Input-output activity of the primate flocculus during visual-vestibular interaction.

In the primate flocculus, unit activity was recorded during vestibular (rotation of the monkey about the vertical axis in complete darkness), optokinetic (rotation of the visual surround around the stationary monkey), and conflicting (rotation of the visual surround and the turntable fixed together) stimulation. Activity indicating two different mossy fiber inputs was recorded. One carried a signal that was similar to that in the vestibular nuclei: during optokinetic stimulation, neurons saturated at a velocity of 60 degrees/second; and during conflicting stimulation, neuronal activity was attenuated only at low accelerations. This input combines vestibular, visual, and oculomotor information. Another mossy fiber input carried information about visual image slip only. This input indicates instances when nystagmus is not compensatory. Purkinje cells were modulated in their simple spike activity during optokinetic stimulation only at high stimulus velocities of 40-60 degrees/second and above, and during conflicting stimulation at high accelerations. This suggests a complementary information processing of the flocculus and the vestibular nuclei during visual-vestibular stimulation. The findings are corroborated by lesion studies in primates.