Prolonged microgravity induces reversible and persistent changes on human cerebral connectivity.

The prospect of continued manned space missions warrants an in-depth understanding of how prolonged microgravity affects the human brain. Functional magnetic resonance imaging (fMRI) can pinpoint changes reflecting adaptive neuroplasticity across time. We acquired resting-state fMRI data of cosmonauts before, shortly after, and eight months after spaceflight as a follow-up to assess global connectivity changes over time. Our results show persisting connectivity decreases in posterior cingulate cortex and thalamus and persisting increases in the right angular gyrus. Connectivity in the bilateral insular cortex decreased after spaceflight, which reversed at follow-up. No significant connectivity changes across eight months were found in a matched control group. Overall, we show that altered gravitational environments influence functional connectivity longitudinally in multimodal brain hubs, reflecting adaptations to unfamiliar and conflicting sensory input in microgravity. These results provide insights into brain functional modifications occurring during spaceflight, and their further development when back on Earth.

Structural reorganization of the cerebral cortex after vestibulo-cerebellar stroke.

OBJECTIVE: Structural reorganization following cerebellar infarcts is not yet known. This study aimed to demonstrate structural volumetric changes over time in the cortical vestibular and multisensory areas (i.e., brain plasticity) after acute cerebellar infarcts with vestibular and ocular motor symptoms. Additionally, we evaluated whether structural reorganization in the patients topographically correlates with cerebello-cortical connectivity that can be observed in healthy participants. METHODS: We obtained high-resolution structural imaging in seven patients with midline cerebellar infarcts at two time points. These data were compared to structural imaging of a group of healthy age-matched controls using voxel-based morphometry (2×2 ANOVA approach). The maximum overlap of the infarcts was used as a seed region for a separate resting-state functional connectivity analysis in healthy volunteers. RESULTS: Volumetric changes were detected in the multisensory cortical vestibular areas around the parieto-opercular and (retro-) insular cortex. Furthermore, structural reorganization was evident in parts of the frontal, temporal, parietal, limbic, and occipital lobes and reflected functional connections between the main infarct regions in the cerebellum and the cerebral cortex in healthy individuals. CONCLUSIONS: This study demonstrates structural reorganization in the parieto-opercular insular vestibular cortex after acute vestibulo-cerebellar infarcts. Additionally, the widely distributed structural reorganization after midline cerebellar infarcts provides additional in vivo evidence for the multifaceted contribution of cerebellar processing to cortical functions that extend beyond vestibular or ocular motor function.

Simultaneous recording of cervical and ocular vestibular-evoked myogenic potentials.

OBJECTIVE: To increase clinical application of vestibular-evoked myogenic potentials (VEMPs) by reducing the testing time by evaluating whether a simultaneous recording of ocular and cervical VEMPs can be achieved without a loss in diagnostic sensitivity and specificity. METHODS: Simultaneous recording of ocular and cervical VEMPs on each side during monaural stimulation, bilateral simultaneous recording of ocular VEMPs and cervical VEMPs during binaural stimulation, and conventional sequential recording of ocular and cervical VEMPs on each side using air-conducted sound (500 Hz, 5-millisecond tone burst) were compared in 40 healthy participants (HPs) and 20 patients with acute vestibular neuritis. RESULTS: Either simultaneous recording during monaural and binaural stimulation effectively reduced the recording time by ≈55% of that for conventional sequential recordings in both the HP and patient groups. The simultaneous recording with monaural stimulation resulted in latencies and thresholds of both VEMPs and the amplitude of cervical VEMPs similar to those found during the conventional recordings but larger ocular VEMP amplitudes (156%) in both groups. In contrast, compared to the conventional recording, simultaneous recording of each VEMP during binaural stimulation showed reduced amplitudes (31%) and increased thresholds for cervical VEMPs in both groups. CONCLUSIONS: The results of simultaneous recording of cervical and ocular VEMPs during monaural stimulation were comparable to those obtained from the conventional recording while reducing the time to record both VEMPs on each side. CLINICALTRIALSGOV IDENTIFIER: NCT03049683.

Delineating function and connectivity of optokinetic hubs in the cerebellum and the brainstem.

Optokinetic eye movements are crucial for keeping a stable image on the retina during movements of the head. These eye movements can be differentiated into a cortically generated response (optokinetic look nystagmus) and the highly reflexive optokinetic stare nystagmus, which is controlled by circuits in the brainstem and cerebellum. The contributions of these infratentorial networks and their functional connectivity with the cortical eye fields are still poorly understood in humans. To map ocular motor centres in the cerebellum and brainstem, we studied stare nystagmus using small-field optokinetic stimuli in the horizontal and vertical directions in 22 healthy subjects. We were able to differentiate ocular motor areas of the pontine brainstem and midbrain in vivo for the first time. Direction and velocity-dependent activations were found in the pontine brainstem (nucleus reticularis, tegmenti pontis, and paramedian pontine reticular formation), the uvula, flocculus, and cerebellar tonsils. The ocular motor vermis, on the other hand, responded to constant and accelerating velocity stimulation. Moreover, deactivation patterns depict a governing role for the cerebellar tonsils in ocular motor control. Functional connectivity results of these hubs reveal the close integration of cortico-cerebellar ocular motor and vestibular networks in humans. Adding to the cortical concept of a right-hemispheric predominance for visual-spatial processing, we found a complementary left-sided cerebellar dominance for our ocular motor task. A deeper understanding of the role of the cerebellum and especially the cerebellar tonsils for eye movement control in a clinical context seems vitally important and is now feasible with functional neuroimaging.

Longitudinal multi-modal neuroimaging in opsoclonus-myoclonus syndrome.

To investigate structural, metabolic, and functional connectivity changes in visual and oculomotor structures in a patient with paraneoplastic opsoclonus-myoclonus syndrome, serial resting-state functional and structural MRI, and FDG-PET data were collected during the acute stage and later on when the opsoclonus had resolved. In the acute stage, an FDG-PET scan demonstrated a substantially increased metabolism in structures around the deep cerebellar nuclei [e.g., fastigial nucleus (FN)] and a relatively reduced metabolism in the bilateral occipital lobes which normalized over 12 months. Functional connectivity increased initially between the seeds of the oculomotor and visual systems, including the primary and motion-sensitive visual cortex, frontal eye fields, superior colliculus, and cerebellar oculomotor vermis (OMV), and then decreased in the chronic stage as the symptoms resolved. The functional connectivity between the OMV and FN showed a positive correlation during the acute stage, which decreased later on. We provide a descriptive presentation of the changes of abnormal functional connectivity throughout visuo-oculomotor brain areas during opsoclonus and suggest directions for further research on the pathogenesis of opsoclonus.