Three methods to elicit sigma-optokinetic nystagmus in Java monkeys.

Sigma-optokinetic nystagmus (sigma-OKN) can be elicited in awake Java monkeys (Macaca fascicularis) when stationary periodic visual patterns (grid of black white stripes, row of equally spaced dots) are illuminated stroboscopically. Three methods were found to be useful in inducing the sigma-OKN: postrotatory nystagmus, optokinetic afternystagmus (OKAN) following normal OKN and a gradual transition from phi-movement (phi-OKN) to sigma-OKN. The properties found for sigma-OKN in man are also present in monkeys with the one exception that monkeys have a long-lasting sigma-OKAN in darkness which is not present in man. The average angular speed Ve of sigma-OKN slow phases was related to the flash frequency fs and the spatial period Ps of the stripe pattern according to the following equation: Ve = k.Ps . fs [degrees . s-1] The constant k was 1 or close to 1.