Eye movements evoked by selective saccular nerve stimulation in cats.

OBJECTIVE: Because of technical obstacles in controlling current spread to adjacent peripheral nerve, eye movements evoked by activation of the otolith organs have not been investigated in detail compared to eye movements evoked by activation of the canal organs. We attempted to solve this problem by applying more sensitive methods using fine needle and strictly controlling stimulus current intensity compare with filed potential for selective stimulation.
METHODS: Eye movements evoked by selective, unilateral saccular (SAC) 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 SAC nerve through implanted acupuncture needles.
RESULTS: In the absence of anesthesia and with stimulus intensities less than (3.1 +/- 2.7) x N(1)T, we found supraduction in both eyes or in either the ipsilateral or contralateral eye of different cats. We observed downward eye movements using a stronger stimulus intensity ((6.2 +/- 2.9)) x N(1)T). The threshold for downward eye movements was significantly greater than that for upward eye movements (P < 0.05). In anesthetized cats, only downward eye movements were observed when stimulus intensities less than 10 x N(1)T ((7.8 +/- 2.3) x N(1)T) were used.
CONCLUSION: These results confirm the known sacculo-ocular anatomical connections, which are involved predominantly in vertical eye movements. Because the sacculo-ocular connections are relatively weak, the normal supraduction evoked by SAC activation can be easily modified by factors such as level of anesthesia and the method of stimulation.