Cerebellar control of visually-guided eye movements by the bilateral mass of activity in the caudal fastigial nuclei

Laurent Goffart (1) , Clara Bourrelly (1) , Julie Quinet (1)

1 INT - Institut de Neurosciences de la Timone

The caudal part of both fastigial nuclei (cFN) plays a crucial role in the ability to foveate a static or moving visual target. Indeed, in the monkey, a unilateral inactivation of cFN by local injection of muscimol impairs the horizontal component of saccadic and pursuit eye movements. An ipsilesional gaze offset is also observed while a target is being fixated or pursued. In the head-unrestrained monkey (and cat), the head exhibits an ipsilesional deviation relative to the trunk (cervical dystonia). In a computational study, Eggert, Robinson & Straube (2016) reported more variability in saccade endpoints after cFN inactivation than before, and interpreted this effect as altered putative "noises" in brain activity: planning noise and/or signal dependent motor noise. From the demonstration that larger current enhances the size and velocity of saccades evoked by electrical cFN microstimulation (Quinet & Goffart 2015), we propose that the variability of saccade endpoints reflects the variable size of the active population of cFN neurons contributing to the movement generation. Indeed, if during the pharmacological experiments, the perturbation is not exactly centered in cFN, the number of inactivated neurons should increase as the muscimol diffuses, resulting in a dysmetria that increases with time. We will show examples of cFN inactivation experiments performed in head-restrained monkeys where the size of dysmetria does not change with time, and other examples where it increases with time. Thus, the variability of saccade amplitude reported by Eggert and colleagues is likely the consequence of grouping saccades launched from variable starting positions and collected during multiple experimental sessions in which the number of inactivated neurons was not identical and possibly also, time-varying. More generally, our observations are consistent with the hypothesis that the cerebellar control of the ability to foveate and pursue a visual target consists of adjusting the firing and the number of active neurons in the cFN for the fine regulation of bilateral activity in territories which, in the brainstem, are recipient of their action potentials (Bourrelly et al. 2018). CFN projections to the paramedian pontomedullary reticular formation would participate in steering of the horizontal component of saccades, those to the reticulospinal neurons in determining head yaw orientation and those to the rostral pontine nuclei (NRTP and DMPN) in accelerating pursuit eye movements. Ascending projections to the rostral superior colliculi would determine gaze direction during fixation and pursuit whereas those to the thalamus remain to be characterized, although a participation in pursuit eye movements is also possible.

Domaines: Psychologie; Neurosciences; Psychologie et comportements

Laurent Goffart, Clara Bourrelly, Julie Quinet. Cerebellar control of visually-guided eye movements by the bilateral mass of activity in the caudal fastigial nuclei. Meeting of the GDR Neural Net, Dec 2019, Bordeaux, France. ⟨hal-03652206⟩