UCL (University College London) researchers have found the first physiological evidence that invisible subliminal images do attract the brain's attention on a subconscious level. The wider implication for the study, published in Current Biology, is that techniques such as subliminal advertising, now banned in the UK but still legal in the USA, certainly do leave their mark on the brain.

Using fMRI, the study looked at whether an image you aren't aware of ¬- but one that reaches the retina - has an impact on brain activity in the primary visual cortex, part of the occipital lobe. Subjects' brains did respond to the object even when they were not conscious of having seen it.

Dr Bahador Bahrami, of the UCL Institute of Cognitive Neuroscience and the UCL Department of Psychology, said: "What's interesting here is that your brain does log things that you aren't even aware of and can't ever become aware of. We show that there is a brain response in the primary visual cortex to subliminal images that attract our attention - without us having the impression of having seen anything. These findings point to the sort of impact that subliminal advertising may have on the brain. What our study doesn't address is whether this would then influence you to go out and buy a product. I believe that it's likely that subliminal advertising may affect our decisions - but that is just speculation at this point."

Subjects wore red-blue filter glasses that projected faint pictures of everyday objects (such as pliers and an iron) to one eye and a strong flashing image known as 'continuous flash suppression' to the other. This recently developed technique effectively erases subjects' awareness of the faint images so that they were unable to localise the faint images on screen. At the same time, subjects performed either an easy task - picking out the letter T from a stream of letters, or a task that required more concentration in which subjects had to pick out the white N or blue Z from the same stream.

During the harder task, the subjects' brains blocked out the subliminal image and the fMRI scan did not detect any associated neural activity. This finding - that the brain does not pick up on subliminal stimuli if it is too busily occupied with other things - shows that some degree of attention is needed for even the subconscious to pick up on subliminal images.

Dr Bahrami said: "This is exciting research for the scientific community because it challenges previous thinking - that what is subconscious is also automatic, effortless and unaffected by attention. This research shows that when your brain doesn't have the capacity to pay attention to an image, even images that act on our subconscious simply do not get registered."

The research challenges the theory of the pioneering American psychologist and philosopher, William James, (1842-1910), who said: "We are conscious of what we attend to - and not conscious of what we do not attend to".

The team's findings show that there are situations where consciousness and attention don't go hand in hand.

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