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The Cerebellum in Eye Movement Control

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The Cerebellum in Eye Movement Control Eye (2015) 29, 191–195 & 2015 Macmillan Publishers Limited All rights reserved 0950-222X/15 www.nature.com/eye CAMBRIDGE OPHTHALMOLOGICAL SYMPOSIUM The cerebellum in VR Patel1 and DS Zee2 eye movement control: nystagmus, coordinate frames and disconjugacy This article has been corrected since Advance Online Publication and an erratum is also printed in this issue Abstract cerebellum in binocular control—both to create disconjugacy when it is necessary, and to In this review we discuss several aspects of prevent ocular misalignment when it is eye movement control in which the unnecessary and disruptive. We will also touch cerebellum is thought to have a key role, but on the implications of evolving into frontal-eyed have been relatively ignored. We will focus creatures, with the competing demands on the mechanisms underlying certain forms of binocular, foveal vs retinal, full-field of cerebellar nystagmus, as well as the stabilization of images. Furthermore, we contributions of the cerebellum to binocular suggest that the phylogenetically old vestibular alignment in healthy and diseased states. anlage persists in a rudimentary form within A contemporary review of our understanding our human brains and its vestiges can be provides a basis for directions of further 1Associate Professor of uncovered in neurological disease.1–4 These inquiry to address some of the uncertainties Ophthalmology, USC Eye issues bear on interpretation of pathological regarding the contributions of the cerebellum Institute, Keck School of nystagmus, which depends on the coordinate Medicine, University of to ocular motor control. system in which the nystagmus is couched Southern California, Eye (2015) 29, 191–195; doi:10.1038/eye.2014.271; Los Angeles, CA, USA (foveal: eye frame vs full-field retina: head published online 14 November 2014 frame), and also on the types of ocular 2Professor of Neurology, misalignment seen in cerebellar patients. Otolaryngology, Head and Neck Surgery, Path 2-200 The cerebellum is an important structure within Johns Hopkins Hospital, a widely distributed neural network that Interpretation of nystagmus Johns Hopkins University controls movements including those of the eyes. School of Medicine, Both the immediate online control of movement Disorders of the cerebellum are associated Baltimore, MD, USA and the adjustments necessary to optimize with many types of nystagmus; downbeat Correspondence: motor performance in the long term are under nystagmus (DBN) is one of the most distinctive. VR Patel, Associate Professor its purview. Not surprisingly, ocular motor Hypotheses abound to explain how DBN might of Ophthalmology, USC Eye abnormalities are prominent on clinical arise from a cerebellar lesion.4,5 It is likely that Institute, Keck School of examination of most cerebellar patients. they are not mutually exclusive, as lesions in Medicine, University of Southern California, 1450 San Attempts to localize particular eye movement several different parts of the cerebellum produce Pablo St #4803, Los Angeles, abnormalities to specific parts of the DBN including the floccular–parafloccular (tonsil) CA 90033, USA cerebellum—traditional clinical–anatomical complex and the nodulus.5 A tone imbalance in Tel: +1 800 872 2273. localization—combined with results of the vestibular system is a common explanation, E-mail: Vivek.Patel@ med.usc.edu experimental lesions in animals, have led to especially as there are inhibitory projections inferring functions to specific cerebellar from the cerebellar flocculus to brainstem Received: 10 October 2014 structures. While these approaches facilitate pathways that mediate the anterior semicircular Accepted in revised form: 10 topical diagnosis, they do not necessarily canal rotational VOR (r-VOR), which produces October 2014 translate to a better understanding of the upward slow phases, but not to the pathways Published online: 14 November 2014 specific role of the cerebellum in eye movement mediating the posterior semicircular canal control. In fact, we are still relatively ignorant r-VOR, which produces downward slow Presented at Cambridge about how the cerebellum performs its phases. Hypothetically, a tone imbalance in Ophthalmological functions. Here, we will consider the role of the pathways that mediate the up–down (bob) Symposium, September 2014 Role of cerebellum in eye movement control VR Patel and DS Zee 192 translational VOR (t-VOR) could also produce DBN, but With these considerations in mind, if DBN is in a head its properties should be different. The r-VOR functions to frame, perhaps coming from an imbalance in the r-VOR, stabilize images on the entire retina and generates slow the observer (remaining aligned with the patient’s visual phases that are referenced to axes of rotation that parallel axis) should observe a torsional component when the the orientation of the semicircular canals in the labyrinth, patient looks laterally