Purpose To investigate the development of smooth pursuit in infants and to assess the influence of different stimulus characteristics.

Methods A total of 131 eye movement recordings were obtained from 71 infants between 1 and 18 months of age using infrared photo-oculography. Smooth pursuit eye movements (SPEM) were stimulated using targets of different sizes (1.2° and 4.7° of visual angle) and velocities (7.5°/s, 15°/s and 30°/s).

Results Smooth pursuit maturation peaked between 2 and 6 months of age with smooth pursuit gain showing a steep rise for all stimulus velocities and target sizes within this age range (p<0.0001). Higher stimulus velocities were associated with shorter durations of the longest smooth pursuit (p<0.0001) and higher saccadic frequencies (p<0.0001). A larger stimulus size led to an increased saccadic frequency (p=0.035). Tracking time was highest when the larger stimulus of 4.7° of visual angle was applied (p=0.022) and when it moved at a medium stimulus velocity of 15°/s (p=0.0002). The choice between a schematic face and a scrambled face did not influence the quality of the infants' smooth pursuit.

Conclusion SPEM show an intensive maturation between 2 and 6 months of life. By 6 months of age SPEM have already reached an almost adult-like gain of 0.8 or higher. Further maturation is slow and still incomplete by the age of 18 months. Stimulus velocity and size have an important impact on the smooth pursuit quality, which should be considered in smooth pursuit testing in infants.