Babies become conscious of their environment by the time they are five months old, according to a new study by French neuroscientists.

By the time infants reach three months of age, their developing brains have trillions of connections and the weight of those firing neurons triples within the first year of life. Scientists have always wanted to know what do these small people know, and when do they know it?

The current study findings, which are published in the journal Science, reveal that at five months, babies have the internal mechanisms to perceive items in adult-like methods – despite the fact they are unable to tell us.

The team of French neuroscientists, led by Dr Sid Kouider of the of École Normale Supérieure in Paris, conducted a study examining the brain activity of 80 infants aged 5, 12 and 15 months old in response to being shown photos of human faces.

Each baby wore a cap filled with electrodes and was then shown pictures several different times, starting with periods of time that were short – a period too short for an adult to be conscious of seeing them – then slowly increasing the time the photos were shown. Each photo was shown on a computer screen and had a sound of a bell administered at the same time to attract the infants’ attention.

Generally, in adults the areas of the brain linked to vision are activated even if the photo is revealed for a short time and the subject is not consciously tuned into it. When the photo is shown for at least 300 milliseconds – a neural signal is transported from the vision center in the back of the brain to the prefrontal cortex – and this is linked to the adult documenting being conscious of the photo. This signal is called the later slow wave – or nonlinear cortical response.

Visual cortex activity and late slow wave response were both seen in every infant, however, in the youngest babies the response was weak and was only present if the image was shown for 900 milliseconds (ms) to over one second. In the oldest babies, the response was more significant and was seen with photos shown for about 750 ms.

The authors believe these findings suggest the nonlinear cortical response could give a marker for conscious thought, since the signal in the prefrontal cortex shows the picture is stored for a short time in the brain’s “working memory” or consciousness.

Results could also aid in testing for people who cannot communicate what they are perceiving, like patients in comas. Similar techniques could also be helpful for medical professionals to better understand perception of pain or the results of anesthetics on kids, and help investigators studying consciousness in non-human animals.

Next the researchers plan to study two-month-old infants in a similar fashion, and to test babies to examine whether familiar objects like toys cause a quicker response.

Written by Kelly Fitzgerald