Appendix
Introduction to Neuroscience
Dr. Gard
How We Study the Brain
The brain can be studied in several ways. The most widely used methods to study the brain, and certainly the mindful brain, are electroencephalography (EEG) and magnetic resonance imaging (MRI). EEG is used to measure electrical activity of the brain by sticking up to 256 electrodes on the scalp. Although the timing of EEG is very precise, it is not very helpful for determining where exactly in the brain activity occurs.
An MRI machine, on the other hand, can quite precisely locate where in the brain activity takes place. Besides measuring brain function, it can also be used to measure the structure of the brain. You may have already been in an MRI scanner in the hospital. It is a very large machine that looks like a giant donut on its side, with a table that slides into its hole. The massive outer part contains a very strong magnet (often 3 tesla), which is about 60,000 times stronger than the magnetic field of the earth. Once put inside the opening of an operating MRI machine, you'll hear beeping and hammering noises, a little bit like minimalist techno music.
The MRI scanner works as follows. The human body, including the brain, consists of a lot of water (H2O), which is made up of two hydrogen atoms (H). The protons of the hydrogen atoms spin around their axes like spin tops, but the spin axes of all the protons randomly point in different directions. When put into the strong magnetic field of the MRI scanner, the spin axes align with the magnetic field. Then a radio wave at the resonance frequency of the protons is sent into the body, which causes the spin axes to flip. When the radio wave is switched off, the spin axes move back to their aligned state with the magnetic field. While moving back, they emit a radio wave that is recorded and used to create the MRI image.
To know exactly where in the brain the radio signal is coming from, so-called gradient coils are used to slightly alter the magnetic field and hence the resonance frequency of the hydrogen protons in different slices of the brain. The loud noises that you hear inside the scanner are caused by the vibration of these gradient coils when rapid pulses of electricity are passed through them to change the magnetic field.1
What Can We See in the Brain?
Neurons
Using different sequences of radio frequency pulses in the MRI scanner, we can create images that display different properties of the brain. The core components of the brain are neurons (nerve cells). The brain consists of on average 86 billion neurons that connect to each other and form networks that enable us to perceive, respond, think, experience emotions, and be otherwise human. Neurons generally consist of the cell body, dendrites, axon, and axon terminals. The dendrites are branches of the cell body that connect to other neurons and receive signals from them. The axon, also called the nerve fiber, is the long and thin tail of the neuron and transmits electrical impulses to other neurons. At the end of the axon branches, there are axon terminals that connect to dendrites of other neurons. The connection between axon and dendrite, where the electrical impulse is transmitted through chemicals called neurotransmitters, is called the synapse.2
Structural MRI and Gray Matter
On structural MRI images of the brain, we can distinguish between gray matter and white matter. The gray matter is mostly made up of the cell bodies, dendrites, axon terminals, synapses, and tiny blood vessels called capillaries. The gray matter is located in the outer layer of the brain (cerebral cortex) and in clusters of nerve cell bodies, deeper inside the brain (nuclei). Using the computer metaphor, the gray matter is the place where the computations take place; it's the switchboard of the brain. When gray matter is measured with MRI, it is expressed in terms of gray matter density, volume, or cortical thickness.
Structural MRI and White Matter
White matter consists mostly of myelinated axons and glia cells. Myelin is a fatty substance that is part of the glia cells and is wrapped around the axons forming a myelin sheath. Myelin is white, hence “white matter,” and its function is to isolate the axons and to speed up electric signal transmission through them. Speaking in terms of the computer metaphor, the axons, or nerve fibers, in the white matter can be viewed as the wires of the brain. Bundles of these myelinated nerve fibers are called neural tracts or pathways and connect distant areas of the brain. These neural tracts can be imaged with an MRI technique called diffusion tensor imaging (DTI) that is based on the principle that water diffuses or moves more rapidly along these bundles.
Functional MRI and Brain Activation
Functional MRI (fMRI) is used to measure brain activity indirectly by detecting activity related changes in blood flow. When neurons fire and transmit signals, oxygen and glucose are required, which are supplied through increased blood flow to active areas. The so-called blood-oxygen-level-dependent (BOLD) contrast that is most often used in fMRI measures the changes in brain activation–related blood flow based on the different magnetic properties of oxygen-rich and oxygen-poor blood. With fMRI, several images of brain activation are made in intervals of typically 1 to 3 seconds so that a time series or movie of brain activation is created. Average brain activations between tasks, events, and groups can then be compared.
Functional Connectivity
Based on the information about how brain activation fluctuates over time in several brain regions, we can calculate how strongly correlated the activation is in these regions or how strongly the brain regions are functionally connected. In other words, functional connectivity refers to how much brain regions talk to each other.
Network Analysis
Based on this functional connectivity data, we then can study complex communication patterns between several or all brain regions by using network or graph analytical methods. Networks do not only exist inside the brain, but they also exist outside of the brain, for example, a social network. With graph theoretical methods, we can calculate all kinds of properties of a network, for example, how many friends you have and how quickly news travels from one person in your network to another person. Similar methods are used to understand brain networks.3
Areas of the Brain Involved in Mindfulness
Using the methods described above, the regions in Figure A.1 have been identified as being involved in mindfulness.
Figure A.1 Brain regions involved in mindfulness
Copyright © David Vago and Fritz Gard. Used with permission.
Side (lateral) and middle (medial) view of the brain with schematically indicated brain regions that are involved in mindfulness and mentioned throughout the book: The anterior cingulate cortex is part of the salience network and plays a role in attention. The lateral prefrontal cortex is part of the central executive network and involved in regulating emotions and cognitions. The medial prefrontal cortex and the posterior cingulate cortex and the adjacent precuneus are part of the default mode network and associated with mind-wandering. The amygdala is part of the salience network and plays a role in emotions. The insula plays a role in (body) awareness and the striatum in habits.