Novel Non-invasive Brain Stimulation Techniques in Neurological Rehabilitation

Novel Non-invasive Brain Stimulation Techniques in Neurological Rehabilitation: Cross-modal Paired Associative Stimulation (PAS) Protocols to Promote Plastic Reorganization in Stroke and Phantom Limb Pain Patients

Paired associative stimulation (PAS) is a non-invasive brain stimulation protocol, where two stimuli (a peripheral and a cortical one, the latter delivered with transcranial magnetic stimulation - TMS) are repeatedly associated to enhance plasticity in the brain. In the present study, a new cross-modal, visuo-motor PAS protocol - called "mirror-PAS"- will be tested as a possible non-invasive brain stimulation treatment in neurological rehabilitation to promote motor recovery and pain reduction.

Participants will perform the standard PAS targeting the motor system and the recently developed mirror-PAS in two separate sessions. The investigators will compare the possible effect of the protocols in terms of neurophysiological and behavioral outcomes to identify the optimal PAS method to enhance plasticity and promote sensory-motor function.

The motor recovery of the upper limb function, particularly of the hand, is challenging in neurological rehabilitation. Transcranial magnetic stimulation (TMS) is a form of non-invasive brain stimulation that can enhance motor recovery, though promoting brain plasticity of the brain.

In this study, researchers will use a TMS protocol called Paired Associative Stimulation (PAS), in which two stimuli (e.g., a cortical stimulus - delivered with TMS - and a peripheral stimulus) are repeatedly associated to promote associative plasticity in the primary motor cortex (M1).

The objective is to explore the clinical efficacy of a newly developed cross-modal PAS protocol -the so-called "mirror-PAS"- as a possible motor rehabilitation treatment in patients with upper limb motor disorders, investigating its neurophysiological and behavioural effects. The innovative aspect of mirror-PAS is to act on a more extended visuomotor network, featured by the functional properties of the mirror neuron system (MNS). Indeed, in the mirror-PAS, a visual stimulus depicting a hand movement is repeatedly paired with a TMS pulse over M1. In the healthy, this protocol was found effective in modulating neurophysiological responses of the motor cortex (i..e, MEPs), hence inducing these effects bypassing the afferent somatosensory pathway exploited by the standard PAS protocols. This may represent a clinical advantage allowing the promotion of motor recovery through the induction of a plastic reorganization in the damaged motor system by gaining access through a potentially spared MNS.

The project's first phase aims to assess the motor and neurophysiological effects of mirror-PAS in stroke patients with upper-limb hemiparesis, compared to the standard excitatory M1-PAS (whose effects are well known in the literature, even if still debated).

In the second phase, we will test the efficacy of the mirror-PAS in a population of amputated patients affected by phantom limb pain (PLP).

Inclusion criteria for stroke patients:

• age from 18 to 85 years
• single, unilateral, hemispheric stroke (cortical or sub-cortical areas)
• stroke confirmed by neuroimaging (CT or MRI)
• stroke at least 4 months before enrollment
• hemiparesis of the upper limb resulting from stroke or hemianesthesia clinically documented

Exclusion criteria for stroke patients:

• contraindications to TMS (Rossi et all, 2021)
• other neurological conditions (e.g., Parkinson's Disease, Multiple Sclerosis, ALS)
• other orthopedic condition that affected the hand (e.g., carpal tunnel syndrome)

Inclusion criteria for PLP patients:

• age from 18 to 90 years
• amputation of one part of the upper limb at least 2 months before enrollment
• presence of stable phantom limb pain at least 2 months prior to enrollment

Exclusion criteria for PLP patients:

• contraindications to TMS (Rossi et all, 2021)
• other neurological, orthopedical or psychiatric conditions