Brainstem and cerebellar fMRI-activation during horizontal and vertical optokinetic stimulation

Preliminary study on the computer-based optokinetic nystagmus analyzer to detect the visual acuity of preschool children

The purpose of this study is to examine the viability, precision, and consistency of a computer-based optokinetic nystagmus analyzer (nystagmus meter) for diagnosing eyesight in preschoolers. A total of 59 subjects who could pass the log of minimum angle of resolution (LogMAR) visual acuity chart were divided into three groups by age, 4-, 5-, and 6-year-old groups, and their visual acuity was tested with nystagmus meter. The percentage of children in each age group that could be detected by nystagmus was recorded along with the differences between these groups. The correlation between the test results from the two methods was found for each age group using the correlation coefficient method. Repeated measurements were used to assess the two visual acuity values of the measured nystagmus, and the repeatability of the two measurement techniques for different age groups was compared. The overall measurability of the visual acuity detected by nystagmus was 93.22%, and the measurability of the 4-, 5-, and 6-year-old groups was 90%, 95%, and 94.74%, respectively. There was no statistically significant difference in the measurability of subjects among all age groups (P = 1.0). The outcomes of the LogMAR visual acuity chart had a negative correlation with the visual acuity measured by the nystagmus meter. The overall correlation coefficient R value was -0.80, and the correlation coefficient R value of the 4-, 5-, and 6-year-old groups was -0.79, -0.76, and -0.87, respectively. The nystagmus meter has good feasibility, accuracy, and stability in visual acuity testing and can be used for visual acuity testing in children.

Ego- and Geo-Centered References: A Functional Neuroimagery Study

INTRODUCTION

The integration of vestibular, visual, and somatosensory cues allows the perception of space through the orientation of our body and surrounding objects with respect to gravity. The main goal of this study was to identify the cortical networks recruited during the representation of body midline and the representation of verticality.

METHODS

Thirty right-handed healthy participants were evaluated using fMRI. Brain networks activated during a subjective straight-ahead (SSA) task were compared to those recruited during a subjective vertical (SV) task.

RESULTS

Different patterns of cortical activation were observed, with differential increases in the angular gyrus and left cerebellum posterior lobe during the SSA task, in right rolandic operculum and cerebellum anterior lobe during the SV task.

DISCUSSION

The activation of these areas involved in visuo-spatial functions suggests that bodily processes of great complexity are engaged in body representation and vertical perception. Interestingly, the common brain networks involved in SSA and SV tasks were comprised of areas of vestibular projection that receive multisensory information (parieto-occipital areas) and the cerebellum, and reveal a predominance of the right cerebral and cerebellar hemispheres. The outcomes of this first fMRI study designed to unmask common and specific neural mechanisms at work in gravity- or body-referenced tasks pave a new way for the exploration of spatial cognitive impairment in patients with vestibular or cortical disorders.

Differential contributions of serotonergic and dopaminergic functional connectivity to the phenomenology of LSD

RATIONALE

LSD is the prototypical psychedelic. Despite a clear central role of the 5HT2a receptor in its mechanism of action, the contributions of additional receptors for which it shows affinity and agonist activity remain unclear.

OBJECTIVES

We employed receptor-enriched analysis of functional connectivity by targets (REACT) to explore differences in functional connectivity (FC) associated with the distributions of the primary targets of LSD-the 5HT1a, 5HT1b, 5HT2a, D1 and D2 receptors.

METHODS

We performed secondary analyses of an openly available dataset (N = 15) to estimate the LSD-induced alterations in receptor-enriched FC maps associated with these systems. Principal component analysis (PCA) was employed as a dimension reduction strategy for subjective experiences associated with LSD captured by the Altered States of Consciousness (ASC) questionnaire. Correlations between these principal components as well as VAS ratings of subjective effects with receptor-enriched FC were explored.

RESULTS

Compared to placebo, LSD produced differences in FC when the analysis was enriched with each of the primary serotonergic and dopaminergic receptors. Altered receptor-enriched FC showed relationships with the subjective effects of LSD on conscious experience, with serotonergic and dopaminergic systems being predominantly associated with perceptual effects and perceived selfhood as well as cognition respectively. These relationships were dissociable, with different receptors showing the same relationships within, but not between, the serotonergic and dopaminergic systems.

CONCLUSIONS

These exploratory findings provide new insights into the pharmacology of LSD and highlight the need for additional investigation of non-5HT2a-mediated mechanisms.

α-Synuclein Induces Progressive Changes in Brain Microstructure and Sensory-Evoked Brain Function That Precedes Locomotor Decline

