Our brains are continuously bombarded with information from the senses, yet our level of vigilance to such input varies, allowing us to selectively focus on one conversation and not another, according to the study published in the journal Neuron.
"If we want to give our full concentration, something happens in the brain to enable us to focus and filter out distractions," said Professor Stephen Williams from the University of Queensland in Australia.
"There must be a mechanism that signals the thing we want to focus on," said Williams.
However, this mechanism is not well understood, he said.
Research has shown that the electrical activity of the neocortex of the brain changes, when we focus our attention.
Neurons stop signalling in sync with one another and start firing out of sync.
This is helpful, said Williams, because it allows individual neurons to respond to sensory information in different ways.
Thus, you can focus on a car speeding down the road or on what a friend is saying in a crowded room.
It's known that the cholinergic system in the brain plays an important role in triggering this desynchronisation.
The cholinergic system consists of clusters of special neurons that synthesise and release a signalling molecule called acetylcholine, he said, and these clusters make far reaching connections throughout the brain.
Not only does this cholinergic system act like a master switch, but mounting evidence suggests it also enables the brain to identify which sensory input is the most salient — ie worthy of attention — at any given moment and then shine a spotlight on that input.
"The cholinergic system broadcasts to the brain, 'this thing is really important to be vigilant to'," said Williams.
He added that the cholinergic system has been proposed to have a far-reaching impact on our cognitive abilities.
"Destruction of the cholinergic system in animals profoundly degrades cognition, and the formation of memory," he said.
"Importantly, in humans a progressive degeneration of the cholinergic system occurs in devastating diseases that blunt cognition and memory, such as Alzheimer's disease," said Williams.
However, precisely which neurons in the cortex are being targeted by this master switch and how it's able to influence their function was unknown.
Williams and researcher Lee Fletcher wondered if layer 5 B-pyramidal neurons, the 'output' neurons of the neocortex, might be involved, because they are intimately involved in how we perceive the world.
"The output neurons of the neocortex perform computations that are thought to underlie our perception of the world," said Williams.
Researchers wanted to know if the cholinergic system is able to influence the activity of these output neurons.