In the chronic phase after stroke, cortical excitability differs between the cerebral hemispheres; the magnitude of this asymmetry depends on degree of motor impairment. It is unclear whether these asymmetries also affect capacity for plasticity in corticospinal tract excitability or whether hemispheric differences in plasticity are related to chronic sensorimotor impairment.

Response to paired associative stimulation (PAS) was assessed bilaterally in 22 individuals with chronic hemiparesis. Corticospinal excitability was measured as the area under the motor-evoked potential (MEP) recruitment curve (AUC) at baseline, 5 minutes, and 30 minutes post-PAS. Percentage change in contralesional AUC was calculated and correlated with paretic motor and somatosensory impairment scores.

PAS induced a significant increase in AUC in the contralesional hemisphere (P＝ .041); in the ipsilesional hemisphere, there was no significant effect of PAS (P＝ .073). Contralesional AUC showed significantly greater change in individuals without an ipsilesional MEP (P＝ .029). Percentage change in contralesional AUC between baseline and 5 m post-PAS correlated significantly with FM score (r＝－0.443; P＝ .039) and monofilament thresholds (r＝0.444, P＝ .044)．

There are differential responses to PAS within each cerebral hemisphere. Contralesional plasticity was increased in individuals with more severe hemiparesis, indicated by both the absence of an ipsilesional MEP and a greater degree of motor and somatosensory impairment. These data support a body of research showing compensatory changes in the contralesional hemisphere after stroke; new therapies for individuals with chronic stroke could exploit contralesional plasticity to help restore function.