Reversed postoptokinetic nystagmus: a model of plasticity in the vestibuloocular system.

A simple manifestation of the memorial processes is the so called reversed postoptokinetic nystagmus (RPN), a trace phenomenon elicited in the rabbit by prolonged (60 min) optokinetic (OK) stimulation. Electrophysiological analysis of RPN indicated that the underlying neural trace is weakened, but not suppressed by spreading depression in the cerebral cortex or superior colliculus. An asymmetry of RPN is brought about by unilateral 6-OHDA lesion on substantia nigra. Electroconvulsive shock applied immediately after a period of OK stimulation blocks the subsequent RPN without interfering with OKN. About 70 percent of neurons in the vestibular complex changed their activity during OKN and RPK. The changes consisted in most cases of an excitation accompanying OKN and inhibition during RPN. The OKN-RPN related reacti6ns were also abundant in flocculus but significant activity changes during RPN were less frequent in this structure. Units in midbrain reticular formation reacted both during OKN and RPN in a similar fashion as the vestibular ones. On the other hand units in the cerebellar deep nuclei and brachium conjunctivum were only weakly influenced by OKN and/or RPN. It is suggested that the neural trace of RPN develops in the vestibular complex and vestibulocerebellum as a part of the process compensating the effect of continued optokinetic stimulation. Flocculus participates in input processing of the optokinetic stimulation whereas reticular formation mediates signal transmission to oculomotor and higher integrating centers. The trace, revealed by sudden cessation of the eliciting stimulus in absence of visual reference signals is probably the neural substrate of the so called motion habituation and visual hallucinations. As other compensatory phenomena in the motor system, RPN has features of instrumental (it improves the organisms control of environment) and classical (it is automatically established and involuntarily emitted) conditioning.