If we told you not to think about a red apple, would an image of a red apple immediately pop into your mind? We're less adept at controlling our thoughts than we might think, according to new research.

Even those who are relatively good at managing what they're thinking about on a conscious level can still have traces of unconscious thoughts they don't know about, this latest study finds: and that could teach scientists more about how much control we have over what's going on in our brains.

A team of researchers from the University of New South Wales in Australia worked with a total of 67 volunteers who were asked to not think about something, then tested to see whether they'd been able to keep their thoughts in check.

"This is an exciting discovery as it tells us for the first time that even when we feel like a thought is not there, because we have successfully suppressed it from our minds – it is actually still there," says one of the researchers, Joel Pearson.

"Which suggests using brute force to not think about something – that cigarette or that drink – simply won't work because the thought is actually there in our brains."

The way the experiment worked was this: volunteers were shown a written cue and told not to think about it. The six cues were red apple, red chilli, red tomato, green broccoli, green cucumber, and green lime.

Next, they were asked to push a button to indicate whether they'd successfully avoided thinking about the cue or not.

The participants were then shown a red-green image in a special binocular rivalry setup (where an image is shown to each eye separately), and told to say which of the colours was dominant.

Here's the surprising bit: there was an above-average chance the study subjects would name the colour of the original cue card, despite the fact they were looking at an image that was equally red and green.

Several previous studies have highlighted how hard it is to specifically not think about something, but that extra test suggests that even when we think we're mastering our own thoughts, that might not be the case.

"Even though they had not thought about the objects, we could still measure the sensory trace of a thought," says Pearson. "In other words, despite not being aware of the thought there was still a representation of it there, most probably in the visual cortex, that was systematically biasing the binocular rivalry illusion."

Various control experiments were used to back up the hypothesis, and further testing showed that thought substitution – asking volunteers to think about a white cloud rather than a red tomato, for example – was a more effective thought control technique.

One of the ways this new research might be used is in treating addiction – helping people to deal with their unconscious as well as conscious thoughts when it comes to avoiding harmful behaviour.

Controlling thoughts is crucial in everything from concentrating on tasks at work to managing our mental well-being, so there are lots of ways this research could be applied. And it ties in with other studies on how unwanted thoughts can be banished. 

Further studies from the same team are planned, which will use functional brain imaging (fMRI) to try and pinpoint the location of these unconscious, suppressed thoughts in the brain.

"This discovery changes the way we think about thoughts of desire and suggests unconscious thoughts can emerge and drive our decisions and behaviour," says Pearson.

The research has been published in Psychological Science.