Abstract
Purpose: Human infants start to learn about and interact with their environment during their first postnatal months. Immaturities in their gross motor responses and spatial vision constrain their visual behavior during this period of rapid development. The goals of this study were to determine the distribution of their saccadic viewing behavior and fixations in depth in a head-free naturalistic setting. Methods: Participants aged 2-15 months wore head-mounted scene and binocular eye-tracking cameras (a modified Pupil Labs Core system) while engaging in naturalistic behavior in an 8ftx8ft home-like lab environment. Calibrated saccadic movements were identified in each recording (3 to 20 mins) using an approach based on Engbert & Mergenthaler (2006) and depths of fixations using an empirically calibrated MIDAS algorithm. Results: Recordings from infants aged 2-3(n=15), 5-6(n=31), 8-9(n=23), 11-12(n=16) and 14-15(n=11) months were analyzed. The distribution of eye positions relative to the head was tighter for the younger infants, particularly in the vertical direction, which was also reflected in the pattern of saccade amplitudes (Levene Test all p<0.0001). The distribution of depths being fixated by the infants in this environment was best described by 2 kernels (KDE) at each age with the probability at the closer distance (approx. 40cm) increasing between 2 & 6 months. Conclusions: The youngest infants have limited head and trunk control and exhibited the most restricted range of eye movements, suggesting they are not compensating for their limited mobility by shifting their gaze on the timescale of these recordings. This likely leads to less active sampling of visual information, slower rates of change in the input, and a tighter link between head- and eye-centered frames of reference for vision. Maturation of motor responses and vision appear to result in a greater range of ocular motor exploration over the first postnatal months.