In the last post, I explained anatomical MRI scans and the differences between the various sequence types (What is an anatomical MRI scan?. This post will explain the basics of… | by Lewis Hotchkiss | Sep, 2022 | Medium). In this post, I will explain what functional MRI (fMRI) scans are, what they are used for, and what they can tell us.
But, before you can understand how to interpret fMRI scans and what they show, you need to be able to understand how the brain functions and interacts with itself.
Regions of the Brain
The brain is composed of four main lobes called frontal, parietal, temporal and occipital which are responsible for different functions within the brain.
The frontal lobe is located at the front of the brain and contains many functions such as emotions, memory, language, problem-solving, attention, sexual behaviour and judgement. It contains most of the dopamine-sensitive neurons in the brain which explains a lot about its function. The dopamine system is associated with reward, attention, motivation and planning. This is why if parts of this region is damaged, then people can experience difficulties in motivation, they may become more impulsive, they might become poor at judging/decision making and they could become easily frustrated. The frontal lobe also contains the prefrontal cortex which is primarily involved in executive function (working memory, flexible thinking, and self-control).
The parietal lobe allows us to be able to make sense of the things we touch (whether something is hot, if it is smooth, if it is painful etc), among other senses. It also allows us to navigate the space around us, stopping us from bumping into things, judging the size, location and number of things that you see around you. That's why damage to this area can cause difficulty for people to make sense of what they are seeing, maybe even forgetting the name or function of an object.
The temporal lobe is located just above the ears, which makes it important when it comes to processing and understanding sounds. It allows us to not only understand speech, but also recognise languages and produce speech ourselves. It also allows us to recognise faces and contains a very important component of the brain, called the hippocampus, which allows us to form new memories and learn new things. If there is damage to this area, then this can result in someone having difficulty to recognise faces and to remember what people say.
The occipital lobe contains the visual processing centre of the brain, it allows us to perceive visual information and make sense of it. It is also associated in functions such as depth perception, colour determination, recognising objects and faces, and memory formation.
So how does this relate to fMRI scans?
These lobes are all associated with specific functions, however it is not as simple as that. These areas of the brain work together and their functions can be spread out across the brain. These are known as functional networks, which represent the interactions which take place between regions, and can be measured by fMRI scans by looking at brain activity when a person is performing specific tasks.
This brings me to the two primary types of fMRI scan studies — resting and task based. One of the core networks of the brain is the Default Mode Network (DMN) which is active when an individual is both either awake or at rest. The regions of the brain which are activated in this network are illustrated below.
This network typically activates when you focus on internal tasks such as day dreaming, remembering memories or thinking of the future. This is one of the most common networks which is often studied in resting state fMRI scans. As you can see from the regions where there is activity, it makes sense that these are the areas used when you are not preforming any tasks. For example, the posterior cingulate cortex is associated with remembering the past, thinking about the future, and processing concepts.
There are also task-based fMRI scans which involve people performing tasks, such as looking at faces, or identifying objects. The electrical activity in the brain is then measured to identify which regions are activated during the performance of that task. To get technical, fMRI scans are usually used to detect Blood-Oxygen-Level-Dependent (BOLD) signals which measure the change in oxygenation. This signal is then used to infer the activity of brain cells, and therefore, which areas are involved in a particular task. The brain activity can be inferred by this since blood supplies oxygen to the brain cells, and when those brain cells are active, then there is an increase in blood flow and hence blood oxygen.
Take the simple task of someone opening and closing their eyes for example. When someone opens their eyes, their visual cortex is activated. That means the brain cells in that area have to work harder, and just like you and me when we exercise, they need more oxygen. Because they need more oxygen, more red blood cells, which carry oxygen, are pumped up into that area of the brain by nearby blood vessels. When someone then closes their eyes, their visual cortex is no longer needed to be used, so there will be less activity here. This means that the brain cells no longer need lots of oxygen anymore. Because of this increase and decrease of oxygen, we can use this to determine which areas are the most active.
This activity is measured over a time period and is represented as a percentage BOLD change signal over time. During the period where the eye is open, and thus the visual cortex is activated, there is a increase in the BOLD change. When there is a decrease in this change, then we know that this region isn't being used. This is how fMRI scans allow us to see how the brain functions and which areas are activated when we perform certain tasks.
