Abstract
Purpose: Humans start to learn about and interact with their visual environment over the first months after birth. At the same time, this visual experience tunes the receptive fields of neurons across the visual field. The goals of this study were to determine the characteristics of fixations in natural viewing during infancy and the distribution of contrast around these fixation points in the visual field. Methods: Participants aged 2-12 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. Binocular fixations ≥400ms were identified in the eye movement recordings using a dispersion algorithm, and then RMS contrast was averaged around these fixation points for each infant and age group (2-3(n=15), 5-6(n=31), 8-9(n=23) & 11-12(n=16) months). Results: The median fixation duration increased minimally with age from 0.51s at 2months to 0.55s at 11months (p=0.025). Additionally, average RMS contrast decreased monotonically with eccentricity from the point of fixation for all age groups, across the analysis radius of 20deg. Interestingly, within a local region of 3deg from the point of fixation, the slope of RMS contrast remained uniform for infants aged seven to twelve months but decreased for the younger age groups (p≤0.0001). Conclusion: For younger infants, the fixation point is, on average, the point of highest local contrast. For older infants, the impetus behind fixation becomes more complex, leading the contrast levels around the point of fixation to be more evenly distributed over time. This result is consistent with the immature spatial vision of the youngest infants and highlights the qualitative change in their visual experience over the first postnatal months. Of note, the duration of fixations did not change dramatically with age.