In vivo functional and structural brain imaging of synucleinopathies in humans have provided a rich new understanding of the affected networks across the cortex and subcortex. Despite this progress, the temporal relationship between α-synuclein (α-syn) pathology and the functional and structural changes occurring in the brain is not well understood. Here, we examine the temporal relationship between locomotor ability, brain microstructure, functional brain activity, and α-syn pathology by longitudinally conducting rotarod, diffusion magnetic resonance imaging (MRI), resting-state functional MRI (fMRI), and sensory-evoked fMRI on 20 mice injected with α-syn fibrils and 20 PBS-injected mice at three timepoints (10 males and 10 females per group). Intramuscular injection of α-syn fibrils in the hindlimb of M83+/- mice leads to progressive α-syn pathology along the spinal cord, brainstem, and midbrain by 16 weeks post-injection. Our results suggest that peripheral injection of α-syn has acute systemic effects on the central nervous system such that structural and resting-state functional activity changes occur in the brain by four weeks post-injection, well before α-syn pathology reaches the brain. At 12 weeks post-injection, a separate and distinct pattern of structural and sensory-evoked functional brain activity changes was observed that are co-localized with previously reported regions of α-syn pathology and immune activation. Microstructural changes in the pons at 12 weeks post-injection were found to predict survival time and preceded measurable locomotor deficits. This study provides preliminary evidence for diffusion and fMRI markers linked to the progression of synuclein pathology and has translational importance for understanding synucleinopathies in humans.SIGNIFICANCE STATEMENT α-Synuclein (α-syn) pathology plays a critical role in neurodegenerative diseases such as Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. The longitudinal effects of α-syn pathology on locomotion, brain microstructure, and functional brain activity are not well understood. Using high field imaging, we show preliminary evidence that peripheral injection of α-syn fibrils induces unique patterns of functional and structural changes that occur at different temporal stages of α-syn pathology progression. Our results challenge existing assumptions that α-syn pathology must precede changes in brain structure and function. Additionally, we show preliminary evidence that diffusion and functional magnetic resonance imaging (fMRI) are capable of resolving such changes and thus should be explored further as markers of disease progression.

Perceptual Postural Imbalance and Visual Vertigo

PURPOSE OF REVIEW

Disorders of posture and balance cause significant patient morbidity, with reduction of quality of life as patients refrain from critical activities of daily living such as walking outside the home and driving. This review describes recent efforts to characterize visual disorders that interact with the neural integrators of positional maintenance and emerging therapies for these disorders.

RECENT FINDINGS

Abnormalities of gait and body position sense may be unrecognized by patients but are correlated with focal neurological injury (stroke). Patients with traumatic brain injury can exhibit visual vertigo despite otherwise normal visual functioning. The effect of visual neglect on posture and balance, even in the absence of a demonstrable visual field defect, has been characterized quantitatively through gait analysis and validates the potential therapeutic value of prism treatment in some patients. In addition, the underlying neural dysfunction in visual vertigo has been explored further using functional imaging, and these observations may allow discrimination of patients with structural causes from those whose co-morbid psychosocial disorders may be primarily contributory.

Encoding of 3D head direction information in the human brain

Head direction cells are critical for navigation because they convey information about which direction an animal is facing within an environment. To date, most studies on head direction encoding have been conducted on a horizontal two-dimensional (2D) plane, and little is known about how three-dimensional (3D) direction information is encoded in the brain despite humans and other animals living in a 3D world. Here, we investigated head direction encoding in the human brain while participants moved within a virtual 3D "spaceship" environment. Movement was not constrained to planes and instead participants could move along all three axes in volumetric space as if in zero gravity. Using functional magnetic resonance imaging (fMRI) multivoxel pattern similarity analysis, we found evidence that the thalamus, particularly the anterior portion, and the subiculum encoded the horizontal component of 3D head direction (azimuth). In contrast, the retrosplenial cortex was significantly more sensitive to the vertical direction (pitch) than to the azimuth. Our results also indicated that vertical direction information in the retrosplenial cortex was significantly correlated with behavioral performance during a direction judgment task. Our findings represent the first evidence showing that the "classic" head direction system that has been identified on a horizontal 2D plane also seems to encode vertical and horizontal heading in 3D space in the human brain.

Use of optokinetic chart stimulation to restore mobility and reduce ataxia in a patient with pseudo-Cushing ataxia

A 61-year-old patient was admitted to hospital after a fall. She presented with bilateral muscle weakness and severe ataxia. She was unable to maintain sitting balance or place feet on the floor and was unable to tolerate hoist transfers due to the severity of her ataxia. Nursing and physiotherapy staff found it difficult to sit her out of bed. Her physiotherapy intervention changed to optokinetic chart stimulation (OKCS) and sensory interaction for balance. After treatment for 5 days, her intention tremor fully resolved. At discharge, she was mobile with a wheeled zimmer walking frame and supervision of one person. At follow-up after 8 months, she was independently mobile without any walking aid in and around her house. She was going out shopping with her son. For recovery from ataxia, it is recommended that further research on restorative intervention at the nervous system level be carried out.

Effects of Dopamine and Norepinephrine on Exercise-induced Oculomotor Fatigue

INTRODUCTION

Fatigue-induced impairments in the control of eye movements are detectable via reduced eye movement velocity after a bout of prolonged, strenuous exercise. Slower eye movements caused by neural fatigue within the oculomotor system can be prevented by caffeine, and the upregulation of central catecholamines may be responsible for this effect. This study explored the individual contribution of dopamine and norepinephrine to fatigue-related impairments in oculomotor control.

METHODS

The influence of a dopamine reuptake inhibitor (methylphenidate) and a norepinephrine reuptake inhibitor (reboxetine) was assessed in 12 cyclists performing 180 min of stationary cycling within a placebo-controlled crossover design. Eye movement kinematics (saccades, smooth pursuit, and optokinetic nystagmus) were measured using infrared oculography. Visual attention was assessed with overt and covert spatial attention tasks.

