Role of the flocculus of the cerebellum in motor learning of the vestibulo-ocular reflex

Does spectacle use lead to vestibular suppression?

BACKGROUND

Laboratory experiments indicate that changes in retinal image size result in adaptive recalibration or suppression of the vestibulo-ocular reflex. Myopia correction with spectacles or contact lenses also leads to retinal image size changes, and may bring about similar vestibulo-ocular reflex alterations.

METHODS

A hypothesis-generating preliminary investigation was conducted. In this cross-sectional study, findings of electronystagmography including bithermal caloric testing were compared between 17 volunteer myopes using spectacles or contact lenses and 17 volunteer emmetropes (with no refractive error).

RESULTS

Bilateral hypoactive caloric responses were demonstrated in 6 of 11 spectacle users, in 1 of 6 contact lens users and in 1 of 17 emmetropes. Hypoactive caloric responses were significantly more likely in spectacle users than in emmetropes (p < 0.01; relative risk = 9.3).

CONCLUSION

A significant proportion of myopes using spectacles have vestibulo-ocular reflex suppression, as demonstrated by the caloric test. This has implications for the interpretation of electronystagmography and videonystagmography results, and highlights spectacle use as a possible cause of vestibular impairment. Further corroboration of these findings is warranted, with more precise and direct vestibulo-ocular reflex tests such as rotational tests and the head impulse test.

An Unusual Presentation of Vertigo: Is Head Titubation the Key to Diagnosis?

Objective. Discuss complex interplay of pathophysiological effects of cerebellar space occupying lesions on the vestibular pathway. Discuss challenges of diagnosis and referral along with differential and final diagnosis of unusual presentation. Case Report. We describe the case of a patient with vertiginous symptoms complicated by neurological features, namely, head titubation and tremor. The patient also had signs of oscillopsia and possible impairment of the vestibulo-ocular reflex. The resulting symptom and sign complex made for a difficult diagnosis, as the interplay of the pathophysiology of these signs, were unusual. Conclusion. The discussion has revealed that the cerebellar lesions themselves may have simultaneously caused head tremor and an inability for the vestibulo-ocular reflex to compensate, resulting in vertigo. However, whether the vertigo was a result of an oscillopsia, nystagmus, or central cause, the referral route should initially be via a general physician to rule out such a life threatening cause as a tumour.

Cerebellar motor learning: when is cortical plasticity not enough?

Classical Marr-Albus theories of cerebellar learning employ only cortical sites of plasticity. However, tests of these theories using adaptive calibration of the vestibulo-ocular reflex (VOR) have indicated plasticity in both cerebellar cortex and the brainstem. To resolve this long-standing conflict, we attempted to identify the computational role of the brainstem site, by using an adaptive filter version of the cerebellar microcircuit to model VOR calibration for changes in the oculomotor plant. With only cortical plasticity, introducing a realistic delay in the retinal-slip error signal of 100 ms prevented learning at frequencies higher than 2.5 Hz, although the VOR itself is accurate up to at least 25 Hz. However, the introduction of an additional brainstem site of plasticity, driven by the correlation between cerebellar and vestibular inputs, overcame the 2.5 Hz limitation and allowed learning of accurate high-frequency gains. This "cortex-first" learning mechanism is consistent with a wide variety of evidence concerning the role of the flocculus in VOR calibration, and complements rather than replaces the previously proposed "brainstem-first" mechanism that operates when ocular tracking mechanisms are effective. These results (i) describe a process whereby information originally learnt in one area of the brain (cerebellar cortex) can be transferred and expressed in another (brainstem), and (ii) indicate for the first time why a brainstem site of plasticity is actually required by Marr-Albus type models when high-frequency gains must be learned in the presence of error delay.