to either sides, because the slow hence the reference frame for the r-VOR is head-fixed. phase of DBN is still rotating around the head-fixed, The r-VOR functions in a similar way for images of interaural (vertical or pitch) axis (see Figure 2). On the objects at all locations in the visual field. In contrast, the other hand, if the nystagmus is in an eye frame, perhaps t-VOR functions to stabilize images on the foveae of both coming from an imbalance in the t-VOR, the nystagmus the eyes and, because of the inevitable effects of head will appear vertical in all horizontal eye positions. In translation on images of targets that are relatively close to other words, if there is a spontaneous horizontal or the head, the t-VOR must modulate its output based on vertical nystagmus in the straight ahead position, one can the distance of the orbits from the visual target.6 detect whether its reference frame is eye- or head-fixed Moreover, during forward translation, as occurs during by examining the nystagmus when the patient looks in a walking or running, the t-VOR must generate a different direction orthogonal to the spontaneous nystagmus. response for images that are located straight ahead Hypothetically at least, one might be able to distinguish (convergence), to the side (horizontal), or above or below whether a vestibular tone imbalance is coming from the eye level (vertical). Thus, because the t-VOR is most canal (r-VOR) or otolith (t-VOR) pathways. However, a concerned with stabilization of images on the fovea, it further complication is the potential role of the generates slow phases that are referenced to horizontal cerebellum in implementing Listing’s law, which could and vertical axes that move with the globe; hence the alter the patterns of torsion that one sees on eccentric reference frame for the t-VOR is eye-fixed. Furthermore, gaze.8 because we have two orbits, any translation of the head DBN often changes its intensity when patients look will require that each eye be adjusted independently. eccentrically or when they change their depth of focus. Thus, the t-VOR must be capable of generating At close viewing, DBN may actually change direction to disconjugate movements to keep the fovea of both the upbeat nystagmus. These complicated effects are not eyes on target (see Figure 1). We also note that the full- easily explained but one can hypothesize that they reflect field, visual tracking (optokinetic, OKN) systems that bungled attempts by the cerebellum to adjust otolith– complement the r-VOR during rotation (r-OKN), and the ocular responses for orbital position and viewing t-VOR during translation (t-OKN), show a similar distance, based on the incorrect assumption the head is difference in reference frames.7 translating. We will invoke an analogous pathogenesis for certain abnormalities of static alignment related to the incorrect assumption that the head is tilted (see below). TARG TARG The analysis of DBN is further complicated by the role of the cerebellum in generating gaze-holding commands.5 The cerebellum projects to the brainstem Convergence Conjugate Horizontal Slow-phase drift upwards Lateral Gaze as examiner looks along the Eye-fixed axes patient’s line of sight (t-VOR, pursuit, t-OKN) Straight ahead Head-fixed axes (r-VOR, r-OKN) Figure 1 The need for image stabilization on the fovea (eye- Figure 2 Effect of direction of gaze on the waveform of fixed frame) of both eyes drives the disconjugacy of the t-VOR nystagmus. (a) DBN in straight-ahead gaze. (b) Pattern of during fore-aft translation when looking straight ahead (left nystagmus with an imbalance in t-VOR or in t-OKN or pursuit panel) or when the target of interest is eccentrically located pathways. (c) Pattern of nystagmus with an imbalance in r-VOR (right panel). or r-OKN pathways. Eye Role of cerebellum in eye movement control VR Patel and DS Zee 193 neural integrator (NI), which is a neural network located (upright-supine test) affirming the supranuclear origin of largely in the medulla for horizontal movements and in the misalignment.21 The OTR has been attributed to the the midbrain for vertical movements. The NI takes emergence of a phylogenetically old response, best velocity commands from the conjugate eye movement appreciated in lateral-eyed animals, as in humans it has systems and creates a position command to hold the eyes been largely superseded by mechanisms that are steady after every movement. Lesions in the flocculus– optimized for binocular, foveate, and frontal-eyed vision. paraflocculus complex interfere with this integrator Consider the response of a rabbit to a lateral roll tilt with function. The integrator may become impaired or ‘leaky’, one ear down and the other up. If the rabbit’s eyes are causing a gaze-evoked nystagmus with velocity- roughly centered in the orbit one eye should go up and decreasing slow-phase wave forms, or become ‘unstable’, the other down to keep the horizontal meridians of the in which case the slow-phase wave forms are velocity retinas of the rabbit aligned along the horizon.
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