RESULTS

Exercise-induced fatigue was associated with a 6% ± 8% reduction in the peak velocity of visually guided, reflexive prosaccades. Importantly, both dopamine reuptake inhibition and norepinephrine reuptake inhibition prevented fatigue-related decrements in the peak velocity of prosaccades. Pursuit eye movements, optokinetic nystagmus, and visual attention tasks were unaffected by exercise or drug treatments.

CONCLUSION

Overall, our findings suggest that alterations in norepinephrinergic and dopaminergic neurotransmission are linked with the development of fatigue within circuits that control eye movements. Psychiatric medications that target central catecholamines can exert a protective effect on eye movements after prolonged exercise.

When one is Enough: Impaired Multisensory Integration in Cerebellar Agenesis

In the last two decades, an intriguing shift in the understanding of the cerebellum has led to consider the nonmotor functions of this structure. Although various aspects of perceptual and sensory processing have been linked to the cerebellar activity, whether the cerebellum is essential for binding information from different sensory modalities remains uninvestigated. Multisensory integration (MSI) appears very early in the ontogenesis and is critical in several perceptual, cognitive, and social domains. For the first time, we investigated MSI in a rare case of cerebellar agenesis without any other associated brain malformations. To this aim, we measured reaction times (RTs) after the presentation of visual, auditory, and audiovisual stimuli. A group of neurotypical age-matched individuals was used as controls. Although we observed the typical advantage of the auditory modality relative to the visual modality in our patient, a clear impairment in MSI was found. Beyond the obvious prudence necessary for inferring definitive conclusions from this single-case picture, this finding is of interest in the light of reduced MSI abilities reported in several neurodevelopmental and psychiatric disorders-such as autism, dyslexia, and schizophrenia-in which the cerebellum has been implicated.

The inion response revisited: evidence for a possible cerebellar contribution to vestibular-evoked potentials produced by air-conducted sound stimulation

This study investigated the effect of eye gaze and head position on vestibular-evoked potentials (VsEPs). Head position would be expected to affect myogenic sources, and eye position is known to affect ocular myogenic responses (ocular vestibular-evoked myogenic potentials), whereas a neurogenic source should behave otherwise. Eleven healthy subjects were recruited, and VsEPs, using 72-channel EEG, were recorded at a fixed intensity above the vestibular threshold. Three eye gaze and three head positions were tested (-20°, 0°, and +20° to the horizontal). Short-latency potentials showed that in addition to the expected effect of gaze on infraocular (IO') leads, where up-gaze gives a maximum response, significant changes in amplitude were also observed in electrodes remote from the eyes and in particular, from contralateral parietal-occipital (PO) and neck (CB') leads. Short-latency potentials of similar latency were observed (p10/n17 and n10/p17, respectively). The pattern of change with gaze in the PO leads was distinct from that observed for the IO' leads. For the PO leads, the maximum response was obtained with neutral gaze, and this was also distinct from that observed for CB' electrodes, where a maximal response was observed with head flexion in the second wave but not the first. Evidence of modulation of N42 and N1 potentials with both eye and head position was also observed. Head- and eye-position manipulation thus suggests that the inion response consists of an early neurogenic component, as well as myogenic responses. The p10/n17 at PO, in particular, may be an indicator of vestibulocerebellar projections.NEW & NOTEWORTHY Loud sounds were used to activate vestibular receptors in human volunteers and the effects of head and eye position studied for short-latency responses. A potential (p10/n17) recorded in the parieto-occipital leads showed behavior not expected for a response with a myogenic origin. Source modeling suggested a possible origin from the cerebellum. It may represent a new indicator of human vestibulocerebellar function.

Fatigue-related impairments in oculomotor control are prevented by norepinephrine-dopamine reuptake inhibition

Fatigue-induced reductions in saccade velocity have been reported following acute, prolonged exercise. Interestingly, the detrimental impact of fatigue on oculomotor control can be prevented by a moderate dose of caffeine. This effect may be related to central catecholamine upregulation via caffeine's action as an adenosine antagonist. To test this hypothesis, we compared the protective effect of caffeine on oculomotor control post-exercise to that of a norepinephrine-dopamine reuptake inhibitor. Within a placebo-controlled crossover design, 12 cyclists consumed placebo, caffeine or a norepinephrine-dopamine reuptake inhibitor (bupropion) during 180 minutes of stationary cycling. Saccades, smooth pursuit and optokinetic nystagmus were measured using infrared oculography. Exercise fatigue was associated with an 8 ± 11% reduction in the peak velocity of prosaccades, and a 10 ± 11% decrement in antisaccade peak velocity. Optokinetic nystagmus quick phases decreased in velocity by 15 ± 17%. These differences were statistically significant (p < 0.05). Norepinephrine-dopamine reuptake inhibition and caffeine prevented fatigue-related decrements in eye movement velocity. Pursuit eye movements and visual attention were unaffected. These findings show that norepinephrine-dopamine reuptake inhibition protects oculomotor function during exercise fatigue. Caffeine's fatigue-reversing effects on eye movements appear to be mediated, at least in part, via modulation of central catecholamines.

Improvement of hemispatial neglect by a see-through head-mounted display: a preliminary study

Background

Patients with right hemisphere damage are often unaware of, inattentive to and fail to interact with stimuli on their left side. This disorder, called hemispatial neglect, is a major source of disability. Inducing leftward ocular pursuit by optokinetic stimulation (OKS) relieves some of the signs of unilateral neglect. However, it is difficult to provide patients with a continuously moving background that is required for OKS. We studied whether OKS projected onto a see-through head-mounted display (HMD) would help treat neglect.

