It is well known that the motor area of one hemisphere of the brain (motor cortex) controls the movement of the opposite side of the body but the role of the motor cortex in the hemisphere spared by stroke (contralesional motor cortex) in motor performance during post stroke recovery is still unclear. It is also not clear whether the motor cortex of both hemispheres of the brain are involved, as movement becomes more complicated. Currently the role of the motor cortex on the same side of the body (referred to as ipsilateral motor cortex) in hand performance remains controversial. In most patients with stroke, only one side of the brain is affected by the stroke (affected hemisphere) resulting in weakness of half of the body opposite to the side of the stroke. Over the recent years, research has discovered that the side of the brain, that is spared by the stroke (non-affected hemisphere) may support recovery after stroke. However, there is also a question whether the non-affected hemisphere may interfere with the process of recovery. It is currently not known what factors influence the activity of non-affected hemisphere to either support or interfere with the recovery of stroke. A better understanding of those events is critical to development of optimal therapeutic strategies. For example, non-invasive stimulation of specific areas of the non-affected hemisphere may help to improve functional recovery following stroke. The objectives of this study are to define the factors that influence the activity in the non-affected hemisphere to either support or interfere with the recovery after stroke. The researchers will study the area of the brain that controls movements of the non-affected hemisphere as it relates to motor function post-stroke.

Transcranial magnetic stimulation (TMS) is a device that allows non-invasive stimulation of the brain. When the brain is stimulated repetitively at a very low rate and low intensity for about 15 minutes, the stimulated brain area becomes less active. This effect lasts 10 minutes and is called a "transient artificial lesion" as it mimics the effects of transiently interfering with the function of the stimulated brain area. The study includes experiments that first identify the extent of stroke and brain areas involved in a motor task using functional MRI of the brain and TMS. The researchers will then determine the functional role of the contralesional motor cortex by studying the effect of low frequency and high frequency repetitive transcranial magnetic stimulation (rTMS) of primary motor cortex (M1) on interhemispheric inhibition (IHI) (resting and active) and motor cortex excitability (short interval intracortical excitability and corticospinal excitability) and behavior. The researchers will thereby also identify rTMS protocols that enhance motor performance in stroke patients.

In this study the researchers will conduct experiments using repetitive TMS to downregulate the activity of the motor area and measures its effect on activity of motor cortex of both hemispheres. In addition to enrolling participants who have had a stroke, the researchers will enroll healthy participants as this collected data will provide normative values for task related changes in M1s and their interactions - a prerequisite to studying abnormalities in stroke patients during motor recovery. The measurements and interventions will occur at two time points in all participants with stroke (1 and 6 months post-stroke). The data will be compared to the results of healthy age matched controls, assessed at a single time point.