Optokinetic eye movements elicited by an apparently moving visual pattern in guinea pigs

Based on known anatomical and physiological properties, guinea pigs could be expected to show a lack of cortical impact on the basic properties of optokinetic nystagmus (OKN) such as eye movements elicited by a rotating striped drum. The present study aimed to clarify the question of whether the guinea pig's brainstem-mediated circuit is capable of generating horizontal OKN with the animal binocularly viewing a stationary stroboscopically illuminated striped pattern (apparent-motion OKN). A striped drum with a pattern periodicity of 2.37 degrees was rotated around the animal. The OKN buildup under real stimulation conditions was found to be largely devoid of a rapid initial rise in slow-phase eye velocity (direct OKN component). Only in a few recordings was a slight initially faster acceleration seen eliciting slow-phase eye velocities of 2 degrees/s at the most. The lack of a rapid initial rise proves that the optokinetic reflex loop does not possess a functionally significant cortically mediated component. To achieve apparent-motion OKN, initially in each recording real-motion OKN was elicited by the constantly illuminated rotating pattern. Then, 60 or 90 s later the drum was stopped and the illumination switched over from constant to flashing light. Following Grüsser and Behrens (1979), we calculated an apparent stimulus velocity (= flash frequency x 2.37 degrees) and an apparent OKN gain. We found that all animals continued in nystagmus under the flashing light condition. The slow-phase eye velocity was the same as under real pattern motion, with gain values of between 0.36 and 0.7. In two animals we produced an apparent gain close to unity by suddenly doubling the flash interval, i.e., halving the apparent drum velocity. This gain, however, did not remain at a high level: The slow-phase eye velocity decreased until the previous lower gain was reached again. The data indicate that cortical mechanisms are not necessary to produce nystagmus under apparent-motion stimulation and neither phi nor sigma phenomena of motion perception satisfactorily explain the apparent-motion OKN in the guinea pig. Our data provide evidence that the brainstem-located velocity storage mechanism plays a decisive role in keeping the eye in motion under apparent-motion stimulation.