Intrinsic membrane properties of vertebrate vestibular neurons: function, development and plasticity

Central vestibular neurons play an important role in the processing of body motion-related multisensory signals and their transformation into motor commands for gaze and posture control. Over recent years, medial vestibular nucleus (MVN) neurons and to a lesser extent other vestibular neurons have been extensively studied in vivo and in vitro, in a range of species. These studies have begun to reveal how their intrinsic electrophysiological properties may relate to their response patterns, discharge dynamics and computational capabilities. In vitro studies indicate that MVN neurons are of two major subtypes (A and B), which differ in their spike shape and after-hyperpolarizations. This reflects differences in particular K(+) conductances present in the two subtypes, which also affect their response dynamics with type A cells having relatively low-frequency dynamics (resembling "tonic" MVN cells in vivo) and type B cells having relatively high-frequency dynamics (resembling "kinetic" cells in vivo). The presence of more than one functional subtype of vestibular neuron seems to be a ubiquitous feature since vestibular neurons in the chick and frog also subdivide into populations with different, analogous electrophysiological properties. The ratio of type A to type B neurons appears to be plastic, and may be determined by the signal processing requirements of the vestibular system, which are species-variant. The membrane properties and discharge pattern of type A and type B MVN neurons develop largely post-natally, through the expression of the underlying ion channel conductances. The membrane properties of MVN neurons show rapid and long-lasting plastic changes after deafferentation (unilateral labyrinthectomy), which may serve to maintain their level of activity and excitability after the loss of afferent inputs.

Fos responses to short-term adaptation of the horizontal vestibuloocular reflex before and after vestibular compensation in the Mongolian gerbil

Fos expression in vestibular brainstem and cerebellar regions was evaluated during vestibular adaptation in the Mongolian gerbil. In addition, vestibular adaptation was evaluated in both normal and compensated animals, as vestibular compensation reorganizes the vestibular pathway constraining adaptive processes. Behaviorally, discordant optokinetic and vestibular input induced appropriate high and low gain in horizontal angular vestibuloocular reflex responses. In normal animals, low gain adaptation was more complete than high gain. However, in compensated animals, only low gain adaptation produced adaptive responses both toward and away from the lesion with appropriate gain shifts. High gain adaptation in compensated animals failed to result in gain adaptation for head movements toward the side of the lesion. Fos expression during acute vestibular adaptation in normal animals was found in the flocculus/paraflocculus, the dorsal cap of the inferior olive (IOK), and the prepositus hypoglossi (PrH). Floccular Fos labeling was increased under both high and low gain conditions. IOK and PrH labeling was increased and correlated during low gain conditions, but was reduced and uncorrelated during high gain conditions. The pattern of Fos labeling in compensated animals was asymmetric-favoring the ipsilesional flocculus and contralesional vestibular brainstem. Both compensated high and low gain adaptation groups displayed increased floccular and IOK Fos labeling, but only compensated high gain adaptation produced increased Fos labeling in the medial vestibular nucleus. The behavioral and Fos labeling results are consistent with visual-vestibular adaptation requiring direct vestibular input.

Reflex control of the spine and posture: a review of the literature from a chiropractic perspective

OBJECTIVE

This review details the anatomy and interactions of the postural and somatosensory reflexes. We attempt to identify the important role the nervous system plays in maintaining reflex control of the spine and posture. We also review, illustrate, and discuss how the human vertebral column develops, functions, and adapts to Earth's gravity in an upright position. We identify functional characteristics of the postural reflexes by reporting previous observations of subjects during periods of microgravity or weightlessness.

BACKGROUND

Historically, chiropractic has centered around the concept that the nervous system controls and regulates all other bodily systems; and that disruption to normal nervous system function can contribute to a wide variety of common ailments. Surprisingly, the chiropractic literature has paid relatively little attention to the importance of neurological regulation of static upright human posture. With so much information available on how posture may affect health and function, we felt it important to review the neuroanatomical structures and pathways responsible for maintaining the spine and posture. Maintenance of static upright posture is regulated by the nervous system through the various postural reflexes. Hence, from a chiropractic standpoint, it is clinically beneficial to understand how the individual postural reflexes work, as it may explain some of the clinical presentations seen in chiropractic practice.

