Abstract
In the literature, it has been suggested that smooth eye tracking with a free head requires that the vestibular reflex (VOR) is suppressed. Does that mean that infants cannot suppress VOR when they are younger than 6 to10 weeks, i.e. when smooth tracking is not observed? This hypothesis was tested in the present study. We studied a group of infants between 2 and 10 weeks of age in 3 different conditions. First, an object oscillated in front of the stationary infant (SP condition). Secondly, the infant oscillated in front of the stationary object (VVOR condition), and, finally, the object and the infant oscillated in synchrony (VORINHIB condition). All oscillations had a frequency of 0.25Hz. In all conditions, eye movements were measured with EOG, and the head+object movements with an optoelectronic system with passive markers (Qualisys, Sweden). The results show that at 2 weeks of age the gain of the smooth pursuit component was low (0.1) in the SP condition but increased significantly at 6–10 weeks of age. In the VVOR condition the smooth tracking-counterrotation- increased between 2 and 4 weeks of age. At 4 weeks of age, suppression of VOR was observed in the VORINHIB condition. For all age groups the gain of the counter rotations of the eyes in this condition could be predicted by a vector addition of the corresponding eye gains in the SP and VVOR conditions