Babies respond to expression
You probably also know how much of an impact direct eye contact can have on you when talking to the people you love: it can convey a huge amount and give you an insight into their emotions.
A study released in 2006[1] found that from as early as four months old, babies process angry faces differently when they are making direct eye contact. All babies are drawn to faces when they are seeing a direct gaze,[2] but this study showed that babies respond the same to happy or neutral faces, no matter if the person is looking at them or away from them. When shown angry faces, however, babies processed them differently if they were also gazing directly at the baby.
Through eye contact, you're sharing your emotions with your baby
Scientists don’t know for sure why this is; it could be that babies are just less used to seeing angry expressions, so when directed towards them babies focus more closely. It may be to do with the complex social learning that babies are doing as they understand the world around them. There may even be another reason just waiting to be discovered as the science of babies' brains is studied in more detail.
For now, just know that when you are making eye contact and sharing your emotions with your baby, their brain is busy making connections and learning about their world.
References:
[1] Striano, T., Kopp, F., Grossmann, T., Reid, VM. (2006) ‘Eye contact influences neural processing of emotional expressions in 4-month-old infants’. Social Cognitive and Affective Neuroscience, Volume 1:Issue 2 (Pages 87–94).
AVAILABLE ONLINE: https://doi.org/10.1093/scan/nsl008
[2] Farroni, T., Csibra, G., Simion, F., Johnson, MH. (2002). ‘Eye contact detection in humans from birth’. Proceeding of the National Academy of Sciences vol.99 (pg.9602-5).