METHOD

We performed a manual search for available relevant textbooks, and a computer search of the MEDLINE, MANTIS, and Index to Chiropractic Literature databases from 1970 to present, using the following key words and phrases: "posture," "ocular," "vestibular," "cervical facet joint," "afferent," "vestibulocollic," "cervicocollic," "postural reflexes," "spaceflight," "microgravity," "weightlessness," "gravity," "posture," and "postural." Studies were selected if they specifically tested any or all of the postural reflexes either in Earth's gravity or in microgravitational environments. Studies testing the function of each postural component, as well as those discussing postural reflex interactions, were also included in this review.

DISCUSSION

It is quite apparent from the indexed literature we searched that posture is largely maintained by reflexive, involuntary control. While reflexive components for postural control are found in skin and joint receptors, somatic graviceptors, and baroreceptors throughout the body, much of the reflexive postural control mechanisms are housed, or occur, within the head and neck region primarily. We suggest that the postural reflexes may function in a hierarchical fashion. This hierarchy may well be based on the gravity-dependent or gravity-independent nature of each postural reflex. Some or all of these postural reflexes may contribute to the development of a postural body scheme, a conceptual internal representation of the external environment under normal gravity. This model may be the framework through which the postural reflexes anticipate and adapt to new gravitational environments.

CONCLUSION

Visual and vestibular input, as well as joint and soft tissue mechanoreceptors, are major players in the regulation of static upright posture. Each of these input sources detects and responds to specific types of postural stimulus and perturbations, and each region has specific pathways by which it communicates with other postural reflexes, as well as higher central nervous system structures. This review of the postural reflex structures and mechanisms adds to the growing body of posture rehabilitation literature relating specifically to chiropractic treatment. Chiropractic interest in these reflexes may enhance the ability of chiropractic physicians to treat and correct global spine and posture disorders. With the knowledge and understanding of these postural reflexes, chiropractors can evaluate spinal configurations not only from a segmental perspective, but can also determine how spinal dysfunction may be the ultimate consequence of maintaining an upright posture in the presence of other postural deficits. These perspectives need to be explored in more detail.

Fos expression in the vestibular brainstem: what one marker can tell us about the network

Fos inducible transcription factor expression in rodent brains (rats and gerbils) during manipulations of vestibular input is reviewed. Stimuli included centripetal hypergravity, unilateral labyrinth lesion or semicircular canal plugging, rotational axis cross-coupling (Coriolis forces), high and low rotational vestibulo-ocular reflex gain adaptation, translabyrinth galvanic stimulation, pharmacological manipulation, and combinations thereof. Each type of stimulation elicited unique but partially redundant response patterns in the vestibulo-olivo-cerebellar (VOC) network that reflect the origin and interaction of the labyrinth inputs. On the basis of these patterns, a trained observer can predict what the animal experienced during testing; the patterns of VOC Fos expression reveal a trace of recent genomic activity. Based on principal component analysis, VOC network modules associated with lesion recovery, spatial representation and the calibration of gravity, and optokinetic influences are proposed. Probable and possible gene targets of the Fos protein are also reviewed.

Report on a workshop concerning the cerebellum and motor learning, held in St Louis October 2004

A one-day meeting on the cerebellum and motor learning was held in St Louis (October 2004), to address issues arising from a previous larger meeting (Tuebingen, June 2004). The learning tasks considered were VOR adaptation, saccadic adaptation and eyeblink conditioning. A theoretical development was reported that indicated how the cerebellum could use sensory error signals for adaptive control, by decorrelating them from an efferent copy of motor commands. The main topics for discussion were the nature of the error signals actually used by the cerebellum, and the evidence for multiple sites of synaptic plasticity. Reports of studies on VOR adaptation confirmed the presence of error signals in addition to retinal slip, in particular the eye-movement related simple-spike firing of floccular PCs. This firing appears to drive synaptic plasticity in the vestibular nuclei. From a theoretical perspective, a second site of plasticity in the brainstem has two advantages: it improves the high-frequency performance of the VOR given a delayed slip signal, and it allows VOR adaptation when smooth pursuit effectively removes the retinal slip signal. In contrast, some of the physiological data reported on saccadic adaptation seemed incompatible with current theoretical ideas about error signals. However, since other reported data were broadly consistent with those ideas, an important area of experimental disagreement was identified. Furthermore, behavioural studies indicated the presence of multiple sites of plasticity, consistent with earlier lesion studies that suggested one such site within cerebellar cortex and another outside it. Data from eyeblink conditioning suggested that the predictability of the error signal was important. Related ideas have previously emerged from studies of skeletal movement, but their theoretical implications for the cerebellar algorithm have yet to be fully explored. Finally, the long-standing controversy concerning sites of plasticity in eyeblink conditioning illustrated the technical difficulties involved in tracking down such sites.

