Here, we investigate whether vertigo and nystagmus is generated at the first stage of vestibular processing in the parieto-insular vestibular cortex (PIVC). It is believed that the PIVC is the first and most important hub for the cortical processing of vestibular information. In this region of the brain, vestibular information from both hemispheres is integrated with somatosensory and visual information. Mismatches among these inputs have been consistently demonstrated to activate the PIVC and the widespread vestibular network. Such mismatches also cause behavioral responses, such as vertigo and nystagmus. However, the role of the PIVC in the generation of these symptoms remains elusive, particularly because behavioral and cortical responses have been found to follow very different time-courses. It therefore remains unclear at which level of information processing vertigo and nystagmus are generated. We performed functional magnetic resonance imaging (fMRI) on 20 healthy subjects during caloric stimulation at different temperatures. We were particularly interested in the changes in the strengths of PIVC activity under these stimulus conditions. By using a non-inferiority analysis, we demonstrate that activity in the PIVC did not increase with the occurence of vertigo and nystagmus. The current data suggest that perceptions of vertigo and nystagmus are not generated at the first integrative stage in the PIVC. We further speculate that nystagmus originates via a direct interaction of vestibular signals with somatosensory and visual information at a subcortical level. In most mammals, posture and gaze are controlled by the complex integration of vestibular, visual and proprioceptive information. This redundancy allows for compensation in the absence of one of these sensory inputs. The downside of this sophisticated and fine-tuned integrative system is that problems arise if the information gathered from different systems or sides of the body is not consistent. Typical examples include reading a book while riding in a car or the dysfunction of one of the vestibular organs. These events sometimes lead to unique feelings that are termed "vertigo" and that are often difficult for patients to describe. The phenomenon of vertigo is difficult to grasp not only for patients but also for neuroscientists. Although the last two decades have greatly improved our understanding of the processing of vestibular information within the brain, our understanding of the perception of vertigo remains incomplete.

Twenty healthy volunteers without any history of neurological, otolaryngologic, or psychiatric diseases participated in this study. All subjects were right-handed according to the Edinburgh Handedness Inventory. One subject revealed movement artefacts during the MRI acquisition of more than 3 mm and was therefore excluded from further analyses, resulting in a final group size of 19 subjects (mean age: 28.7 ± 8 years, range: 21-55 years). Finally, we must consider the possible influence of habituation, as the current study used a fixed design, and in this design, the 37°C stimulus preceded the 44°C caloric stimulus. It is therefore well conceivable that the non-inferiority might not have occurred if the order of the experimental stimuli were reversed. However, the effects of habitation did not affect our main research hypothesis, namely the link between activity in the PIVC and the rise in vertigo and nystagmus.

The study was approved by the local ethics committee, and all subjects gave their written informed consent according to the Declaration of Helsinki. We demonstrated that a vestibular stimulus that did not caused vertigo and nystagmus elicited non-inferior activity in the PIVC compared to a vestibular stimulus that caused nystagmus and vertigo. We therefore suggest that the activity in the PIVC does not determine the generation of nystagmus and vertigo. Moreover, we speculate that a nystagmus is not generated by the cortical processing of vestibular information but is generated at the subcortical level.

Best Wishes,
Joseph Kent
Journal Manager
Journal of Brain Behavior & Cognitive Sciences