Background
While strong vestibular input to the fastigial nucleus has been demonstrated, little is known on how this is integrated in the oculomotor region of the caudal area of fastigial nucleus. In this study, single units in the caudal region of the fastigial nucleus were recorded in the rat, while applying sinusoidal galvanic stimuli to the labyrinths.
Material and Method
The left and right labyrinths were polarized with either monaural or binaural sinusoidal stimuli (0.5 hertz), with stimulus phase shifts between right and left in 90 degrees steps (90 degrees, 180 degrees, 270 degrees, 360 degrees). The study describes the interaction and integration between the information of both labyrinths in the caudal fastigial nucleus.
Results
In 23 neurons, the relationship between input (galvanic stimuli to the labyrinths) and output (neuronal discharge) was analyzed for a series of stimuli (102 recordings in total). During monaural stimulation, phase shifts greater than 45 degrees were observed in 14 of 23 neurons (60.9%) during stimulation to the left and right labyrinths. To explain this phenomenon and the previously observed tuning characteristics in the fastigial nucleus, the neuronal responses were regarded as vectors. Thus, the observed response for binaural stimulation appeared to correspond to a vector addition of the left and right vestibular afferents. Of the 23 neurons, 14 neurons (60.9%) indicated a difference between predicted and measured values of less than 14.1 degrees.
Conclusions
The findings support a working model based on vector addition of vestibular afferents in the caudal fastigial nucleus.

Keywords: Vestibule, Labyrinth - physiology, Acoustic Stimulation