This investigation has revealed that certain kinds of rhythmic magnetic stimulation to localized areas of the human brain modulate our sensory capacity. Despite this particular study has focused on the sensation of touch, the results open new avenues to use rhythmic magnetic stimulation in other sensory domains such as vision, audition, and even to modulate other cognitive abilities. The results have been published in the journal Current Biology, on January 16th.

Salvador Soto-Faraco, ICREA researcher at DTIC and head of the Multisensory Research Group (MRG) one of the authors, stated that "so far, Transcranial Magnetic Stimulation to the brain has been mostly used to produce interference in mental function. We have shown that magnetic pulses can also produce specific improvements in (touch) perception."

The researchers focused the Transcranial Magnetic Stimulation (TMS) pulses to the Posterior Parietal area of the cortex in healthy human volunteers. These brief pulses produce temporary and reversible alterations in the normal activity of neurons in the target brain area, which was previously localized anatomically in each participant using Magnetic Resonance Imaging. The paramount advantage of TMS, compared to other popular neuroimaging techniques, is the possibility to infer causality between a particular brain area and concrete mental function, above and beyond simple correlation.

Stimulation trains must be delivered at 10Hz to be effective

One of the significant findings of this study is that in the Posterior Parietal cortex of humans, only a very specific pattern of stimulation, at 10Hz (that is, 1 pulse every 10th of a second) seems to produce effective modulation.

In particular, this stimulation pattern improved detection of tactile events at one hand and worsened perception at the contrary hand, mimicking the effects of focusing attention to one side of the body.

When the pattern of stimulation was not rhythmic, or delivered at a different frequency (20Hz), or in a different brain area, it failed to produce these effects. This implies that t he brain uses precisely tuned rhythmic neural activity to encode specialized functions.

What is more, the study reports yet another curious result. This sensory modulation produced after magnetic stimulation expressed in a spatial domain that compensated for the different body postures that participants adopted. That is, stimulation of the right parietal cortex led to sensory improvement at the hand that was placed to the right side of the body midline, whichever hand it happened to be; Either the right hand extended forward, or the left hand crossed over to the right.  This reveals that the human Posterior Parietal cortex computes rather elaborate representations of space which could be useful to link touch with other modalities, or to planning actions.

"Many previous studies using magnetic stimulation of the brain have used stimulation patterns that did not take into account the natural neural firing patterns in the brain", added Salvador Soto-Faraco. Hence, one of the main contributions of this study has been to establish a causality link between a particular kind of neural activity and a concrete mental function.

This research has been funded by a Starting Grant from the European Research Council (ERC) to Salvador Soto-Faraco. The study counted with the collaboration of forty-two volunteer observers who agreed to be included in the study after reading and signing the informed consent form. The protocols were approved by the Ethical Committee at Parc de Salut Mar.

Reference:

Manuela Ruzzoli i Salvador Soto-Faraco (2014), " Alpha Stimulation of the Human Parietal Cortex Attunes Tactile Perception to External Space", Current Biology, 16 de gener , http://dx.doi.org/10.1016/j.cub.2013.12.029 .