Methods

14 patients with neglect after cerebral infarction performed line bisections on a computer screen, both with and without OKS that was either delivered by the HMD or on the same screen that was displaying the lines that were to be bisected.

Results

The line bisection performances were significantly different in the four conditions (P < 0.001). The post hoc analyses indicated that the rightward deviation observed in the control conditions on the line bisection tasks without OKS, improved significantly with the use OKS in both the HMD and screen conditions (α < 0.05). The results between the screen and HMD conditions were also different (α < 0.05). The OKS in the HMD condition corrected patients’ rightward deviation more toward the actual midline than did the OKS provided during the screen condition.

Conclusions

OKS projected onto the see-through HMD improved hemispatial neglect. The development of a portable device may aid in the treatment of neglect.

Consensus paper: the role of the cerebellum in perceptual processes

Various lines of evidence accumulated over the past 30 years indicate that the cerebellum, long recognized as essential for motor control, also has considerable influence on perceptual processes. In this paper, we bring together experts from psychology and neuroscience, with the aim of providing a succinct but comprehensive overview of key findings related to the involvement of the cerebellum in sensory perception. The contributions cover such topics as anatomical and functional connectivity, evolutionary and comparative perspectives, visual and auditory processing, biological motion perception, nociception, self-motion, timing, predictive processing, and perceptual sequencing. While no single explanation has yet emerged concerning the role of the cerebellum in perceptual processes, this consensus paper summarizes the impressive empirical evidence on this problem and highlights diversities as well as commonalities between existing hypotheses. In addition to work with healthy individuals and patients with cerebellar disorders, it is also apparent that several neurological conditions in which perceptual disturbances occur, including autism and schizophrenia, are associated with cerebellar pathology. A better understanding of the involvement of the cerebellum in perceptual processes will thus likely be important for identifying and treating perceptual deficits that may at present go unnoticed and untreated. This paper provides a useful framework for further debate and empirical investigations into the influence of the cerebellum on sensory perception.

Smooth Pursuit Eye Movement Training Promotes Recovery From Auditory and Visual Neglect

Background. No treatment for auditory neglect and no randomized controlled trial evaluating smooth pursuit eye movement therapy (SPT) for multimodal neglect are available. Objective. To compare the effects of SPT and visual scanning therapy (VST) on auditory and visual neglect in chronic stroke patients with neglect. Methods. A randomized, prospective trial was conducted. Fifty patients with left auditory and visual neglect were randomly assigned. Twenty-four patients completed SPT therapy and 21 patients VST. Five patients (4 VST, 1 SPT) were lost. Each group received 1-hour sessions of neglect therapy for 5 consecutive days totaling 5 hours. Outcome measures in visual neglect (digit cancellation, visuoperceptual- and motor line bisection, paragraph reading) and auditory neglect (auditory midline) were assessed twice before therapy, thereafter, and at 2-week follow-up. The SPT group practiced smooth pursuit eye movements while tracking stimuli moving leftward. The VST group systematically scanned the same but static stimuli. Both groups were divided into subgroups, and effects were separately investigated for mild and severe neglect. Results. Both groups did not differ before therapy in clinical/demographic variables or neglect severity (auditory/visual). After treatment, the SPT group showed significant and lasting improvements in all visual measures and normal performance in the auditory midline. Neither visual nor auditory neglect impairments changed significantly after VST. Moreover, the treatment effect sizes (Cohen’s d) were considerably higher for visual and auditory neglect after SPT versus VST, both for mild and severe neglect. Conclusions. Repetitive contralesional, smooth pursuit training induces superior, multimodal therapeutic effects in mild and severe neglect.

Age-related changes of blood-oxygen-level-dependent signal dynamics during optokinetic stimulation

The present study aimed to reveal the effects of age on the temporal profile of the positive blood-oxygen-level-dependent response (PBR) during low-velocity optokinetic nystagmus (OKN) in healthy subjects. We were specifically interested in comparing these effects with the effects of age on the PBR elicited by pure visual and motor tasks. Therefore, we conducted 2 additional control experiments: a checkerboard experiment (visual stimulation) and a finger-tapping (motor task) experiment. Whereas the oculomotor performance of the subjects remained unaltered, the temporal profile of the PBR changed significantly with increasing age in visual and oculomotor areas. None of the control experiments revealed significant age-related PBR changes. Thus, this study demonstrates that the PBR changes during OKN occur before any changes in the oculomotor performance can be detected. These effects of age are specific for the OKN task and probably reflect both changes in the neurovascular coupling and changes in the neural processing during OKN.

Novel use of optokinetic chart stimulation to restore muscle strength and mobility in patients with subdural haemorrhage: two case studies

BACKGROUND AND PURPOSE

The aim of these case studies is to report on the novel use of optokinetic chart stimulation to restore muscle strength and functional independence in severely disabled patients with subdural haemorrhage.

METHODOLOGY

An optokinetic chart was moved in front of the patient: from side to side, up and down and finally forwards and backwards. Once the patient could stand optokinetic chart stimulation was supplemented by sensory interaction for balance.

