Brain Primer

Viewing the Human Brain Through Noninvasive Tools

  • Reviewed9 Mar 2023
  • Author Susan Rojahn
  • Source BrainFacts/SfN
A patient in a MEG scanner
A patient sits in a magnetoencephalography (MEG) scanner. When groups of neurons fire in synchrony, MEG scanners can rapidly detect weak magnetic fields from the neurons’ electrical currents.
NIMH via Wikimedia Commons

Scientists add new techniques and technologies to their repertoire as they emerge, helping to deepen our understanding of the brain and suggest new ways to assist people whose lives are affected by brain disorders and diseases. And while many of the methods and tools used in neuroscience research are too invasive to use in humans, several methods for imaging human brain function do not require holes in the skull or other lasting physical changes.

Functional MRI (fMRI) can be used to follow changes in the brain activity of a person lying inside an MRI scanner. The machine is tuned so that it detects blood flow as well as differences in oxygen-rich and oxygen-poor blood, based on the idea that more active regions of the brain need more oxygen and nutrients, which are supplied by fresh oxygenated blood. While this is an indirect indication of neuron activity, it can pinpoint brain activity to fairly small regions.

fMRI provides an indirect view of neuron activity, but magnetoencephalography (MEG) detects actual electrical currents coursing through groups of neurons. When neuron activities are synchronized, their combined electrical currents produce weak magnetic fields that MEG equipment can detect. A person undergoing the procedure sits or lies down, with his or her head surrounded by a helmet-shaped device that can sense magnetic fields. MEG has been useful in a variety of studies: from how the auditory cortex responds to sounds to identifying where epileptic seizures start in a patient’s brain. MEG is useful for detecting rapid changes in brain activity (temporal resolution), but it does not provide the precise location of that activity (spatial resolution). For this reason, researchers can combine MEG data with fMRI data to obtain good anatomical detail from fMRI and high-speed readings of brain activity from MEG.

Near-infrared spectroscopy (NIRS) is similar to fMRI in that it monitors the flow of oxygenated blood as a way to estimate neuron activity. A major difference is that NIRS is only useful for measuring activity near the surface of the brain and does not provide as much detail; however, it is far less expensive and cumbersome than fMRI. NIRS is also more comfortable for the person undergoing the procedure, as the setup essentially involves wearing a cap with wiring hooked to it. Some of the wires transmit harmless laser beams (~1 megawatt of power or less) into the brain while others detect the light after it travels through the brain. NIRS can be used to determine the extent of brain injuries and to monitor oxygen levels in the brains of patients under anesthesia. Because of its portability, NIRS is very useful for studying brain activity during tasks — such as driving down the highway — that can’t take place inside an fMRI scanner.

Positron emission tomography (PET) detects short-lived radioactive compounds that have been injected into the bloodstream. The radioactive compounds could be oxygen or glucose, or they might be a neurotransmitter. PET traces where these compounds go in the body. The location of labeled oxygen can indicate blood flow, while a labeled neurotransmitter can show which brain regions are using that signaling molecule. PET can also detect the amyloid plaques that are associated with Alzheimer’s disease; this technique could one day enable us to identify the disease in its early stages. Although PET has good temporal resolution like MEG, it lacks the detailed spatial resolution of MRI.

Some methods used in human research can change brain activity. In transcranial magnetic stimulation (TMS), a coil that generates a magnetic field is placed near a person’s head. The magnetic field can penetrate the skull, temporarily activating or silencing a region of the cortex. TMS is used to treat psychiatric disorders such as anxiety, depression, and post-traumatic stress disorder and could be an effective option for patients with conditions that do not respond to medications.

Although neuroscience has progressed by leaps and bounds since neurons were first viewed under a microscope, many phenomena observed with these techniques are not fully understood. For example, mysteries still surround data obtained with electroencephalography (EEG). EEG shows that several different brain regions have characteristic rhythms or oscillations — one pattern in the visual cortex, another in the sensory motor cortex, and so on. Even though this method of examining brain activity has been used since 1929, the generation of these patterns (sometimes called brain waves) at the level of neural circuits is not well understood.

One branch of neuroscience that can help bridge findings from the microscopic to the whole-brain level is computational neuroscience. Researchers in this field develop theories or models about how the brain processes information, then test these models against real-world data. For example, they can examine the data and images from EEG or fMRI, then develop mathematical models to explain the underlying neuron and circuit activity. Data from many methods — electrophysiology, molecular studies, anatomy, and functional brain scans — can all contribute to these computational models.

Adapted from the 8th edition of Brain Facts by Susan Rojahn.




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