Eye movements evoked by the selective stimulation of the utricular nerve in cats

Abstract

Objective: Eye movements evoked by otolith organ are not well-investigated compare with canal related eye movements due to the technical difficulties. We try to solve this problem by means of our methods. Methods: Eye movements evoked by selective utricular (UT) nerve stimulation were investigated using both electrooculography (EOG) and video recording in decerebrated cats in the presence or absence of anesthesia. Electrical stimulation was applied to the UT nerve through implanted acupuncture needles. Results: In the absence of anesthesia and with stimulus intensities less than 2.6±0.7×N 1T, we found ipsilaterally directed horizontal eye movements in both eyes in one cat, abduction in the ipsilateral eye in two cats, and adduction in the contralateral eye in another cat. Other types of eye movements (e.g., supraduction or diagonal eye movements) were observed in both eyes of cats in the absence of anesthesia at a stimulus intensity of 12.2±7.6×N 1T, an intensity in which current spread to the adjacent nerve could not be ruled out. In the presence of anesthesia, UT nerve stimulation alone failed to evoke horizontal eye movements, but with an intensity 13.8±6.4×N 1T, supraduction or diagonal eye movements were evoked. UT nerve stimulation at 2–3×N 1T facilitated horizontal eye movements induced by ipsilateral abducens (AB) nucleus stimulation or contralateral horizontal canal nerve stimulation. Conclusion: This is the first report to our knowledge in which UT nerve-evoked horizontal eye movements are documented. These results confirm the known monosynaptic and disynaptic anatomical connections from utricular primary afferents to the ipsilateral AB nucleus neurons.