RESULTS

In first case study contractures in the knees improved from 90° short of full passive knee extension bilaterally to full active extension of the right knee joint and 25° short of full active extension of the left knee joint. The Barthel index improved from 0/20 to 18/20. In the second case study the Barthel Index improved from 0/20 to 18/20. From being bed bound the patients were independently mobile with a walking aid, on discharge.

DISCUSSION

With optokinetic chart stimulation leading to reversal of knee flexor contractures in one case and restoration of independent mobility in both cases, more research is needed to investigate use of optokinetic chart stimulation in patients with severe subdural haemorrhages and other traumatic brain injuries.

Altered connectivity of the balance processing network after tongue stimulation in balance-impaired individuals

Some individuals with balance impairment have hypersensitivity of the motion-sensitive visual cortices (hMT+) compared to healthy controls. Previous work showed that electrical tongue stimulation can reduce the exaggerated postural sway induced by optic flow in this subject population and decrease the hypersensitive response of hMT+. Additionally, a region within the brainstem (BS), likely containing the vestibular and trigeminal nuclei, showed increased optic flow-induced activity after tongue stimulation. The aim of this study was to understand how the modulation induced by tongue stimulation affects the balance-processing network as a whole and how modulation of BS structures can influence cortical activity. Four volumes of interest, discovered in a general linear model analysis, constitute major contributors to the balance-processing network. These regions were entered into a dynamic causal modeling analysis to map the network and measure any connection or topology changes due to the stimulation. Balance-impaired individuals had downregulated response of the primary visual cortex (V1) to visual stimuli but upregulated modulation of the connection between V1 and hMT+ by visual motion compared to healthy controls (p ≤ 1E-5). This upregulation was decreased to near-normal levels after stimulation. Additionally, the region within the BS showed increased response to visual motion after stimulation compared to both prestimulation and controls. Stimulation to the tongue enters the central nervous system at the BS but likely propagates to the cortex through supramodal information transfer. We present a model to explain these brain responses that utilizes an anatomically present, but functionally dormant pathway of information flow within the processing network.

Visuomotor cerebellum in human and nonhuman primates

In this paper, we will review the anatomical components of the visuomotor cerebellum in human and, where possible, in non-human primates and discuss their function in relation to those of extracerebellar visuomotor regions with which they are connected. The floccular lobe, the dorsal paraflocculus, the oculomotor vermis, the uvula-nodulus, and the ansiform lobule are more or less independent components of the visuomotor cerebellum that are involved in different corticocerebellar and/or brain stem olivocerebellar loops. The floccular lobe and the oculomotor vermis share different mossy fiber inputs from the brain stem; the dorsal paraflocculus and the ansiform lobule receive corticopontine mossy fibers from postrolandic visual areas and the frontal eye fields, respectively. Of the visuomotor functions of the cerebellum, the vestibulo-ocular reflex is controlled by the floccular lobe; saccadic eye movements are controlled by the oculomotor vermis and ansiform lobule, while control of smooth pursuit involves all these cerebellar visuomotor regions. Functional imaging studies in humans further emphasize cerebellar involvement in visual reflexive eye movements and are discussed.

Neural circuit involved in idiopathic infantile nystagmus syndrome based on FMRI

PURPOSE

To identify the neural circuitry of idiopathic infantile nystagmus syndrome (INS), characterized by an early onset alternating series of slow and rapid eye movements that can manifest in different waveforms and genetic lines. The neural circuitry of INS is currently unknown.

METHODS

A novel functional magnetic resonance imaging (fMRI) method, referred to as the null zone fMRI technique, was used to identify the neural circuitry for INS. In the null zone fMRI technique, a gaze position with minimal nystagmus within the null zone was linked to the fMRI "off" condition and a gaze position with robust nystagmus outside of the null zone was linked to the fMRI "on" condition. Eye movements were monitored with an fMRI compatible eye tracker and observed in real time to ensure subject compliance in "on" and "off" states. Subjects with INS (n = 4) included three family members (a mother and two daughters) with presumed autosomal dominant INS, as well as age- and gender-matched normal controls (n = 3).

RESULTS

Three of four subjects with INS demonstrated significant increased activation of the declive of the cerebellum, whereas no normal subjects under identical conditions showed activation of the declive of the cerebellum. Both groups showed significant activation in the occipital lobe (Brodmann areas 17, 18, 19, and cuneus).

CONCLUSION

A novel fMRI method demonstrated that the declive of the cerebellum is actively involved in INS. These are the first results to identify the cerebellum, and specifically the declive, as a possible site involved in the ocular motor dysfunction known as INS.

Cerebellar and visual gray matter brain volume increases in congenital nystagmus

Structural brain abnormalities associated with congenital nystagmus (CN) are still unknown. In some patients with CN additional sensory, metabolic, or gross structural alterations can be detected. In the present study voxel-based morphometry was used to compare the gray matter (GM) brain volumes of 14 individuals with CN without associated sensory, metabolic, or obvious structural alterations (i.e., idiopathic CN) to those of a group of controls. Further, GM brain volumes were correlated with nystagmus severity as measured by sway path. Intergroup comparison exhibited significant volume increases in the human motion sensitive complex V5/MT+, the fusiform gyrus, and the middle occipital gyrus bilaterally in CN. These volume increases may be associated with excess visual motion stimulation due to involuntary retinal slip of the visual scene. A positive correlation (linear model) of nystagmus sway path with cerebellar GM volume was seen in the following areas: vermal parts VIII-X as well as hemisphere lobule II, hemisphere VI, crus I, crus II, and lobule VII-IX bilaterally. There is evidence that the reported GM volume changes in the vestibulo-cerebellum, which correlated with nystagmus sway path, might be related to the subjects‘ attempt to maintain fixation, rather than be due to the generation of nystagmus.

