Suppression of optokinetic velocity storage in humans by static tilt in roll

The effects of static tilts about the roll (anterior-posterior) axis on human horizontal optokinetic afternystagmus (HOKAN) were examined. Static tilts in roll, with subjects lying on their left side, produced significant tilt-dependent HOKAN suppression. Only the slow (indirect pathway) component time constant (1/D) of the double exponential model for human HOKAN decreased with angle of roll tilt. The effect was direction specific in that suppression occurred only following a leftward-going stimulus. These findings provide further support for the postulate that otolith-organ-mediated activity can couple to the horizontal velocity storage mechanism in humans. A slight trend towards a tilt-dependent reduction of coefficient A (initial slow phase velocity of fast component decay) was revealed, suggesting the possibility that otolith-organ-mediated activity could couple to direct (pursuit-mediated?) pathways as well. No horizontal-to-vertical cross-coupling occurred, indicating that this aspect of the 3-dimensional model for velocity storage proposed by Raphan & Cohen (1988) may not completely apply to humans.