Magnetic resonance imaging (MRI)
Magnetic resonance imaging is a tool that allows us to visualize different aspects of brain function and anatomy. Individuals lie on a scanner bed and high-resolution pictures of their brain are taken using a high magnetic field.
Functional MRI enables us to measure brain activity indirectly by detecting changes in the concentration of oxygen in the blood. When you are flexing your fingers, regions in your brain that control movement (e.g., the motor cortex) are being used; therefore, oxygen and blood flow increase to that region. The signal we measure is called BOLD, which stands for blood-oxygenation-level dependent.
Figure 1: Motor brain regions are active when we listen to musical rhythms.
Diffusion MRI allows us to examine white matter of the brain. Using tractography analyses, we can visualize white matter anatomical connections. For example, we can look at the corticospinal tract, which connects motor cortex with the spinal cord, and is often affected in individuals with stroke. In particular, we can assess the integrity of these white matter pathways, which can be used as a marker of disease.
Figure 2: Integrity of corticospinal tract is important for stroke recovery.
Resting state fMRI
Resting state fMRI is a technique that allows us to measure how well the BOLD response is correlated across time in different brain regions. We measure this when the brain is “resting,” that is, when the participant is not performing a task. Resting state fMRI enables us to gain an understanding of which brain regions are interacting together in a network. These resting state networks represent patterns of brain connections that can be changed by learning or disease, and can thus be a marker of brain health.
Figure 3: Neural activity between left and right motor cortex are temporally coupled.
Transcranial magnetic stimulation and transcranial direct current stimulation
Transcranial magnetic stimulation and transcranial direct current stimulation (TDCS) are noninvasive tools that allow us to stimulate the brain through the scalp when participants are awake. Transcranial magnetic stimulation uses a rapidly changing magnetic field to induce a weak electric current, whereas TDCS uses a small electric direct current. Both tools can be used to facilitate or inhibit neural activity. In individuals with stroke we can apply these tools to excite stroke-affected brain regions. In contrast, when a stroke causes overactivity in healthy brain regions, these tools can be used to dampen or inhibit activity levels. Transcranial magnetic stimulation can also be used to look at how different brain regions influence each other.
The Optotrak Certus is a motion-capture system that allows us to evaluate movements precisely. Markers or infrared emitting diodes are attached to a person and a sensor detects motion from these markers in real time. This enables us to determine the speed, accuracy and efficiency with which an individual with stroke moves his or her arm. We can also evaluate the degree to which movements are coordinated.
Electrogoniometers enable us to measure changes in joint range of motion during movement. An electrogoniometer consists of a spring cable joining two segments. Each segment is attached to a body part (e.g., arm and forearm), with the spring cable going across a joint (e.g., elbow). As the elbow joint moves, the spring bends or extends. The tension or strain placed on the spring results in a change in resistance, which is measured in volts and then converted to joint angles.
Thus, motion-capture systems and electrogoniometers allow us to quantify how movements have been affected by stroke and whether they improved after therapy.