Analysis of the nystagmus evoked by cross-coupled acceleration (Coriolis phenomenon)

Motion sickness and spatial disorientation represent two outstanding challenges in aviation medicine. In both cases, the vestibular system plays a fundamental role in their genesis. One of the most common ground-based simulations utilised in aero-physiological training is the sense of vertigo and tilt generated by the cross-coupled stimulation of the semicircular canals, while exposed to rotation in the yaw axis (Coriolis’ Phenomenon: CP). However, the complex stimulus induced on the two labyrinths by this manoeuvre still deserves investigation. Nine male subjects sitting on a rotatory chair were asked to tilt their head back and forth during a yaw – axis clock – (CW) or counter-clock-wise (CCW) rotation at a constant speed of 70°/sec, generating the CP. Eye movements were recorded via Video-Oculo-Scopy and qualitatively analysed. A second camera simultaneously recorded the subject’s and chair’s movements. The observed nystagmus (Ny) was then analysed and related to the actual head/chair position and motion. A clear relationship was detected between Ny, head movements and direction of chair rotation. During CW rotation, backward head tilts systematically induced a CW-Ny, while a CCW-Ny was observed while returning to the upright position, or during forward head tilt. Opposite patterns were detected during CCW chair rotation. Minor lateral eye movements were also observed, due to the activity of horizontal semicircular canals, but no vertical ones. Due to the neural connections between extra-ocular muscles and each labyrinth sensor, the semicircular canals involved in the genesis of the Ny during this form of stimulation could be identified. In agreement with the third Ewald’s law, our results indicated a dominant left labyrinth during backward tilt and CW motion, or forward tilt and CCW rotation. On the contrary, during forward tilt and CW rotation, or backward tilt and CCW rotation, the right vertical canals produced the main contribution to ocular response.