What’s the science?

Stroke that affects the brain’s motor cortex can cause motor impairment and disability. Low frequency oscillatory activity (LFO; rhythmic electrical activity in the brain) in the motor cortex is known to be involved in motor movements such as reaching. In particular, LFO is related to movement timing, and may be responsible for fast, accurate movements. The role of LFO in recovery from strokes that affect motor function is not known. This week in Nature Medicine, Ramanathan and colleagues studied LFO in rats and humans to understand their potential role in stroke.

How did they do it?

Four rats were trained to perform a skilled reaching task and then had microwire arrays (arrays of electrodes measuring electrical signal) implanted in their primary motor cortices. Signals were recorded from the electrodes while they performed the reaching task. The authors then performed a distal middle cerebral artery occlusion as a model of stroke (this model results in damage to sensorimotor cortex) and recorded brain activity and reaching behaviour 5 days post-stroke. However, the middle cerebral artery occlusion model of stroke results in widely variable damage to the motor cortex. Therefore, the authors next used a different stroke model (focal photothrombotic stroke) to study damage in a specific area of the motor cortex and how recovery of brain tissue at the edge of a stroke-related lesion (perilesional cortex) might be related to LFO. They did this by using a microwire array placed just anterior to the site of injury. The authors also assessed electrocorticography data in humans (when electrodes are placed on the brain’s surface to record activity – in this case in patients with epilepsy undergoing monitoring) and had them perform a reaching task. Two of the patients were otherwise healthy (‘non-stroke subjects’), while one patient had had a stroke in sensorimotor cortex four years prior (‘stroke subject’). Finally, the authors applied direct current stimulation to the sensorimotor cortex in rats to assess whether this stimulation would change LFO or could improve reaching behaviour.

What did they find?

As expected, LFO was found in rats during the reaching task, both in terms of spiking (action potentials from neurons in the brain), and local field potentials (the summation of local electrical currents around neurons), especially at lower frequencies (~<4Hz). For example, neurons showed coherent spiking at low frequencies prior to the onset of reaching during the task. Five days following the middle cerebral artery occlusion stroke, the animals had impaired motor skills. However, at least some electrodes in the microwire array were in undamaged or viable tissue that was still able to demonstrate reach-related increases in activity similar to pre-stroke activity. Prior to stroke, the strength of local field potentials tracked the phase (phase locking, ie. synchrony of firing) of neuron spiking. However, after stroke, local field potential modulation was reduced and was no longer related to neuron spiking.