Electrical tongue stimulation normalizes activity within the motion-sensitive brain network in balance-impaired subjects as revealed by group independent component analysis

Multivariate analysis of functional magnetic resonance imaging (fMRI) data allows investigations into network behavior beyond simple activations of individual regions. We apply group independent component analysis to fMRI data collected in a previous study looking at the sustained neuromodulatory effects of electrical tongue stimulation in balance-impaired individuals. Twelve subjects with balance disorders viewed optic flow in an fMRI scanner before and after 5 days of electrical tongue stimulation. Nine healthy controls also viewed the visual stimuli but did not receive any stimulation. Multiple regression of the 47 estimated components found two that were modulated by the visual stimuli. Component 7, comprised primarily of the primary visual cortex (V1), responded to all visual stimuli and showed no difference in task-related activity between the healthy controls and the balance-impaired subjects before or after stimulation. Component 11 responded only to motion in the visual field and contained multiple cortical and subcortical regions involved in processing information pertinent to balance. Two-sample t-tests of the calculated signal change revealed that the task-related activity of this network is greater in balance-impaired subjects compared with controls before stimulation (p=0.02), but that this network hypersensitivity decreases after electrical tongue stimulation (p=0.001).

An assessment of optokinetic nystagmus (OKN) in persons with Down syndrome

Down syndrome (DS), the most common genetically defined cause of intellectual disability, is the phenotypic consequence of a supernumerary chromosome 21. Persons with DS commonly display deficits in visuomotor integration, motor coordination, and balance. Despite the key roles of the optokinetic and vestibular systems in these submodalities of motor function, a systematic investigation of the optokinetic nystagmus (OKN) and vestibulo-ocular reflex (VOR) in persons with DS had lacked in the literature. Accordingly, this study generated quantitative data on oculomotor function in persons with DS under optokinetic and sinusoidal smooth pursuit stimulation. Thirty-two participants with DS (14-36 years old, equally divided by gender) and 32 chronological age- and gender-matched typically developing controls were recruited from the community. Eye movements were recorded by binocular video oculography and an LCD projector produced visual stimulation. Assessments of the gain and frequency of slow phase of OKN beats and number and mean amplitude of intruding saccades during smooth pursuit were performed. Individuals with DS displayed angular velocity-dependent reduction in OKN gain and number of produced nystagmus beats compared to controls. The gain of the smooth pursuit was not significantly different between participants with DS and control participants. However, the number and mean amplitude of intruding saccades during smooth pursuit were increased in participants with DS compared to control participants. These findings may have implications to the understanding of the neurological basis of the motor dysfunction that affects performance in many practical tasks persons with DS encounter in their everyday lives.

Functional neuroimaging of the oculomotor brainstem network in humans

The cortical systems involved in eye movement control in humans have been investigated extensively using fMRI. In contrast, there is virtually no data concerning the functional status of the human oculomotor brainstem nuclei. This lack of evidence has usually been explained by technical constraints of EPI based imaging and anatomical characteristics of the brainstem. Against this assumption, we successfully localised nuclei of the oculomotor system using high-resolution fMRI based on standard EPI sequences in a group of healthy subjects executing reflexive horizontal saccades. A random-effects group analysis revealed task-related BOLD increases in the superior colliculus, the oculomotor nucleus, the abducens nucleus and in the paramedian pontine reticular formation. This group analysis was complemented by individual positive findings in up to 94% of single subject analyses. A visual control paradigm led to increased signal levels in the superior colliculus consistent with its visual properties but no corresponding signal changes in other brainstem nuclei. These results are consistent with findings in animal studies and demonstrate the feasibility to detect BOLD signal increases associated with oculomotor tasks even in the human brainstem using conventional EPI imaging techniques.

Sustained cortical and subcortical neuromodulation induced by electrical tongue stimulation

This pilot study aimed to show that information-free stimulation of the tongue can improve behavioral measures and induce sustained neuromodulation of the balance-processing network in individuals with balance dysfunction. Twelve balance-impaired subjects received one week of cranial nerve non-invasive neuromodulation (CN-NINM). Before and after the week of stimulation, postural sway and fMRI activation were measured to monitor susceptibility to optic flow. Nine normal controls also underwent the postural sway and fMRI tests but did not receive CN-NINM. Results showed that before CN-NINM balance-impaired subjects swayed more than normal controls as expected (p ≤ 0.05), and that overall sway and susceptibility to optic flow decreased after CN-NINM (p ≤ 0.005 & p ≤ 0.05). fMRI showed upregulation of visual sensitivity to optic flow in balance-impaired subjects that decreased after CN-NINM. A region of interest analysis indicated that CN-NINM may induce neuromodulation by increasing activity within the dorsal pons (p ≤ 0.01).

