens*, A.L. Mathoera and J. Van der Steen Department of Physiology, Erasmus University Rotterdam, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands Introduction Compensatory eye movements serve to minimize retinal slip. By rotating the eyes in a direction opposite to movements of the head, the orientation of the eyes in space remains relatively stable, which prevents blurring of the retinal image by move- ments of the visual scene relative to the head. In head-restrained afoveate species, such as the rabbit, these reflexive eye movements (most importantly the vestibulo-ocular reflex (VOR) and the optoki- netic reflex (OKR)) form the sole oculomotor output. Therefore the rabbit is an ideal animal to study compensatory eye movements in isolation, without interference of pursuit or saccades. Both the VOR and the OKR are highly plastic, and changing conditions can modify the perform- ance of the reflexes within short periods of time. Nonetheless, somewhat surprisingly the gain (the ratio of eye movement amplitude and stimulus amplitude) of the reflexes of the rabbit is usually less than one. In other words, the response is not perfect but rather shows a consistent undershoot (Collewijn, 1969). Only during combined vestibu- lar and visual stimulation the gain reaches a value that is close to unity. Plasticity of compensatory eye movements has been most extensively studied in the VOR (e.g. Collewijn and Grootendorst, 1979; Demer et al., 1989; De Zeeuw et al., 1998). For instance, a gain- increase paradigm in which the animal is *Corresponding author. Fax: (31) 10 408 9457; e-mail: frens@fys.fgg.eur.nl sinusoidally rotated, while the visual environment is rotated in the opposite direction, leads to an increased amount of retinal slip with respect to a rotation in a stable environment. The VOR adapts to this new condition within hours by increasing the amplitude of the oculomotor response (Collewijn and Grootendorst, 1979). This increase persists even when the animal is subsequently rotated in the dark. Likewise, a gain-decrease paradigm in which the visual environment moves in phase with the vestibular stimulation leads to a decrease in the amplitude of the eye movements. In the rabbit VOR-adaptation is somewhat specific for the frequency of the adapting stimulus. At other stimulus frequencies the response changes less (Collewijn and Grootendorst, 1979). Also the OKR has plastic properties. This has been shown when the animal is subjected to prolonged optokinetic stimulation. The gain of the OKR increased from 0.65 to 0.8 after four hours of stimulation at a frequency of 1/6 Hz (Collewijn and Kleinschmidt, 1975). This type of plasticity is, similar to the VOR, frequency specific. Fur- thermore it not only increases the performance of the OKR but also of the VOR. Although there is controversy about the exact neural substrate of VOR-plasticity, it is generally accepted that the flocculus of the cerebellum plays an important role. The flocculus receives two major inputs. One source is the parallelfiber system that consists of afferents from the vestibular nuclei. This input consists of mixed visual and vestibular information, as well as eye velocity signals. Furthermore, the flocculus receives input from the