Development of gaze tracking of small and large objects.

A longitudinal study was designed to address the relationships between the smooth pursuit (SP) and the optokinetic response (OKR). Eye and head movements were measured in infants between 7 and 14 weeks of age. They were placed in front of a moving object subtending visual angles of 2.5, 5, 7, 14, 21, 28, and 35 degrees. The object oscillated sinusoidally along a horizontal path with a frequency of 0.25 Hz and an amplitude of +/-25 degrees visual angle. It was found that the number of saccades was dependent on object size: at 6 and 9 weeks of age there were more saccades for the smallest objects. With increasing age, the number of saccades decreased. The composite eye movement gain (smooth tracking + saccades) did not change with age but for the 6.5-week group the gain was higher for smaller objects. The gain of the smooth eye tracking increased with age and showed no dependency on object size. In conclusion, the results do not support the concept of two separate systems, the OKR and the SP, each of them processing eye tracking of small and large objects. Finally, it was observed that two infants at 6.5 weeks of age who used considerable head movements did not inhibit the vestibular-ocular response.