Probably the most important advance in neuroplasticity in the last two decades has been the development of sophisticated methods to measure brain activity. Traditional CT scans and MRI tell us about the structure of the brain while newer methods tell us how areas of the brain are functioning. These are very different capabilities. Techniques that show brain structure (CT scan, MRI) operate like visualizing the wiring in your house; the inspector can examine all the wires, plugs and connections and approve they are where they should be. In fact the physical arrangement of the wiring can be quite ‘normal’ and pass inspection but won’t operate properly until the power is supplied to the system. In this case there is adequate structure (wiring) but no function (power).
Tools that measure brain activity (PET, fMRI, TMS) actually detect the location and intensity of brain activation (function). In the case of our house wiring analogy, these methods can see how power is moving around the house-which dimmers are on, how intense they are, the degree to which the appliances are pulling power. In the late 1990’s these brain imaging techniques could detect change in brain regions but now the technology is advancing such that we can detect changes in very small groups of neurons. I think we are only beginning to understand imaging capabilities and how we can use them to improve treatments to optimize recovery after brain injury.
Positron Emission Tomography (PET) measures the flow of blood in brain regions. The person being scanned inhales, injects or ingests a special tracer. This tracer will attach itself to the molecule or cell of interest. Timing is important because the image has to be taken when the tracer reaches the brain and before the body rids itself of it. Increased or decreased tracer presence in the brain reflects increased or decreased metabolic activity. For example when you speak, it would make sense that there would be increased tracer in areas of your brain responsible for speech production. If this does not happen in areas that you would expect then there may be something going on with how your wiring is working.
Transcranial Magnetic Stimulation (TMS) is a technique in which a magnetic pulse is applied to the outside of the skull using a special coil. This stimulus activates neurons near where the coil is positioned resulting transmission of a nerve impulse to its final destination-usually a muscle or group of muscles. This technique has been around for awhile so researchers have mapped out typical activation patterns. Using TMS, we can see if brain regions are active or not, how responsive they are (how much current needs to be applied to get a response) and also how efficient they are (how fast the current travels down nerves to muscles). The responses are usually picked up by electrodes placed on the target muscle, called motor evoked potentials or MEPs. Higher and faster MEP responses to TMS would be indicative of good transmission.
Functional Magnetic Resonance Imaging (fMRI) measures subtle changes in blood flow within brain tissue that correspond to areas that are active. When groups of neurons are working, they require oxygen and therefore arterial blood so fMRI capitalizes on this phenomenon. This is similar to PET however PET measures the brain’s use of a molecule or neurotransmitter of interest while fMRI really measures oxygen requirement. Recently, MRI and fMRI use stronger magnetic waves to give better and better image resolution.