Network Architecture of Verticality Processing in the Human Thalamus.

OBJECTIVE: Thalamic dysfunction in lesions or neurodegeneration may alter verticality perception and lead to postural imbalance and falls. The aim of the current study was to delineate the structural and functional connectivity network architecture of the vestibular representations in the thalamus by multimodal magnetic resonance imaging. METHODS: Seventy-four patients with acute unilateral isolated thalamic infarcts were studied prospectively with emphasis on the perception of verticality (tilts of the subjective visual vertical [SVV]). We used multivariate lesion-symptom mapping based on support-vector regression to determine the thalamic nuclei associated with ipsiversive and contraversive tilts of the SVV. The lesion maps were used to evaluate the white matter disconnection and whole brain functional connectivity in healthy subjects. RESULTS: Contraversive SVV tilts were associated with lesions of the ventral posterior lateral/medial, ventral lateral, medial pulvinar, and medial central/parafascicular nuclei. Clusters associated with ipsiversive tilts were located inferiorly (ventral posterior inferior nucleus) and laterally (ventral lateral, ventral posterior lateral, and reticular nucleus) to these areas. Distinct ascending vestibular brainstem pathways terminated in the subnuclei for ipsi- or contraversive verticality processing. The functional connectivity analysis showed specific patterns of cortical connections with the somatomotor network for lesions with contraversive tilts, and with the core multisensory vestibular representations (areas Ri, OP2-3, Ig, 3av, 2v) for lesions with ipsiversive tilts. INTERPRETATION: The functional specialization may allow both a stable representation of verticality for sensorimotor integration and flexible adaption to sudden changes in the environment. A targeted modulation of this circuitry could be a novel therapeutic strategy for higher level balance disorders of thalamocortical origin. ANN NEUROL 2023;94:133-145.

White matter volume loss drives cortical reshaping after thalamic infarcts.

OBJECTIVE: The integration of somatosensory, ocular motor and vestibular signals is necessary for self-location in space and goal-directed action. We aimed to detect remote changes in the cerebral cortex after thalamic infarcts to reveal the thalamo-cortical connections necessary for multisensory processing and ocular motor control. METHODS: Thirteen patients with unilateral ischemic thalamic infarcts presenting with vestibular, somatosensory, and ocular motor symptoms were examined longitudinally in the acute phase and after six months. Voxel- and surface-based morphometry were used to detect changes in vestibular and multisensory cortical areas and known hubs of central ocular motor processing. The results were compared with functional connectivity data in 50 healthy volunteers. RESULTS: Patients with paramedian infarcts showed impaired saccades and vestibular perception, i.e., tilts of the subjective visual vertical (SVV). The most common complaint in these patients was double vision or vertigo / dizziness. Posterolateral thalamic infarcts led to tilts of the SVV and somatosensory deficits without vertigo. Tilts of the SVV were higher in paramedian compared to posterolateral infarcts (median 11.2° vs 3.8°). Vestibular and ocular motor symptoms recovered within six months. Somatosensory deficits persisted. Structural longitudinal imaging showed significant volume reduction in subcortical structures connected to the infarcted thalamic nuclei (vestibular nuclei region, dentate nucleus region, trigeminal root entry zone, medial lemniscus, superior colliculi). Volume loss was evident in connections to the frontal, parietal and cingulate lobes. Changes were larger in the ipsilesional hemisphere but were also detected in homotopical regions contralesionally. The white matter volume reduction led to deformation of the cortical projection zones of the infarcted nuclei. CONCLUSIONS: White matter volume loss after thalamic infarcts reflects sensory input from the brainstem as well the cortical projections of the main affected nuclei for sensory and ocular motor processing. Changes in the cortical geometry seem not to reflect gray matter atrophy but rather reshaping of the cortical surface due to the underlying white matter atrophy.

Vestibular thalamus: Two distinct graviceptive pathways.

OBJECTIVE: To determine whether there are distinct thalamic regions statistically associated with either contraversive or ipsiversive disturbance of verticality perception measured by subjective visual vertical (SVV). METHODS: We used modern statistical lesion behavior mapping on a sample of 37 stroke patients with isolated thalamic lesions to clarify which thalamic regions are involved in graviceptive otolith processing and whether there are distinct regions associated with contraversive or ipsiversive SVV deviation. RESULTS: We found 2 distinct systems of graviceptive processing within the thalamus. Contraversive tilt of SVV was associated with lesions to the nuclei dorsomedialis, intralamellaris, centrales thalami, posterior thalami, ventrooralis internus, ventrointermedii, ventrocaudales and superior parts of the nuclei parafascicularis thalami. The regions associated with ipsiversive tilt of SVV were located in more inferior regions, involving structures such as the nuclei endymalis thalami, inferior parts of the nuclei parafascicularis thalami, and also small parts of the junction zone of the nuclei ruber tegmenti and brachium conjunctivum. CONCLUSIONS: Our data indicate that there are 2 anatomically distinct graviceptive signal processing mechanisms within the vestibular network in humans that lead, when damaged, to a vestibular tone imbalance either to the contraversive or to the ipsiversive side.

The role of the thalamus in the human subcortical vestibular system.

Most of our knowledge concerning central vestibular pathways is derived from animal studies while evidence of the functional importance and localization of these pathways in humans is less well defined. The termination of these pathways at the thalamic level in humans is even less known. In this review we summarize the findings concerning the central subcortical vestibular pathways in humans and the role of these structures in the central vestibular system with regard to anatomical localization and function. Also, we review the role of the thalamus in the pathogenesis of higher order sensory deficits such as spatial neglect, pusher syndrome or thalamic astasia and the correlation of these phenomena with findings of a vestibular tone imbalance at the thalamic level. By highlighting thalamic structures involved in vestibular signal processing and relating the different nomenclatures we hope to provide a base for future studies on thalamic sensory signal processing.