Packed Lunch: Turning your brain off

14 January 2011

 Photograph of man undergoing Transcranial Magnetic Stimulation
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When Simon had his brain switched off by SBishop, on Flickr

Packed Lunch returns soon with more tales of research from local scientists. To get you in the mood, we’re catching up with some of last year’s, via the Packed Lunch podcast. In November, Benjamin Thompson went along to hear about a scientist whose work involves using a giant magnet to stop you thinking…

The theme of November’s first Packed Lunch at the Wellcome Collection was the neurological basis of language recognition. Dr Joe Devlin from the UCL Division of Psychology and Language Sciences came by to talk about a technique he uses to investigate the inner workings of the brain.

Joe’s research involves using strong magnetic fields to stimulate different areas of the brain known as Transcranial Magnetic Stimulation (TMS). The machine used to provide the magnetic field looked fairly innocuous, two doughnut shaped rings of wound copper covered with insulating tape. But as Joe described what it was capable of initially, it seemed rather terrifying…

In order to create the magnetic field a huge current needs to be passed through the coil. This current is lethal, so it is imperative that the insulating tape is checked frequently! The magnetic field that is created is as strong as those used in junkyards to pick up cars, but it only lasts a fraction of a second and extends for a few centimetres.

So what can this strong magnetic field achieve? By placing the TMS machine next to a volunteer’s head Joe is able to externally stimulate a very precise area of the brain. The magnetic pulse causes brain cells to fire, interrupting the task they would normally be doing and introducing ‘noise’ to that particular brain area. This noise only lasts for a fraction of a second before returning to normal, but this time period allows for some interesting experiments.

The experiment that Joe described showed how the understanding of spoken words and the ability to read are linked. The experiment involved playing a volunteer a list of words, some of which were real and some which were false, but sounded real. The volunteer had to press one button when they heard a real word and another when they heard a false word. This allowed Joe to measure how quickly and accurately the volunteer responded to the words.

Typically the volunteers were highly accurate and had a response time of around one second. This included half a second needed to hear the word and about half a second to respond to it. When subjected to TMS, the response time of the volunteer increased by about a third of a second, but only for certain words.

Amazingly, it was simple words with a regular spelling that the volunteers were slower to recognise during TMS. This seems totally counterintuitive so I asked Joe if he could explain it to me a bit more. He told me that adding brain noise using the TMS machine makes normally easy to understand words not so easy to understand.

He likened the recognition of a word to being on a hill. The word is found at the foot of the hill and you have to descend the slope in order to recognise it. With difficult words, that are spelt differently to how they sound, you’d begin at the top of the hill, with a long distance to travel, but with easy words you begin much lower down the hill so the time needed for recognition is much lower.

During TMS, you get pushed back up the hill when recognising simple words which results in the increase in time seen in the button pressing experiments.

It turns out that learning to read changes the way we recognise speech. We get better at recognising speech when the word and its spelling correspond closely. This increase in speed is not down to visualising the words, as using TMS to disrupt the visual recognition centres has no effect on the speed of word recognition, it’s all down to the way in which you hear the words themselves.

Joe’s talk really clicked with the audience, who raised many questions about a variety of topics, such as adult illiteracy, Alzheimer’s disease and synaesthesia – associating colours with sounds. People were also intrigued by the potential of using TMS for therapeutic purposes. The one question that everyone was thinking, but only one asked was  “Are you looking for volunteers?”

Benjamin Thompson is a writer at the Wellcome Trust.