Short-term habituation of eye-movement responses induced by galvanic vestibular stimulation (GVS) in the alert guinea pig.

In a recent study, we showed that primary afferent neurons innervating all vestibular end organs were sensitive to galvanic vestibular stimulation (GVS) in guinea pigs. In order to determine the three-dimensional character of eye movements induced by GVS, changes in eye position were recorded using digital video oculography during delivery of bilateral GVS ranging in intensity between 20 and 80 microA. Pulses were delivered in repetitive trains in order to also ascertain the involvement of vestibular habituation. At low intensities of GVS (up to 40 microA), maintained changes in eye position were induced toward the anode and away from the cathode. These eye movements were predominantly vertical with some horizontal eye movement and very little or no torsion. At higher intensities of GVS (>40 microA), horizontal nystagmus was initially observed, as well as an overall deviation of the beating field toward the anode. Nystagmus was found to habituate rapidly over successive presentations of GVS, whereas the tonic deviation of the eye remained consistent without any detectable habituation. The direction of eye movements induced by GVS was similar to that observed in humans during trans-mastoidal GVS, and the threshold differences between tonic and phasic components for GVS were also similar to previous human GVS studies. The observed habituation appears to be more specific to the phasic VOR component in quadrupedal animals such as guinea pigs, and this may reflect a considerable emphasis placed on otolithic stimulation in these animals during complex locomotor activities.