Visuo-vestibular interactions in the rabbit: the role of the flocculus and its mono-aminergic inputs

This thesis deals with two gaze-stabilizing oculomotor reflexes, the vestibula-ocular reflex (VOR) and the optokinetic reflex (OKR), which serve to reduce retinal slip during movements of the head, or relative movements of the surroundings respectively. The anatomical connections related to these two reflexes have been extensively studied. Both the VOR and the OKR pathways are mainly located in brain stem circuits, indicating their early phylogenetical origins. The VOR, which is activated by vestibular stimuli, is a feedforward system, and sends its signals to the oculomotor neurons after synapting in the vestibular nuclei. The OKR, on the other hand, is activated by retinal slip, and induces a compensatory eye movement by a feedback circuit which contains a number of brain stem areas including the· vestibular nuclei (see Fig. 1.1). Both systems have been demonstrated to supply collaterals to a parallel cerebellar loop, which passes through the flocculus. These connections strongly suggest involvement of the cerebellar flocculus in the generation of the VOR and the OKR, and probably also in visuovestibular interaction and adaptive changes in the VOR gain. On the basis of anatomical and physiological experiments, it has indeed been demonstrated that the flocculus is involved in three aspects of the VOR and OKR: 1) the flocculus affects the basic dynamic characteristics of the VOR and OKR; 2) direct visuo-vestibular interaction, i.e., the immediate improvement of the VOR by vision, is under floccular control; 3) long-term adaptive changes of the VOR, which can be induced by persistent unusual combinations of visual and vestibular inputs, require an intact flocculus. This thesis presents new evidence on all of these three functions of the flocculus in VOR and OKR control