Modeling and Monitoring of Otolith Organ Performance in US Navy Operating Environments

Previous mathematical modeling work has produced a transfer function that relates otoconial layer displacement to stimulus acceleration. Due to the complexity of this transfer function, time domain solutions may be obtained only through numerical methods. In the current work, several approximations are introduced to the transfer function that result in its simplification. This simplified version can be inverted to yield analytic time domain solutions. Results from a frequency response analysis of the simplified transfer function are compared with the same results from the complete transfer function, and with mammalian first-order neuron frequency response data. There is good agreement in the comparisons. Time domain solutions of the approximation are compared to numerical solutions of the full transfer function, and again there is a good match. System time constants are calculated from the simplified transfer function.

A 2-D finite element model of a mammalian utricular macula is presented. Physical dimensions used in the model are taken from mammalian anatomical studies. Values for the material properties of the problem are not readily available; however, ranges are chosen to produce realistic physiologic behavior. Deflections predicted by this model show that a single value for hair bundle stiffness throughout the organ is inadequate for the organ to respond to the entire range of human acceleration perception. Therefore, it is necessary for a range of hair bundle stiffnesses to exist in each organ. Natural frequencies calculated in this model support previous studies on vestibular damage due to low frequency sound.

Divers exposed to high-intensity underwater sound have experienced symptoms attributed to vestibular stimulation. An in-water video-oculography (VOG) system was developed to monitor divers' eye movements, particularly torsional, during exposure to varying underwater sound signals. The system included an underwater closed-circuit video camera with infrared lights attached to the diver's mask with an adjustable mounting bracket. The video image was sent to a surface control room for real-time and post-experiment processing. Six divers at 60 feet in open water received 15 minutes daily cumulative exposure of 240-320 Hertz underwater sound at 160 dB re 1 mPa for 10 days. No spontaneous primary position nystagmus, horizontal, vertical or torsional, was detected in any diver. This experiment was the first successful attempt to record and analyze eye movements underwater.