Interaction between cervico-ocular and vestibulo-ocular reflexes in normal adults

The interaction of the cervico-ocular reflex (COR) and the vestibulo-ocular reflex (VOR) was studied in 20 human Subjects (Ss) during application of synergistic and antagonistic combinations of neck and vestibular stimuli, and during two different psychophysical tasks related to the Ss' self-motion sensation. Slow and quick eye movement responses were analyzed separately. Neck stimulation produced by horizontal rotation of the trunk about the stationary head elicited slow COR eye movements of very low gain; COR direction was anticompensatory, unlike the compensatory one of the VOR. During either a synergistic combination of neck and labyrinthine stimuli (head rotation on stationary trunk) or an antagonistic combination (head-to-trunk rotation counter to head-in-space rotation), the resulting slow eye movements were slightly larger than those during labyrinthine stimulation alone (whole body rotation). This weak neck contribution could be described by a directionally non-specific enhancement of VOR gain and a linear summation of VOR and COR slow phases. These effects were essentially independent of whether the Ss estimated the magnitude of their head turning or trunk turning in space. If Ss were estimating their trunk turning, neck stimulation also evoked quick eye movements, but these were small and hardly affected the VOR quick phases during the combined stimulations. In contrast, if Ss estimated their head turning, neck stimulation evoked large quick phases, which interfered with the quick phases of the VOR; during the synergistic combination of head and neck stimuli. COR quick phases added to those of the VOR, thereby shifting the gaze in the direction of head rotation (reorientation of gaze). With the antagonistic combination they subtracted, so that the VOR slow phase could compensate the head rotation in space (stabilization of gaze). These findings suggest that (1) the slow phase of the COR has no functional significance in intact humans and (2) the quick phase of the COR plays a role for both stabilization and reorientation of gaze depending on the behavioural context.