New study explores why babies make irrelevant movements

A new Goldsmiths, University of London study into the unnecessary arm and leg movements of babies could help us better understand and monitor development in a child’s early years.

When moving with a purpose – to grab an object, for example – an adult’s brain will select whether a hand or foot is needed for the task, and whether we should use one or more of our limbs.

But babies tend to use ‘goal-irrelevant’ limbs as well. When grabbing an object with one hand, other limbs join in, and the baby will move their feet or reach out with two hands, for example.

While previous studies show that babies produce these ‘extraneous movements’, little was known about how and when we grow out of them.

Researchers from the Goldsmiths InfantLab in the Department of Psychology carried out two experiments with 9-month- and 12-month-olds to get a clearer idea of when babies stop using unnecessary limbs while they’re carrying out an action, and how it relates to other aspects of their motor development.

In the first test, most of the time 9-month-olds activated multiple limbs while reaching for a small ball. While extending one arm to grab the ball, they lifted, splayed or wiggled their toes, jerked their legs, or did the same with their other arm.

The infants who were just three months older were much more likely to restrict their movement to using a single, goal-directed, hand and arm.

In a second experiment, the same babies were presented with a rattle in one hand. Nine-month-olds were more likely than the older babies to move their other hand in the same way as the hand shaking the rattle, as if their arm movements were mirrored between the two sides of their body even though only one hand was actually doing anything with the rattle.

During the tests, participants were strapped to a baby seat to restrict torso movement, and had their actions recorded on video, and tracked with reflective markers using motion-tracking equipment like that used to reconstruct human movements in CGI films.

The infants also had their motor experience – such as whether they could sit without support, crawl, stand with help or walk with help – assessed by the research team and through a parents’ report.

Hana D’Souza, a PhD student in the Goldsmiths InfantLab and lead author of the study explains:

“Our results document, for the first time, a substantial decrease in infants’ extraneous movements between the ages of 9 and 12 months. The increased specialisation of limb movements takes place as the brain develops from being ‘broadly tuned’ to the environment, to becoming more specialised to react to stimuli.”

Professor Andrew Bremner, Head of the Department of Psychology and director of Goldsmiths’ InfantLab, adds:

“It’s likely that this original ‘broad tuning’ in the first few months of life fulfils an adaptive function. It provides new-born babies with a wide repertoire of responses to their environment from which they can select the most effective over the coming months of life.

“Brain activation becomes increasingly specialised over time through interactions between various brain regions and with the environment. So a 9-month-old might activate multiple limbs when reaching for an object, but it’s likely that feedback about which limb was successful, over many repetitions, gives rise to the ability to learn what the most efficient action is and how they can retrieve an object using just the required limb.”

Although these extraneous movements appear to be part of normal development, Hana D’Souza and her colleagues also conclude that close examination of these movements could be helpful in early diagnosis of developmental disorders.

Specialization of the motor system in infancy: from broad tuning to selectively specialized purposeful actions by Hana D’Souza, Andrew Bremner (Goldsmiths), Dorothy Cowie (University of Durham) and Annette Karmiloff-Smith (Birkbeck) is published in the journal Developmental Science.