Duration of Effective Thermal Stimulation in Caloric Test

It is difficult to determine the actual period of time that the vestibular nerve is activated after applying a caloric stimulus. Several processes are activated by the caloric stimulus, including convection currents in the canals as well as direct heating or cooling of the vestibular nerves. In turn, the neural activity excites both direct and indirect (velocity storage) vestibulo-ocular pathways, all of which contribute to produce nystagmus. Velocity storage has integrative properties that can be altered by the presence of stationary visual surroundings, which discharge stored activity and shorten the velocity storage time constant. Therefore, eye velocity generated by velocity storage can be used as a test to determine whether there is still activity in the nerve. To eliminate convection currents in the semicircular canals, we used an all-six-canal-plugged cynomolgus monkey. Eye movements were recorded using a dual-search coil system. Eye velocity was expressed as a vector comprising horizontal, vertical and torsional components. Caloric nystagmus was induced by irrigating the eardrum in light in the upright position with 10 ml water at 20°C irrigation for 15 sec. The animal was left in light for periods of time ranging from 10-110 s. At the conclusion of the period in light, the animal was put in darkness and caloric nystagmus, if any, was analyzed. If there was nerve activity, it would charge the velocity storage and nystagmus would return. For periods of light dump up to 110 s, caloric nystagmus ended 130 s from the start of irrigation. For periods of light dump more than 110 s, no caloric nystagmus appeared when the animal went back into darkness. The time of culmination, i.e., the time to reach the peak value of caloric vector, was 26 s in light, and the peak magnitude of the caloric vector was 86°/s. In darkness, the culmination was 11 sec from the point where the lights were extinguished. The magnitude of the caloric vector was 256°/s initially when there was no light dump, and it decayed exponentially according to the period of preceding visual suppression. The time constant of caloric nystagmus also decayed exponentially depending on the period of the preceding light dump. These results suggest that the duration of the effective thermal change for inducing caloric nystagmus was 110 seconds, and velocity storage contributes an additional 20 s (15%) to the duration of the first phase.