Scientists say they have found the brain regions that help us to decide whether to look someone in the eye or look away.
Two areas of the brain were involved
Using brain imaging, the Imperial College London team pinpointed specific areas deep within the frontal cortex.
One region works when we decide to look at something, while another works if we change our mind at the last minute.
It might help doctors better understand certain brain diseases, they told the journal, Current Biology.
Where we choose to look is fundamental to how we interact with other people.
On some occasions we might want to look someone straight in the eye, but at other times we might decide to avert our gaze and look away.
It has been thought that the frontal lobe is involved in helping a person select between conflicting voluntary actions such as this.
It is also known that people who have damage to their frontal lobe have difficulties in spontaneously initiating voluntary actions.
To investigate further, Dr Masud Husain and colleagues asked volunteers to perform a visual task while undergoing a brain scan.
The volunteers were shown an image and asked to either choose which part they looked at and fixate on that, or change their minds and look elsewhere.
The magnetic resonance imaging (MRI) scans showed that two different areas of the medial frontal cortex were active depending on whether the individual made a free choice or changed their minds.
The medial frontal cortex is highlighted in red in this picture of the head sliced in half
Dr Husain said: "Different parts of the medial frontal cortex become active when we choose to make an eye movement or our own free will and when we face a difficult choice involving conflicting alternatives.
"These decisions can have quite important consequences.
"The simplest analogy is whether you decide to look your boss in the eye or not.
"Your eye movement behaviour can tell a lot about what you are thinking.
"What we are really beginning to do is to identify the highest levels of command and control within the human brain."
Professor Chris Frith, of the Institute of Neurology at University College London, said the results were important for understanding the behaviour of patients when this region of the brain is damaged.
"One of the things that can happen when our frontal lobe is damaged is that we become slaves to our environment.
"Patients with severe damage in this part of the brain no longer behave like free agents. They cannot decide for themselves what to do and cannot change their minds at the last minute.
"The researchers have been able to identify specific roles for small regions of frontal cortex deep within the brain.
"This shows that making free choices is not simply a matter of selecting between alternative actions."