Effect of gravity on horizontal optokinetic after-nystagmus in humans

The change in horizontal optokinetic after-nystagmus (HOKAN) with regard to gravity was evaluated in 5 normal subjects who had not suffered from vertiginous diseases. Following optokinetic stimulation for 30 sec, HOKAN was recorded in total darkness by using electro-nystagmography. Four parameters of HOKAN, such as duration, number, initial slow phase velocity and time constant, were measured upright or two 90° lateral tilted positions (right lateral head position or left lateral head position). In contrast with duration, number and initial slow phase velocity, time constants of HOKAN during both statically tilted positions were markedly reduced compared with that during upright position. Furthermore, secondary HOKAN was found in only one lateral head position, neither in the other side nor in upright position in 1 out of 5 subjects. These findings suggest that otolith input may modify not only vertical OKAN but also HOKAN in humans.