Dougal Waters/ DigitalVision / Getty Images.
The sense of touch holds many secrets for neuroscientists. Only a few weeks ago, the Nobel prize in medicine went to scientists who, in the last two decades, discovered how the brain processes the perception of pressure, temperature and of the body’s position in space, the ‘somatosensory domain’.
Now, a study led by Pietro Avanzini, from the CNR Institute of Neuroscience in Parma, adds a new piece to the puzzle. His team had previously identified a group of neurons that respond to tactile stimuli with a characteristic ‘tonic’ response, a relatively slow, unprecise and long-lasting neuronal activity. They did this by analysing stereo electro-encephalography (stereo-EEG) data from 60 patients suffering from drug-resistant epilepsy, whose brain activity was mapped in preparation for surgery.
The researchers theorized that tonic response may also be associated with the awareness of touch. To prove this idea, they teamed with Francesca Garbarini’s group at the University of Turin, and studied 153 stroke patients, whose brain damage was causing a phenomenon called tactile extinction. When both sides of the body are stimulated at the same time, these patients cannot feel any contact on one of the two sides. “We realized that in these patients the deficit was not primarily somatosensory,” explains Avanzini. It was not the actual perception of tactile stimuli that was damaged, but rather the awareness of it.
To find what was causing the disruption, the scientists compared the stereo-EEG data from the epileptic patients with magnetic resonance imaging (MRI) from the post-stroke ones. The results, published in Brain1, show that damage in patients with tactile extinction is concentrated in the same area that is responsible for tonic responses, an area called the parietal operculum. They also found that both the tonic response and the tactile perception are enhanced in healthy subjects, as well as post-stroke subjects, when a visual stimulus is associated with the tactile one. If these findings are confirmed, behavioural therapies that match visual and tactile stimulation could be used to improve the conscious experience of touch or mitigate the chronic picture in post-stroke patients.
“The study is well designed and is an additional piece of evidence in a complex field,” says Edward de Haan, Professor of Neuropsychology at the University of Amsterdam. “But it shows a correlation, and it may be that tactile awareness is not supported by a single cortical system.”
Avanzini says that “within the area responding to somatosensory tactile stimuli, we looked for similar correlations with other response patterns and found that the tonic response is indeed the mechanism compromised in patients with tactile extinction.” More studies will be needed to test whether a similar mechanism also regulates other systems involved in consciousness.