The effect of repeated visual motion stimuli on visual dependence and postural control in normal subjects

Patients with vestibular dysfunction, migraine and/or anxiety may experience visual vertigo (VV), whereby symptoms are provoked by disorienting visual environments (e.g. supermarkets). Patients with VV over rely on vision for balance (i.e. visually dependent). Visual vertigo significantly improves when vestibular rehabilitation incorporates exposure to optokinetic stimulation (OKS). However, whether OKS exposure induces a reduction in visual dependency is unknown. This study investigated this issue by measuring visual dependency before and after repeated OKS exposure. Twenty-six healthy subjects (10 males; mean age 29.8 years, range 20-42 years) were randomly allocated into an OKS group who underwent graded OKS exposure for five consecutive days, or a no intervention control group. Assessment included the 'Rod and Frame' and 'Rod and Disc' tests where subjects set the subjective visual vertical in darkness, facing a tilted luminous frame or luminous rotating disc, respectively. Postural sway measures were obtained with eyes open, closed and facing the rotating disc. Results showed significant reductions in subjective vertical tilt with the frame and rotating disc for the OKS group only (p≤0.01). Total sway path and mean deviation induced by the rotating stimulus decreased significantly only for the OKS group (p<0.01), as did the Kinetic Quotient (disc rotation/eyes open sway path ratio; p=0.04). The Romberg Quotient (eyes closed/eyes open ratio) showed no change. Findings suggest visual dependency, both at a perceptual and a postural level, can be reduced with short-term graded OKS exposure in healthy subjects. This has important implications for treatment of patients with VV and balance disorders.

The use of optokinetic stimulation in vestibular rehabilitation

Individuals with vestibular dysfunction may experience visual vertigo (VV), in which symptoms are provoked or exacerbated by excessive or disorienting visual stimuli (eg, supermarkets). Individuals with VV are believed to be overly reliant on visual input for balance (ie, visually dependent). VV can significantly improve when customized vestibular rehabilitation exercises are combined with exposure to optokinetic stimuli. However, the frequency of treatment sessions (twice weekly for 8 weeks) and the equipment used (expensive and space consuming) make it difficult to incorporate these techniques into everyday clinical practice where exercises may be practiced unsupervised. The aim of this focused review is to provide an overview of recent findings investigating (a) responses of individuals with vestibular deficits to a customized exercise program incorporating exposure to optokinetic stimuli via a "high-tech" visual environment rotator or a "low-tech" DVD with and without supervision, and (b) the mechanism of recovery. Optokinetic stimulation will also be discussed in relation to other new innovations in vestibular rehabilitation techniques and future work.

Eye movement abnormalities in Joubert syndrome

PURPOSE

Joubert syndrome is a genetic disorder characterized by hypoplasia of the midline cerebellum and deficiency of crossed connections between neural structures in the brain stem that control eye movements. The goal of the study was to quantify the eye movement abnormalities that occur in Joubert syndrome.

METHODS

Eye movements were recorded in response to stationary stimuli and stimuli designed to elicit smooth pursuit, saccades, optokinetic nystagmus (OKN), vestibulo-ocular reflex (VOR), and vergence using video-oculography or Skalar search coils in 8 patients with Joubert syndrome. All patients underwent high-resolution magnetic resonance imaging (MRI).

RESULTS

All patients had the highly characteristic molar tooth sign on brain MRI. Six patients had conjugate pendular (n = 4) or see-saw nystagmus (n = 2); gaze holding was stable in four patients. Smooth-pursuit gains were 0.28 to 1.19, 0.11 to 0.68, and 0.33 to 0.73 at peak stimulus velocities of 10, 20, and 30 deg/s in six patients; smooth pursuit could not be elicited in four patients. Saccade gains in five patients ranged from 0.35 to 0.91 and velocities ranged from 60.9 to 259.5 deg/s. Targeted saccades could not be elicited in five patients. Horizontal OKN gain was uniformly reduced across gratings drifted at velocities of 15, 30, and 45 deg/s. VOR gain was 0.8 or higher and phase appropriate in three of seven subjects; VOR gain was 0.3 or less and phase was indeterminate in four subjects.

CONCLUSIONS

The abnormalities in gaze-holding and eye movements are consistent with the distributed abnormalities of midline cerebellum and brain stem regions associated with Joubert syndrome.

The human premotor oculomotor brainstem system - can it help to understand oculomotor symptoms in Huntington's disease?

Recent progress in oculomotor research has enabled new insights into the functional neuroanatomy of the human premotor oculomotor brainstem network. In the present review, we provide an overview of its functional neuroanatomy and summarize the broad range of oculomotor dysfunctions that may occur in Huntington's disease (HD) patients. Although some of these oculomotor symptoms point to an involvement of the premotor oculomotor brainstem network in HD, no systematic analysis of this functional system has yet been performed in brains of HD patients. Therefore, its exact contribution to oculomotor symptoms in HD remains unclear. A possible strategy to clarify this issue is the use of unconventional 100-microm-thick serial tissue sections stained for Nissl substance and lipofuscin pigment (Nissl-pigment stain according to Braak). This technique makes it possible to identify the known nuclei of the premotor oculomotor brainstem network and to study their possible involvement in the neurodegenerative process. Studies applying this morphological approach and using the current knowledge regarding the functional neuroanatomy of this human premotor oculomotor brainstem network will help to elucidate the anatomical basis of the large spectrum of oculomotor dysfunctions that are observed in HD patients. This knowledge may aid clinicians in the diagnosis and monitoring of the disease.

