The human vestibular cortex: functional anatomy, connectivity and the effect of vestibular disease

Area OP2 in the posterior peri-sylvian cortex has been proposed to be the core human vestibular cortex. We defined the functional anatomy of OP2 using spatially constrained independent component analysis of functional MRI data from the Human Connectome Project. Ten distinct subregions were identified. Most subregions showed significant connectivity to other areas with vestibular function: the parietal opercula, the primary somatosensory cortex, the supracalcarine cortex, the left inferior parietal lobule and the anterior cingulate cortex. OP2 responses to vestibular and visual-motion were analysed in 17 controls and 17 right-sided unilateral vestibular lesion patients (vestibular neuritis) who had previously undergone caloric and optokinetic stimulation during functional MRI. In controls, a posterior part of right OP2 showed: (a) direction-selective responses to visual motion; and (b) activation during caloric stimulation that correlated positively with perceived self-motion, and negatively with visual dependence. Patients showed abnormal OP2 activity, with an absence of visual or caloric activation of the healthy ear and no correlations with dizziness or visual dependence despite normal brainstem responses to caloric stimulation (slow-phase nystagmus velocity). A lateral part of right OP2 showed activity that correlated with chronic dizziness (situational vertigo) in patients. Our results define the functional anatomy of OP2 in health and disease. A posterior subregion of right OP2 shows strong functional connectivity to other vestibular regions and a visuo-vestibular profile that becomes profoundly disrupted after vestibular disease. In vestibular patients, a lateral subregion of right OP2 shows responses linked to the challenging long-term symptoms which define poorer clinical outcomes.

Subjective Visual Vertical during Caloric Stimulation in Healthy Subjects: Implications to Research and Neurorehabilitation

Background. The subjective visual vertical (SVV) is a perception often impaired in patients with neurologic disorders and is considered a sensitive tool to detect otolithic dysfunctions. However, it remains unclear whether the semicircular canals (SCCs) are also involved in the visual vertical perception.Objective. The aim of this study was to analyze the influence of horizontal SCCs on SVV by caloric stimulation in healthy subjects.Methods. SVV was performed before and during the ice-cold caloric stimulation (4°C, right ear) in 30 healthy subjects.Results. The mean SVV tilts before and during the caloric stimulation were 0.31° ± 0.39 and −0.28° ± 0.40, respectively. There was no significant difference between the mean SVV tilts before and during stimulationp=0.113.Conclusion. These results suggest that horizontal SCCs do not influence SVV. Therefore, investigations and rehabilitation approaches for SVV misperceptions should be focused on otolithic and cognitive strategies.