Background. Stroke is a major cause of disability worldwide, and effective rehabilitation is crucial to regain skills for independent living. Recently, novel therapeutic approaches manipulating the excitatory-inhibitory balance of the motor cortex have been introduced to boost recovery after stroke. However, stroke-induced neurophysiological changes of the motor cortex may vary despite of similar clinical symptoms. Therefore, better understanding of excitability changes after stroke is essential when developing and targeting novel therapeutic approaches.
Objective and Methods. We identified recovery-related alterations in motor cortex excitability after stroke using magnetoencephalography. Dynamics (suppression and rebound) of the ~20-Hz motor cortex rhythm were monitored during passive movement of the index finger in 23 stroke patients with upper limb paresis at acute phase, 1 month, and 1 year after stroke.
Results. After stroke, the strength of the ~20-Hz rebound to stimulation of both impaired and healthy hand was decreased with respect to the controls in the affected (AH) and unaffected (UH) hemispheres, and increased during recovery. Importantly, the rebound strength was lower than that of the controls in the AH and UH also to healthy-hand stimulation despite of intact afferent input. In the AH, the rebound strength to impaired-hand stimulation correlated with hand motor recovery.
Conclusions. Motor cortex excitability is increased bilaterally after stroke and decreases concomitantly with recovery. Motor cortex excitability changes are related to both alterations in local excitatory-inhibitory circuits and changes in afferent input. Fluent sensorimotor integration, which is closely coupled with excitability changes, seems to be a key factor for motor recovery.
Approximately 75% of stroke survivors suffer from permanent disability; thus, stroke causes significant human suffering and poses a major economic burden on the society.1 Recovery from stroke is based on brain’s plasticity. Studies in both animals and humans have shown that a period of enhanced plasticity occurs 1-4 weeks after stroke.2–5 After this sensitive period, the effectiveness of poststroke rehabilitation diminishes dramatically. Recently, there have been promising attempts to prolong or enhance the sensitive period with pharmacological manipulations6–8 or with noninvasive brain stimulation,9,10 both aiming at changing the cortical excitation-inhibition balance. However, patients with initially similar clinical symptoms may recover differently, possibly because the underlying neurophysiological changes vary between these patients. Thus, understanding and monitoring recovery-related neurophysiological mechanisms and their temporal evolution is crucial for developing efficient, personalized rehabilitation.
Fluent upper limb motor function is important for independency in daily life. Integration of proprioceptive and tactile input with motor plans forms the basis of smooth and precise movements.11 Afferent input mediates its effect on motor functions by modulating the motor cortex excitability.12 Accordingly, our previous study in healthy subjects indicated that proprioceptive input strongly modulates the ~20-Hz motor cortex rhythm, causing an initial suppression followed by a strong and robust rebound.13 Prior studies have suggested that the ~20-Hz rebound reflects deactivation or inhibition of the motor cortex.14–17 Moreover, a combined magnetiencephalography (MEG) and magnetic resonance spectroscopy study showed that the ~20-Hz rebound strength is associated with the concentration of the inhibitory neurotransmitter GABA (γ-aminobutyric acid).18
To study alterations in motor cortex excitability after stroke and its association with motor recovery, we measured the dynamics of ~20-Hz motor cortex oscillations during passive movement of the index fingers in 23 stroke patients at the acute phase and during 1-year recovery. The motivation of this study was to understand the neurophysiological mechanisms underlying stroke recovery, which is instrumental for developing novel therapeutic interventions.