Canadian researchers have witnessed the emergence of a conscious thought. Using imaging scanners to measure brain activity, they recorded the moment when volunteers became conscious of images appearing in front of them-and found that it all depends on how many neurons are firing in the brain.
Ravi Menon and his colleagues at the University of Western Ontario showed people patterns of black and white stripes. While keeping the width of the stripes the same, they increased the contrast slightly with each image they showed. The volunteers were asked to press a button as soon as they could see a pattern appearing.
At first, the observers spotted no pattern at all. But when the contrast reached about 2 per cent they started pressing the button to show that they could see the stripes. The researchers imaged the brains of the volunteers continuously during the experiment, using a non-invasive method of brain imaging called fMRI. This visualises the amount of oxygen in the blood, a measure which correlates with neural activity. There was an increase in brain activity in the primary visual cortex of all the volunteers, corresponding to each button press, and the signal got stronger as the pattern became easier to see. "The neurons fire more as the contrast increases," says Menon.
But the researchers could spot the signals even before the volunteers started pressing the button. "They first pushed the button at 2 per cent. But we saw a detectable response at 1 per cent," says Menon. He argues that the difference between an unconscious response to the stripes and a conscious one is all down to the level of brain activity. "You need a certain number of neurons to fire to cross the threshold into consciousness," he says.
Cognitive neuroscientist David Perrett of the University of St Andrews agrees. "Considering the number of neurons firing is the only way to think about consciousness," he says. "Consciousness is brain activity." But psychologist Max Velmans of Goldsmiths College in London says that is just part of the story. He argues that other elements, such as which neurons are firing and at what frequency, influence consciousness. "If you go to the cinema," he says, "all of your visual neurons are firing. Then you feel something crawling up your leg. The signals from your leg in terms of energy are far less than the flooding of your visual system from the film. But it is more important to attend to your leg."
Neuroscientist Susan Greenfield from the University of Oxford says the work supports her idea that consciousness is like a "dimmer switch" that can be turned on gradually. "I am delighted that they are looking at something quantitative, so that we can look at degrees of consciousness," she says.
Author: Joanna Marchant
In New Scientist issue: 12th August 2000
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