For the first time, one of the tics that bedevil people with Tourette's has been induced in volunteers who don't themselves have the disorder, an experiment that might help us to understand and even treat the condition.
Jennifer Finis of Heinrich Heine University in Düsseldorf, Germany, and her colleagues suspected that a type of Tourette's tic called echophenomena, which involves mimicking other's movements, may be caused by over-excitation of the supplementary motor area (SMA) – a brain region involved in the initiation of movement.
To investigate further, her team used a non-invasive technique called repetitive transcranial magnetic stimulation (rTMS), which involves delivering brief but strong magnetic pulses to the scalp. By changing the frequency of rTMS, the stimulation could either inhibit or excite the SMA.
Thirty seconds after rTMS, 30 volunteers were shown video clips of someone making a spontaneous movement. Those who'd had their SMA excited were three times as likely to imitate the kind of behaviour they saw in the clips than those who'd had it suppressed.
"We suspect that this is a mechanism that might underlie tics more generally than just echophenomena in people with Tourette's syndrome," says Peter Enticott of Monash University in Melbourne, Australia, who worked on the study.
Recent studies have found mirror neurons in the human SMA (Current Biology, doi.org/dfp4j6). Mirror neurons are known to fire when we watch others perform actions. Could these SMA mirror neurons be involved in inducing echophenomena when the SMA is stimulated in volunteers without Tourette's?
"Yes, this could indeed provide an explanation," says Marcel Brass of Ghent University in Belgium, who studies echophenomena. "Alternatively, the SMA has also been related to internal control of action. So interfering with this region might make participants more susceptible to [echophenomena]."
The team is now planning to study whether inhibiting the SMA using rTMS may help reduce symptoms in people with Tourette's.
Matthias Wittfoth of the Hannover Medical School in Germany, says that he sees the findings as "a valuable first step towards the understanding of specific symptoms of people suffering from Tourette's syndrome".
But he advises caution while interpreting the results. He points out that the researchers did not use computer-assisted technology to deliver the rTMS signals precisely to the SMA, something that the researchers themselves acknowledge. "We cannot be sure that the same brain areas were stimulated in each participant," says Wittfoth. Moreover, of the 30 individuals who participated in the experiment, only seven were males. "Tourette's syndrome affects proportionally more men, thus their experimental group including mostly women does not reflect this," says Wittfoth.
Journal reference: Cortex, doi.org/jmj
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