Babies can link sight and touch at four months

To play sport — or simply walk safely down the street — we need an awareness of how our body interacts with other objects. This requires us to integrate sensory signals about things happening around us (such as the sight of a ball approaching) with sensory signals on the body (such as the touch of a ball thwacking into our hands). Though most adults have a good awareness of their 'peripersonal space' and can manage this without any problem, exactly when this ability emerges has been unclear. 

Now, a new paper in Scientific Reports finds that even four-month-old babies are capable of linking the sight of an object in this space with touch sensations on their hands. The team, led by Giulia Orioli at the University of Birmingham, also reports that the brains of eight-month-olds react with surprise when expected patterns of visual-touch sensations don't happen, suggesting that even at this young age, we make predictions about how our bodies should interact with the world around us. 

In the experiments, 20 four-month-olds and 20 eight-month olds had small vibration devices bandaged to their palms, and EEG caps fitted to their heads to measure brain activity. The infants sat on a parent's lap to watch a screen, which showed an attention-grabbing animated animal face in the upper part of the screen. In the bottom half, meanwhile, a red ball seemed to either move towards them or move away, then disappear. In some trials, the devices on their palms vibrated shortly after the ball disappeared. This touch stimulation happened at the time when, if a real ball had rolled all the way to them, it would have reached their hands. 

The team used the EEG caps to monitor bursts of activity in somatosensory areas, which process touch. They found that for the four-month-olds (and younger 8-month olds), there was more activity when the touch to their hands followed the sight of the ball moving towards them than in other conditions. This difference suggests that the babies could coordinate sensory information about activity in different locations (the world around them, and their body) and occurring at different times. 

This finding was quite surprising, given the fact that the younger participants were too young to reliably interact with anything in their environment; most babies learn to consistently grasp objects only at about 5 months of age. "This finding indicates that fundamental aspects of the multisensory process underpinning peripersonal space representations, and self-awareness more generally, are in place prior to the onset of skilled action," the researchers write. 

Results for the older 8-month-olds were different, however. Their brains showed bigger somatosensory responses when they were touched after the ball had seemed to move away from them. This bigger response to unexpected contact might reflect their ability to recognise unexpected events in their peripersonal space, the team thinks. "Based on this finding, we may speculate that if a group of adults were presented with these same stimuli, they would show a pattern of responses like that shown by the older 8-month-old participants reported here," the researchers write.

It would of course be interesting to see if that is indeed the case. As the younger babies' brains responded more to the expected patterns of visual and touch signals, perhaps they too are capable of appreciating their peripersonal space — but it seems that the way that space is represented in the brain becomes more sophisticated with age. These are indeed, as the authors write, "exciting new clues" to the development of human self-awareness in the first year of life. 

Read the paper in full: https://doi.org/10.1038/s41598-023-45897-4