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Abstract

High strength static magnetic fields are known to induce vertigo, believed to be via stimulation of the vestibular system. The leading hypothesis (Lorentz forces) predicts that the induced vertigo should depend on the orientation of the magnetic field relative to the head. In this study we examined the effect of static head pitch (−80° to +40°; 12 participants) and roll (−40° to +40°; 11 participants) on qualitative and quantitative aspects of vertigo experienced in the dark by healthy humans when exposed to the static uniform magnetic field inside a 7T MRI scanner. Three participants were additionally examined at 180° pitch and roll orientations. The effect of roll orientation on horizontal and vertical nystagmus was also measured and was found to affect only the vertical component. Vertigo was most discomforting when head pitch was around 60° extension and was mildest when it was around 20° flexion. Quantitative analysis of vertigo focused on the induced perception of horizontal-plane rotation reported on-line with the aid of hand-held switches. Head orientation had effects on both the magnitude and the direction of this perceived rotation. The data suggest sinusoidal relationships between head orientation and perception with spatial periods of 180° for pitch and 360° for roll, which we explain is consistent with the Lorentz force hypothesis. The effects of head pitch on vertigo and previously-reported nystagmus are consistent with both effects being driven by a common vestibular signal. To explain all the observed effects, this common signal requires contributions from multiple semi-circular canals.