Visual-vestibular interaction in the flocculus of the alert monkey. II. Purkinje cell activity

The activity of Purkinje cells (P-cells) was recorded in the flocculus of alert Rhesus monkeys under different conditions of visual-vestibular stimulation. Stimulations conditions were vestibular, optokinetic, combined and conflicting. About 10--20% of all P-cells were activated in their simple spike activity during conflicting stimulation to the recording side (type I) and gave no response or much less during vestibular stimulation. About half of these P-cells were also activated during optokinetic stimulation to the recording side at velocities above 40--60 deg/s. Simple and complex spike activity behaved in a reciprocal way with overlapping but not identical working ranges. Simple spike modulation was unidirectional, complex spike activity always bidirectional. Modulation of simple spike activity cannot be related to one single parameter of the sensory input or the oculomotor output. The hypothesis is put forward that the vestibular nuclei and the flocculus behave in a complementary fashion in processing visual-vestibular information, the flocculus being specialized for high velocity optokinetic nystagmus and suppression of vestibular nystagmus.