Volunteers and equipment in the VOR experiment.
3 volunteers volunteered to participate in the VOR experiment, and gave written informed consent to their participation prior to the experiment. Besides, the statement that all volunteers agreed to publish their identifiable information or images in an online open-access publication was included in the written informed consent. They were informed of the experimental procedures, and were allowed to stop the experiment at any time. All of them had normal vestibular function without history of vestibular or ocular diseases. The experiments were approved by the ethics committee of Dalian university of technology, and were in line with the principles of the Declaration of Helsinki; the registration number was 2020-077.
As shown in Fig. 7A, a volunteer was sitting on the rotatable chair. The experimental equipment including eyepatch, gyroscope, wireless transmission module and battery was used for recording eye movement (see Fig. 7B). A small infrared camera was embedded in the eyepatch to monitor the left eye. Besides, the gyroscope fixed on the right side of the eyepatch could measure instantaneous angular velocity; the sampling frequency of the gyroscope was up to 200 Hz, and we used 50 Hz. There was no relative movement between the eyepatch and the head when the volunteers wearing the eyepatch sensed the angular motion of head. Then, the gyroscope could real-time monitor the angular velocity of volunteers’ head. The wireless transmission module included transmitter and receiver module; the transmitter module connecting to the infrared camera transmitted the video signals of recording eye movement to the receiver module which was connected to the computer; the eye movement videos were stored on the computer. Additionally, the headrest was fixed to the chair to reduce the relative movement of the volunteers' heads during rotation, and the volunteers were secured to the chair with safety belts. Figure 7C shows the working process of the refrigerating device which has the function of producing cold air and transmitting it to the external auditory canal of the volunteers. The refrigerating device consisted of five modules including power source, wind-supply department, refrigerating equipment, heat dissipation device, and air transmission apparatus. Lithium battery was selected as power source, which was portable and supplied appropriate electricity for other modules. The flowing air was generated by the wind-supply department, which was transmitted to the refrigerating equipment through the air transmission apparatus. The refrigerating equipment produced cold air depending on three semiconductor chilling plates in it. There were several air ducts constituting the air transmission apparatus. The terminal of the air ducts were inserted into the soft earplugs which were probed into the ear to transmit the cold air (below 26℃) to the external auditory canal. The earplugs had multiple air holes, which ensured that the air flowed out of the earplug air holes normally without harm for the volunteers. The heat generated by the electronic equipment was dissipated by the cyclic water in the module of the heat dissipation device.
The volunteers sitting on the rotatable chair fastened the safety belts, and wore the eyepatch which was adjusted to record the pupil movement. As shown in Fig. 8A and 8C, the volunteers maintained a head position with the axis of rotation perpendicular to the ground and passing through the middle point P1 of both ears. We marked the auxiliary signs on the chair railings to keep volunteer’s head in the correct position. The chair rotated anticlockwise for 7 s with a constant angular acceleration of 30°/s2, and the initial rotational velocity of 0. The gyroscope recorded the real-time angular velocity of the volunteer’s head when the chair was rotating. All the volunteers firstly participated in the experiment without using the refrigerating device (normal experiment), and then participated in the experiment using the refrigerating device (control experiment). To eliminate the interference between the same volunteers in normal and control experiments, the volunteers rested for at least 30 minutes before the next experiment. All experiments were performed in a dark room to exclude the disturbance of light to eye movement. Besides, the volunteers used refrigerating device for 5 minutes in advance before the control experiment to ensure that the temperature of the SCCs decreased and achieved the balance between heat and cold.
The recorded videos of eye movement were processed by MATLAB R2017b to track and locate the center of the pupil (see Fig. 7D). We discarded the first and last data of nystagmus slow phases, and removed the phases shorter than 50 ms to reduce the statistical error22. Besides, the first 3 s of the nystagmus was discarded because the SPV generally increases within 3 s after the head senses the change in acceleration18. We also removed the nystagmus data when the volunteers blinked. The SPV was calculated based on the method provided by Wu et al.15.
We reconstructed the 3D geometric model of SCCs in human left ear according to the parameters provided by Ifediba et al.23. The geometric model of SCCs included utricle, three semicircular canals, and three cupulae (see Fig. 8B). The geometric model of SCCs was meshed by Hypermesh (version 12.0). The endolymph domain consisted of 214 k tetrahedral elements and 48 k nodes. The three cupulae domains consisted of 41 k tetrahedral elements and 9 k nodes. Additionally, the results of numerical simulation were almost the same when we refined mesh in endolymph domain with 492 k tetrahedral elements and 103 k nodes, indicating that the selected number of elements for the original model was appropriate.
The cupula is a gelatinous structure10,24,25. Its density is 1000 kg/m3 26,27; the Young’s modulus is 5 Pa6; the Poisson ratio is 0.486,8,28. The endolymph is similar to water25,29−31, with the density of 1000 kg/m3 and viscosity of 0.0085 Pa8,26. These parameters commonly represented the physical properties of the SCCs at the temperature of 37℃. When the temperature of the SCCs dropped to 36℃, the density and viscosity of endolymph increased by 0.036% and 1.83% respectively based on the properties of water32.
We constructed the computer models including endolymph and three cupulae in ANSYS Workbench (version 16.0) based on the method provided by Goyens et al.10. When the SCCs are rotating, the endolymphatic movement in the earthbound reference frame can be expressed by Navier-Stokes equations33:
where ρ is the fluid density, u is the flow velocity vector, P is the static pressure, and µ is the dynamic viscosity. Besides, considering the relative reference frame moving with the walls of SCCs, the walls of endolymph are stationary. Hence, the movement of endolymph in the relative reference frame can be described by the following equations33:
where v is the fluid velocity vector relative to the velocity of the moving reference frame, Ω = (0, 0, ω) is the angular velocity vector of the moving reference frame, and r is the radial coordinate of the fluid element.
We used the Ansys Workbench (version 16.0) to simulate the rotation of SCCs at temperatures of 36℃ and 37℃, respectively. The numerical model was loaded with the anticlockwise rotational acceleration of 30°/s2, and the initial rotational velocity of 0. Furthermore, we observed that the results of cupula response were stable after 2 s in the early calculations. Thus, the computational time was set as 2 s in order to reduce unnecessary calculating time. The time step was set as 0.001 s.