Relationships between functional and structural corticospinal tract integrity and walking post stroke.

Objective: Studies on upper limb recovery following stroke have highlighted the importance of the structural and functional integrity of the corticospinal tract (CST) in determining clinical outcomes. However, such relationships have not been fully explored for the lower limb. We aimed to test whether variation in walking impairment was associated with variation in the structural or functional integrity of the CST.

Modulating locomotor adaptation with cerebellar stimulation.

Human locomotor adaptation is necessary to maintain flexibility of walking. Several lines of research suggest that the cerebellum plays a critical role in motor adaptation. In this study we investigated the effects of noninvasive stimulation of the cerebellum to enhance locomotor adaptation.

Human locomotor adaptive learning is proportional to depression of cerebellar excitability.

Human locomotor adaptive learning is thought to involve the cerebellum, but the neurophysiological mechanisms underlying this process are not known. While animal research has pointed to depressive modulation of cerebellar outputs, a direct correlation between adaptive learning and cerebellar depression has never been demonstrated. Here, we used transcranial magnetic stimulation to assess excitability changes occurring in the cerebellum and primary motor cortex (M1) after individuals learned a new locomotor pattern on a split-belt treadmill.

Modulation of cerebellar excitability by polarity-specific noninvasive direct current stimulation.

The cerebellum is a crucial structure involved in movement control and cognitive processing. Noninvasive stimulation of the cerebellum results in neurophysiological and behavioral changes, an effect that has been attributed to modulation of cerebello-brain connectivity. At rest, the cerebellum exerts an overall inhibitory tone over the primary motor cortex (M1), cerebello-brain inhibition (CBI), likely through dentate-thalamo-cortical connections.

The effects of transcranial stimulation on paretic lower limb motor excitability during walking.

Balanced transcallosal inhibition sustains symmetrical corticomotor excitability and assists the performance of bimanual voluntary movements. After stroke, transcallosal inhibition becomes asymmetric. This finding raised the notion that reducing poststroke asymmetry in transcallosal inhibition might prime the motor system before training and lead to improvements in walking recovery.

Contralesional paired associative stimulation increases paretic lower limb motor excitability post-stroke.

Following stroke, an abnormally high interhemispheric inhibitory drive from the contralesional to the ipsilesional primary motor cortex (M1) is evident during voluntary movement. Down-regulating motor excitability of the contralesional M1 using inhibitory neuromodulatory protocols has demonstrated a correlation between balanced interhemispheric interactions and increased motor recovery. In 2005, our laboratory first reported bidirectional modulation of healthy subjects' tibialis anterior (TA) motor excitability during walking, using a stimulation paradigm known as paired associative stimulation (PAS).

Spike-timing-dependent plasticity induced in resting lower limb cortex persists during subsequent walking.

Transcranial magnetic stimulation (TMS) of human lower limb motor cortex paired with common peroneal nerve electrical stimulation produces a lasting modulation of motor cortex excitability following the principles of spike-timing-dependent plasticity. We previously demonstrated that this "paired associative stimulation" (PAS) protocol applied during walking induced a bidirectional modulation of cortical excitability. The present study tested the hypothesis that the excitability of lower limb motor cortex assessed during walking is increased when PAS is applied to the resting cortex.