Neurological disorders including addiction, depression, and obsessive-compulsive disorder (OCD) are often characterised by complex pathologies involving multiple brain regions and circuits.

These conditions are difficult to treat due to the intricate and poorly understood nature of brain functions and the challenge of delivering therapies to deep brain structures without invasive procedures.

Now, an interdisciplinary team of researchers led by Friedhelm Hummel and postdoc Pierre Vassiliadis are pioneering transcranial Temporal Interference Electric Stimulation (tTIS). The method targets deep brain regions that are the control centres of several important cognitive functions and involved in different neurological and psychiatric pathologies. The research is detailed in Nature Human Behaviour.

Invasive deep brain stimulation [DBS] has already successfully been applied to the deeply seated neural control centres in order to curb addiction and treat Parkinson, OCD or depression, Hummel said in a statement. The key difference with our approach is that it is non-invasive, meaning that we use low-level electrical stimulation on the scalp to target these regions.

Lead author Vassiliadis, a medical doctor with a joint PhD, describes tTIS as using two pairs of electrodes attached to the scalp to apply weak electrical fields inside the brain.

"Up until now, we couldnt specifically target these regions with non-invasive techniques, as the low-level electrical fields would stimulate all the regions between the skull and the deeper zones, rendering any treatments ineffective. This approach allows us to selectively stimulate deep brain regions that are important in neuropsychiatric disorders," he said.

The technique is based on the concept of temporal interference, initially explored in rodent models, and now translated to human applications by the EPFL team.

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In this experiment, one pair of electrodes is set to a frequency of 2,000Hz, while another is set to 2,080Hz. Thanks to detailed computational models of the brain structure, the electrodes are specifically positioned on the scalp to ensure that their signals intersect in the target region.

The frequency disparity of 80Hz between the two currents becomes the effective stimulation frequency within the target zone. The high base frequencies do not stimulate neural activity directly, leaving the intervening brain tissue unaffected and focusing the effect solely on the targeted region.

According to EPFL, the focus of this latest research is the human striatum, which is involved in decision making functions, such as reward.

"We're examining how reinforcement learning, essentially how we learn through rewards, can be influenced by targeting specific brain frequencies," said Vassiliadis. By applying stimulation of the striatum at 80Hz, the team found they could disrupt its normal functioning, directly affecting the learning process.

The therapeutic potential of their work could have a positive impact on conditions like addiction, apathy and depression, where reward mechanisms play a crucial role.

"In addiction, for example, people tend to over-approach rewards. Our method could help reduce this pathological overemphasis," said Vassiliadis.

Furthermore, the team is exploring how different stimulation patterns can not only disrupt but also potentially enhance brain functions. "This first step was to prove the hypothesis of 80Hz affecting the striatum, and we did it by disrupting its functioning. Our research also shows promise in improving motor behaviour and increasing striatum activity, particularly in older adults with reduced learning abilities," said Vassiliadis.

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Non-invasive brain stimulation shows promise for neurological therapy - The Engineer