Distorted Gravity and Distorted Eyes: Who Is at Fault – The Cerebellum or Brainstem?

Abstract

Sensory input from the otolith organs and the semicircular canals is combined in the vestibular nuclei and forwarded to other brainstem, cerebellar, and higher cortical areas by the central graviceptive pathways. This network forms the basis for internal estimates of the direction of gravity. Lateralized lesions along these pathways will result in vestibular tone imbalance in the roll plane. This can be observed both at the level of brainstem reflexes as the ocular tilt reaction (OTR; consisting of ocular torsion, head tilt, and skew deviation) and at the higher level of cortical behavioral paradigms (such as the subjective visual vertical, SVV). Recent research has demonstrated a wide network of cerebellar and brainstem areas contributing to these internal estimates. While cerebellar lesions including the dentate nucleus, nodulus, or flocculus result in contraversive OTR and shifts of perceived vertical, lesions affecting the biventer lobule, the middle cerebellar peduncle, the tonsil, and the inferior semilunar lobule are associated with ipsiversive shifts of OTR and perceived vertical. Patients with brainstem lesions below the crossing of the graviceptive pathways at the level of the pons present with ipsilesional OTR and tilts of perceived vertical, while lesions above the crossing demonstrate contraversive shifts. Specifically, ipsiversive shifts were noted for lesions affecting the medial longitudinal fasciculus (MLF) and the medial vestibular nucleus, whereas contraversive shifts were associated with lesions of the rostral interstitial nucleus of the MLF and the interstitial nucleus of Cajal. Distortions in verticality perception and OTR therefore can be mediated both by brainstem and cerebellar lesions.