A burst of brain activity recorded by scientists could offer clues to a baby's level of understanding of the world around it.
Babies were fitted with nets of sensors
The researchers involved, from Birkbeck College, and University College London, believe their finding could begin to settle a controversial argument on baby brain development.
When an object is shown to six-month-old babies, then hidden, they often behave as if it is no longer present.
It appears to be "out of sight, out of mind", as far as their level of understanding is concerned.
But scientists still suspect the baby, to some extent, does understand the object is still around, just hidden, even if it shows no physical signs of awareness.
The London team wired up their babies to a harmless "hair-net" of sensors which measured electrical activity in the brain.
They were looking for a burst of activity that might correspond to the infant thinking about the object while it was hidden.
One of the traditional tests used for these experiments is a toy train that is pushed into and out of a tunnel.
What they found was a distinctive burst of electrical activity over a part of the brain called the temporal lobe at key stages in the game.
It happened both when the object was "occluded", or hidden, and again at around the time the baby might expect the train to reappear from the tunnel.
The researchers believe these activity bursts represent a clear process in which the baby is thinking about the train - even though he or she cannot see it.
"Whatever the exact neural basis of the effects we have observed, our finding that increased 'gamma-band' activity is associated with the representation of hidden objects will inform fundamental issues about how infants process their visual world," they wrote.
But the study is unlikely to solve the puzzle completely.
Dr Alan Slater, a psychologist from Exeter University who specialises in this field, said it was always a big jump to link either infant behaviour - or their brain activity - with what they may be understanding about the world.
He said: "Many people think it's an amazing leap to make.
"What we really have to do is to rule out all the alternatives that may be leading to this activity."
He said the area of research was a vital one, which might provide insights into how the brain develops in its earliest days.
"If anything is going wrong in infancy, we have a chance of correcting it," he said.
The study was published in the Royal Society journal Biology Letters.