Reduced self-motion perception in patients with midline cerebellar lesions

Vestibular input to the cerebellum mediates balance and eye movement control. Recent functional MRI studies, however, show midline cerebellar activation during visually induced illusions of self-rotation, thus suggesting that the cerebellum may also contribute to self-motion perception. Here, we investigate self-motion perception directly in patients with vermal (or midline) cerebellar ataxia. Participants were rotated in the dark (90 degrees /s velocity steps) and the time constant of decay of the postrotational angular velocity sensation was measured. The perceptual vestibular time constant in patients was considerably reduced (7.8 s) with respect to control values in this (25.6 s) and several previous studies. In addition to the processing of vestibular signals for motor control, the cerebellar vermis is involved in vestibular processing of self-motion perception.

Ocular motor syndromes of the brainstem and cerebellum

PURPOSE OF REVIEW

The brainstem and cerebellum contain many neuronal types that play a critical role in eye movement control. In a physiological approach, understanding how these neuronal assemblies cooperate provides strong insight into general brain functions. Furthermore, eye movements provide an interesting model for understanding neural mechanisms of sensorimotor learning, and a knowledge of the mechanisms underlying oculomotor plasticity is essential for correctly diagnosing and effectively managing patients. Finally, knowledge of the ocular motor syndromes frequently helps localize the pathological abnormality.

RECENT FINDINGS

We review the recently published works dealing with the physiological organization and pathology of slow and rapid eye movements at a brainstem and cerebellar level.

SUMMARY

The main recent findings of great interest for clinical practice or research concern the physiopathology of head shaking nystagmus, downbeat nystagmus and oculopalatal tremor; the neural substrates of three-dimensional control of eye movements and of optokinetic nystagmus; the understanding of saccade generation and of its consequences on physiological and pathological eye oscillations; and, finally, the physiological basis of saccadic adaptation.

Functional neuroanatomy of the human premotor oculomotor brainstem nuclei: insights from postmortem and advanced in vivo imaging studies

Considerable progress has been made recently in the field of the functional neuroanatomy of the primate oculomotor system, which has also improved our understanding of the structure, organization and function of the human oculomotor system. In the present review we provide for the first time an overview of the neuroanatomical basis of eye movement control in humans as revealed by a series of post-mortem studies in which the human premotor oculomotor brainstem nuclei were identified using unconventional 100 μm thick serial tissue sections stained for Nissl substance and lipofuscin pigment (Nissl-pigment stain according to Braak). Data from control brains and from patients suffering from spinocerebellar ataxia type 3, a neurodegenerative disease that severely impairs oculomotor function are discussed and recommendations for the identification of human premotor oculomotor brainstem nuclei in post-mortem studies are given. To visualize premotor brainstem nuclei in living patients, modern brain imaging techniques have been employed, albeit with limited success. Establishing topographic markers of brainstem nuclei may be a necessary next step to further elucidate the functional neuroanatomy of the premotor oculomotor brainstem network in human patients. This will help radiologists to identify these nuclei in living patients and will enable clinicians to monitor the progression of neurological disorders affecting the oculomotor system.

Vestibular Nuclei and Cerebellum Put Visual Gravitational Motion in Context

Animal survival in the forest, and human success on the sports field, often depend on the ability to seize a target on the fly. All bodies fall at the same rate in the gravitational field, but the corresponding retinal motion varies with apparent viewing distance. How then does the brain predict time-to-collision under gravity? A perspective context from natural or pictorial settings might afford accurate predictions of gravity's effects via the recovery of an environmental reference from the scene structure. We report that embedding motion in a pictorial scene facilitates interception of gravitational acceleration over unnatural acceleration, whereas a blank scene eliminates such bias. Functional magnetic resonance imaging (fMRI) revealed blood-oxygen-level-dependent correlates of these visual context effects on gravitational motion processing in the vestibular nuclei and posterior cerebellar vermis. Our results suggest an early stage of integration of high-level visual analysis with gravity-related motion information, which may represent the substrate for perceptual constancy of ubiquitous gravitational motion.

Nicotine-induced nystagmus correlates with midpontine activation

The pathomechanism of nicotine-induced nystagmus (NIN) is unknown. The aim of this study was to delineate brain structures that are involved in NIN generation. Eight healthy volunteers inhaled nicotine in darkness during a functional magnetic resonance imaging (fMRI) experiment; eye movements were registered using video-oculography. NIN correlated with blood oxygen level-dependent (BOLD) activity levels in a midpontine site in the posterior basis pontis. NIN-induced midpontine activation may correspond to activation of the dorsomedial pontine nuclei and the nucleus reticularis tegmenti pontis, structures known to participate in the generation of multidirectional saccades and smooth pursuit eye movements.

Functional brain imaging: a window into the visuo-vestibular systems

PURPOSE OF REVIEW

Advances have been made in identifying how areas involved in processing vestibular, ocular motor, and visual information are represented in the human cortex as well as the cortical interaction between these systems in healthy subjects.

RECENT FINDINGS

While we know how some vestibular and ocular motor disorders modify visuo-vestibular interaction by changing the 'normal' cortical activation-deactivation patterns, it is still early days in functional magnetic resonance imaging studies of patients with specific disorders. Findings from current brain imaging studies of several vestibular, ocular motor, and cerebellar disorders are presented.

SUMMARY

The promise of more insights into the complex neuronal networks of the human cortex is great.