Effect of roll and pitch on flash-induced nystagmus: support for involvement of retinal direction-selective elements?

The direction of eye movement (i.e., the angle of the resultant velocity vector calculated from the horizontal and vertical components) during flash-induced nystagmus was evaluated in static roll and pitch positions. It appeared that this direction coincided with the orientation of the visual streak, as influenced by the ocular countertorsion which is known to occur in the various body positions. This supports the hypothesis that flash-induced nystagmus results from stimulation of direction-selective retinal elements. The intensity of flash-induced nystagmus, evaluated either by measuring slow-phase velocity (i.e., magnitude of resultant vector) or by beat frequency, was minimal in approximately 90 and 270 degrees roll or pitch positions, presumably because the total combined (utricular and saccular, left and right) excitatory otolith stimulation was maximal in these positions, the effect of this stimulation being inhibitory to the flash-induced nystagmus.