Purkinje cell synapses target physiologically unique brainstem neurons

The cerebellum controls motor learning via Purkinje cell synapses onto discrete populations of neurons in the deep cerebellar nuclei and brainstem vestibular nuclei. In the circuitry that subserves the vestibulo-ocular reflex, the postsynaptic targets of Purkinje cells, termed flocculus target neurons (FTNs), are thought to be a critical site of learning. Little is known, however, about the intrinsic cellular properties of FTNs, which are sparsely distributed in the medial vestibular nucleus. To identify these neurons, we used the L7 promoter to express a tau-green fluorescent protein fusion protein selectively in Purkinje cells. Fluorescent Purkinje cell axons and terminal boutons surrounded the somata and proximal dendrites of a small subset of neurons, presumed FTNs, in the medial vestibular nucleus. Targeted intracellular recordings revealed that FTNs fired spontaneously at high rates in brain slices (mean, 47 spikes/sec) and exhibited dramatic postinhibitory rebound firing after the offset of membrane hyperpolarization. These intrinsic firing properties were exceptional among brainstem vestibular nucleus neurons but strikingly similar to neurons in the deep cerebellar nuclei, indicating a common role for intrinsic firing mechanisms in cerebellar control of diverse behaviors.

Floccular modulation of vestibuloocular pathways and cerebellum-related plasticity: An in vitro whole brain study

The isolated whole brain (IWB) preparation of the guinea pig was used to investigate the floccular modulation of vestibular-evoked responses in abducens and oculomotor nerves and abducens nucleus; for identification of flocculus target neurons (FTNs) in the vestibular nuclei and intracellular study of some of their physiological properties; to search for possible flocculus-dependent plasticity at the FTN level by pairing of vestibular nerve and floccular stimulations; and to study the possibility of induction of long-term depression (LTD) in Purkinje cells by paired stimulation of the inferior olive and vestibular nerve. Stimulation of the flocculus had only effects on responses evoked from the ipsilateral (with respect to the stimulated flocculus) vestibular nerve. Floccular stimulation significantly inhibited the vestibular-evoked discharges in oculomotor nerves on both sides and the inhibitory field potential in the ipsilateral abducens nucleus while the excitatory responses in the contralateral abducens nerve and nucleus were free from such inhibition. Eleven second-order vestibular neurons were found to receive a short-latency monosynaptic inhibitory input from the flocculus and were thus characterized as FTNs. Monosynaptic inhibitory postsynaptic potentials from the flocculus were bicuculline sensitive, suggesting a GABA(A)-ergic transmission from Purkinje cells to FTNs. Two of recorded FTNs could be identified as vestibulospinal neurons by their antidromic activation from the cervical segments of the spinal cord. Several pairing paradigms were investigated in which stimulation of the flocculus could precede, coincide with, or follow the vestibular nerve stimulation. None of them led to long-term modification of responses in the abducens nucleus or oculomotor nerve evoked by activation of vestibular afferents. On the other hand, pairing of the inferior olive and vestibular nerve stimulation resulted in approximately a 30% reduction of excitatory postsynaptic potentials evoked in Purkinje cells by the vestibular nerve stimulation. This reduction was pairing-specific and lasted throughout the entire recording time of the neurons. Thus in the IWB preparation, we were able to induce a LTD in Purkinje cells, but we failed to detect traces of flocculus-dependent plasticity at the level of FTNs in vestibular nuclei. Although these data cannot rule out the possibility of synaptic modifications in FTNs and/or at other brain stem sites under different experimental conditions, they are in favor of the hypothesis that the LTD in the flocculus could be the essential mechanism of cellular plasticity in the vestibuloocular pathways.