Research suggests there's a good scientific reason why some of us struggle to look someone in the eye and hold a conversation with them.

It turns out we're not just awkward, our brains actually can't handle the tasks of thinking of the right words and focussing on a face at the same time.

The effect becomes more noticeable when someone is trying to come up with less familiar words, which is thought to use the same mental resources as sustaining eye contact.

Scientists from Kyoto University in Japan put this to the test in 2016 by having 26 volunteers play word association games while staring at computer-generated faces.

When making eye contact, the participants found it harder to come up with links between words.

"Although eye contact and verbal processing appear independent, people frequently avert their eyes from interlocutors during conversation," wrote the researchers.

"This suggests that there is interference between these processes."

The volunteers were tested while looking at both animations of faces making eye contact and animations of faces looking away. They were also asked to think of links between easily associated words and words where there are a lot of competing associations.

For example, thinking of a verb for 'knife' is relatively easy, because you can't do much more than cut or stab with one. Coming up with an associated verb for 'folder' is harder, considering you could open, close, or fill them.

The volunteers took longer to think of words when they were making eye contact, but only when difficult word associations were involved. The researchers suspect the hesitation indicates the brain is handling too much information at once.

So while making eye contact and holding a conversation is certainly possible, this is evidence that they can both draw on the same pool of cognitive resources, and sometimes that pool starts to run a little dry.

The sample size used was pretty small, but it's an interesting hypothesis. And it's also not the only study to suggest the brain gets slightly freaked out by eye contact.

In 2015, Italian psychologist Giovanni Caputo demonstrated that staring into someone else's eyes for just 10 minutes induced an altered state of consciousness. Participants saw hallucinations of monsters, their relatives, and even their own faces.

It seems that a process called neural adaptation is the cause, where our brains gradually alter their response to a stimulus that doesn't change – so when you put your hand on a table, you immediately feel it, but that feeling lessens as you keep your hand there.

The volunteers making eye contact and associating words may also be experiencing some kind of neural adaptation, but for now the Kyoto University researchers are calling for further study into the links between verbal and non-verbal communication.

And in the meantime, if someone looks away while they're talking to you, they might not be being rude – they could just have an overloaded cognitive system.

The findings were published in the journal Cognition.

A version of this article was first published in November 2016.