When do babies track objects?

It is not uncommon to see an adult hold up a finger and move it across a baby’s field of view to see if the baby can fixate and follow the finger’s movement with his eyes. There is great interest in babies meeting milestones in order to assess health and, if necessary, provide support. Can, in fact, my baby track a finger with his eyes? At what age is he supposed to be able to do this? Before looking at the academic literature to answer when babies can track objects, it is important to understand three types of eye movements necessary to follow an object. These are saccades, smooth pursuit, and vergence eye movements.

Types of Eye Movements

Saccades

Saccades are rapid eye movements that quickly change a person’s fixation to a different interest area. In fact, “saccade” is French for jerk or jolt. A saccade can be voluntary (within our control) or reflexive (out of our control), and saccade lengths can also be small (e.g., as seen during reading) or large (e.g., as evident while scanning a football game).

Smooth Pursuit

Smooth pursuit eye movements are used to fixate and follow a moving object. These eye movements are slow and function to maintain the moving object on the fovea, which allows for highly detailed object information to be processed. Typically, a moving object is necessary to elicit smooth pursuit eye movements; we are generally unable to make these movements voluntarily in the absence of an object to follow. In other words, smooth pursuit eye movements are closely linked with object tracking.

Vergence

For smooth pursuit and saccade eye movements, both eyes synchronously move in the same direction. In contrast, for vergence eye movements, the eyes move in opposite directions, so the object falls directly on the fovea of each eye to enable detailed vision. If the object is near, the pupils will converge towards each other for the target to fall on the foveas. On the other hand, if the object is further away, the pupils will diverge from each other for this to happen. Vergence (convergence or divergence) along with lens and pupil size adjustments allow people to clearly focus on an object in three-dimensional space. The three together (vergence, lens adjustments, and pupil size) are known as the accommodation reflex.

What Age do Babies Track Objects?

During object tracking, people use smooth pursuit eye movements to follow a moving object, saccades to catch up to the object when smooth pursuit is off-track, and vergence shifts to manage the depth of the tracked object. Babies’ vergence responses appear to be online in their second month (although the accommodation system, as a whole, is still relatively insensitive to depth changes).

A comprehensive eye-tracking study with infants by Christina Pieh, Frank Proudlock, and Irene Gottlob provides some insight into the development of smooth pursuit and saccadic eye movements. Within the first two months of age, smooth pursuit eye movements are present (and this result is consistent across a number of studies, e.g., here, here, and here); however, at this early age, smooth pursuit eye movement are quickly over and substantially lag behind the moving object. Object tracking is, in fact, very saccadic during this period.

Christina Pieh, Frank Proudlock, and Irene Gottlob, however, indicate that the ability to track objects quickly develops between 2 and 6 months of life. Smooth pursuit gain (eye velocity divided by the object velocity during the longest single smooth pursuit movement – 1.0 indicates perfect pursuit) is 0.8 or higher. In addition, the number of catch-up saccades decreases during this period. The authors indicate that any further development is still slow and incomplete by 18 months. In fact, children’s smooth pursuit gain does not reach adult levels until seven or eight years of age or even older. It is important to note that, just as with adults, the ability of infants and children to track objects depends on a wide range of factors, including an object’s speed and shape (e.g., square versus dot).

What Time Period Do Babies Develop Object Tracking?

So, at what age do infants tack objects with their eyes? Perhaps a better question would be – what is the time period over which object tracking develops? As with most things, an ability does not simply appear overnight. Object tracking develops rapidly between two and six months of age. By seven months, infants are generally tracking objects with their eyes rather impressively but will not reach adult levels until well into childhood.
 

 

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References

Mitkin, A. & Yamstshikov, A. (1984). Oculomotor and visual functions in early infancy, 7(1), 253. doi: https://doi.org/10.1016/S0163-6383(84)80315-X

Pieh, C., Proudlock, F., & Gottlob, I. (2011). Smooth pursuit in infants: maturation and the influence of stimulation, 96(11), 73-77. doi: http://dx.doi.org/10.1136/bjo.2010.191726

Rütsche, A., Baumann, A., Jiang, X., Mojon, D. S. (2006). Development of visual pursuit in the first 6 years of life. Graefe’s Archive for Clinical and Experimental Ophthalmology 244, 1406–1411 (2006). doi: https://doi.org/10.1007/s00417-005-0248-4

Salman, M. S., Sharpe, J. A., Lillakas, L., Dennis, M. & Steinbach., M. J. (2006). Smooth pursuit eye movements in children. Experimental Brain Research, 169, 139–143. doi: https://doi.org/10.1007/s00221-005-0292-7

Seemiller, E. S., Wang, J. T., Rowan Candy, T. R. (2016). Sensitivity of vergence responses of 5- to 10-week-old human infants, 16: 20. doi: https://doi.org/10.1167/16.3.20

Strand-Brodd, K., Ewald, U., Grönqvist, H., Holmström, G., Strömberg, B., Grönqvist, E., von Hofsten, C., & Rosander, K. (2011). Development of smooth pursuit eye movements in very preterm infants: 1. General aspects. Acta Paediatrica, 100(7): 983–991. doi: https://doi.org/10.1111/j.1651-2227.2011.02218.x

Vinuela-Navarro, V., Erichsen, J. T., Williams, C., & Woodhouse, J. M. (2019) Quantitative characterization of smooth pursuit eye movements in school-age children using a child-friendly setup. TVST, 8(8), 1-12. doi: doi:https://doi.org/10.1167/tvst.8.5.8

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