Head and body structure infants’ visual experiences during mobile, naturalistic play

Infants’ visual experiences are important for learning, and may depend on how information is structured in the visual field. This study examined how objects are distributed in 12-month-old infants’ field of view in a mobile play setting. Infants wore a mobile eye tracker that recorded their field of view and eye movements while they freely played with toys and a caregiver. We measured how centered and spread object locations were in infants’ field of view, and investigated how infant posture, object looking, and object distance affected the centering and spread. We found that far toys were less centered in infants’ field of view while infants were prone compared to when sitting or upright. Overall, toys became more centered in view and less spread in location when infants were looking at toys regardless of posture and toy distance. In sum, this study showed that infants’ visual experiences are shaped by the physical relation between infants’ bodies and the locations of objects in the world. However, infants are able to compensate for postural and environmental constraints by actively moving their head and eyes when choosing to look at an object.


Reviewer 1's Comments
1. Head-centered field of view How was the center of the scene camera image aligned to the center of the field of view of the infant's right eye (i.e., how was the head-centered field of view defined)? I believe that calibration in the eye tracker is done so as to match the position of the pupil in the eye-camera image and the location of the object in the scene camera image. The eye tracker can track the gaze accurately even if the orientation of scene camera is not perfectly aligned to the center of the actual field of view of the right eye. Slight changes in the orientation of the scene camera could result in different "center" in the scene camera image. In other words, although the eye tracker is able to pick up the location of gaze (green cross in Fig. 4) accurately, the center of the scene camera image and resulting distance between the image center and the green cross may not be so accurate. We appreciate the comment. We agree that the field of view was not perfectly head-centered as the scene camera was positioned slightly over the right eye. However, given that the distance between the right eye and head midline is very small, the scene could be used as an approximation of the head-centered field of view. We are also aware of the concern about whether the scene camera is in fact oriented towards the center. We added to the method section to detail the calibration procedure that the original study used to ensure that the scene camera was positioned correctly.

Non-looking episodes
By definition, the toys in non-looking episodes were what randomly got in infant's field of view. They were not something that infant's visual system was oriented to. As such, I'm not sure whether and how the analysis of the data of "non-looking" episode could deepen our understanding the nature of infants' visual experiences during naturalistic tasks. I have difficulty in interpreting the results of the analysis of what infants were not looking at, and the comparison between looking and non-looking episodes.
Although we agree that what is in infant's field of view when not looking may be unintentional, we disagree with calling it random. Our aim is to show that infants' head and body movements shape their visual experiences, which may then shape their opportunities for learning about (and looking at) different stimuli in the environment. This is analogous to studies of auditory experiences-audio recorders count how much language infants are exposed to without knowing how much infants attend to. We added a paragraph at the end of the current study section to address this concern.
3. What does the result imply that far toys are less centered compared to close toys not only in looking episodes but also in "non-looking" episodes when infants are in prone (Fig. 6A)? It implies that infants while prone have a more constrained field of view in general compared to in sitting and upright positions. Far toys are more likely to become out of view (as they are located at the top of the field of view) for infants while prone during both looking and non-looking episodes. It may be because lifting the head to look at distant objects in prone is effortful. We added a clearer interpretation of this result to the discussion.
4. Why did toy locations in the field of view become more spread from prone to sitting and from sitting to upright during "non-looking" episodes, but not during looking episodes (Fig. 6B)? What underlies the increased spread in the gaze toward closely located toys while infants are prone in looking episodes ( Fig  6B), which was the exact opposite to the tendency found in "non-looking" episode just mentioned above? We think that infants in sitting and upright positions have a relatively higher point of view than prone and have visual access to a larger area of the environment. That's why the spread becomes greater from prone to sitting and from sitting to upright-a larger viewing area allows a larger spread of object positions. This trend is not shown during looking episodes because infants actively center the object in view, reducing the spread of positions while looking.
Regarding why the spread of close toys while prone is larger than in other positions during looking episodes, we suggest that infants have more difficulty stabilizing the field of view while prone. Tilting the head up and down is effortful while prone. As a result, infants are less likely to keep the toys at consistent locations in the field of view when looking. However, we believe this warrants future testing. We added these interpretations of the results to the discussion.