By Christian Jarrett

If the courts wanted to know if a suspected sex offender was attracted to children, they could ask him or her, or they could ask experts to measure signs of the suspect’s sexual arousal while he or she looked at different images. But a devious suspect would surely lie about their interests, and they could distract themselves to cheat the physical test.

Brain scans offer an alternative strategy: research shows that when we look at images that we find sexually attractive, our brains show distinct patterns of activation. But of course, the same issues of cheating and deliberate distraction could apply.

Unless, that is, you could somehow prevent the suspect from knowing what images they were looking at, by using subliminal stimuli that can’t be seen at a conscious level. Then you could see how their brain responds to different types of image without the suspect even knowing what they were looking at.

This is the essence of a strategy tested in a new paper in Consciousness and Cognition. Martina Wernicke at Asklepios Forensic Psychiatric Hospital of Gottingen and her colleagues have provided a partial proof of principle that it might one day be possible to use subliminally presented images in a brain scanner to provide a fraud-proof test of a person’s sexual interests. It’s a potentially important break-through for crime prevention – given that deviant sexual interest is one of the strongest predictors of future offences – but it also raises important ethical questions.

Wernicke and her team scanned the brains of 24 healthy, heterosexual men with fMRI while they looked at images of naked men and women, or more explicitly pornographic images that depicted either naked men or women masturbating. The idea was that, given their sexual orientation, the participating men’s brains would show a distinct pattern of activity when looking at the naked women compared with when looking at naked men, especially in the more explicit context.

Another detail is that the researchers attempted to make some of the images subliminal – that is, not visible at a conscious level. To do this, they presented the target images twice extremely briefly – just 16.7 milliseconds at a time – and they used one of two types of “mask”. A mask in this context is a second, distracting image shown immediately after the first instance of the target image and designed to interfere with the processing of that target image. After the scanning session, a memory test was used to find out how much the participants had processed the subliminal images.

Consistent with past research, the men’s brains showed a distinct pattern of activity across a range of areas when they viewed consciously visible images of naked women compared with naked men. But what about when images were made subliminal?

Here the challenge was to make the images inaccessible to conscious awareness, but still visible enough for the brain to process them. One type of mask the researchers used was different neutral scenes, such as a leaf or a rustic street scene. In a sense, these masks interfered too much with the processing of the naked pictures. Not only were the participants unable to identity which of the naked images they’d seen before, but also there was no difference in their brain activity whether they were looking at images of naked men or naked women, sexually explicit or otherwise. In this condition, the participants were also unable to identify afterwards, from a mix of new and previously seen images, which ones they’d seen before.

However, the researchers also tested a second type of mask – scrambled versions of the naked images – and these were more successful. The participants said they couldn’t see the naked images rendered subliminal in this way, yet their brains showed distinct patterns of activity in response to the images of naked women compared with naked men. In other words, the brain scans seemed to betray their sexual preference, even though they didn’t know what images they’d been looking at – the foundations of a fraud-proof test. In a memory quiz afterwards, the participants showed some very limited ability to distinguish previously seen naked images from new ones, but the researchers think this could have been based on memory traces stored earlier at a non-conscious level, though this is open to debate.

More work will be needed to find the precise sweet spot where images are not visible at a conscious level, but are still processed enough by the brain to betray neural signs of sexual preference. This study was a proof of principle with heterosexual men and further research would also need to examine the sensitive issue of what kinds of images could be used to investigate suspected sexual interest in children – for instance would benign images provoke the necessary neural effects to make this a feasible forensic test?

More broadly, while it’s easy to see how this approach could help in the detection of deviant sexual interest, it’s a strategy that raises serious ethical issues. It’s well known that the link between physical sexual arousal and subjective sexual interest is often far from straight-forward. It seems highly likely that the link between neural markers of sexual interest and subjective sexual preference will be even more complex. As a society, are we ready to accept the idea of psychologists measuring our sexual preferences by scanning our brain activity?

—Neural correlates of subliminally presented visual sexual stimuli

Christian Jarrett (@Psych_Writer) is Editor of BPS Research Digest