fMRI and PET Scans

Introduction: Seeing the Brain in Action

students, when psychologists want to understand behaviour from a biological approach, they often ask a big question: what is happening inside the brain when a person thinks, feels, or acts? 🧠 Two important tools for answering this question are fMRI and PET scans. These methods let researchers study the living brain without surgery, which is a major advance in psychology and neuroscience.

By the end of this lesson, you should be able to:

explain what $\mathrm{fMRI}$ and $\mathrm{PET}$ scans are and how they work,
use correct biological psychology terms such as brain activity, blood flow, and glucose metabolism,
connect these methods to the Biological Approach to Understanding Behaviour,
give examples of how researchers use these scans in studies,
compare strengths and limitations of each method in IB Psychology SL.

In real life, these scans are used in hospitals, universities, and research labs. They help scientists study conditions like stroke, epilepsy, Alzheimer’s disease, depression, and language processing. They also help psychologists understand which brain areas are active during memory tasks, decision-making, or emotion. 📊

What Is fMRI?

$fMRI$ stands for functional magnetic resonance imaging. The word functional means it measures activity, not just structure. Unlike a regular $\mathrm{MRI}$, which gives a picture of brain anatomy, $\mathrm{fMRI}$ shows which parts of the brain are more active during a task.

$fMRI$ works by detecting changes in blood oxygen levels. Active brain cells need more oxygen, so when a brain area works harder, blood flow to that region increases. The scan tracks the $\mathrm{BOLD}$ signal, which means blood-oxygen-level-dependent signal. This is based on the fact that oxygen-rich blood and oxygen-poor blood affect the magnetic properties of the brain differently.

Here is the key idea: if a person is reading words, solving a puzzle, or looking at a face, some brain areas use more oxygen than others. The scanner measures those changes and creates a picture showing the active regions. This allows psychologists to infer which brain areas are involved in certain behaviours.

A simple example is studying memory. If a participant is asked to remember a list of words, the $\mathrm{fMRI}$ scan may show greater activity in the hippocampus and related memory networks. This does not mean the scan directly reads thoughts; it shows patterns of brain activation linked to the task.

What Is PET?

$\mathrm{PET}$ stands for positron emission tomography. A $\mathrm{PET}$ scan is a brain imaging method that uses a small amount of radioactive tracer. This tracer is usually attached to a substance such as glucose, because active brain cells use glucose as fuel.

When the tracer is injected, it travels through the body and builds up in areas with higher metabolic activity. The scanner detects the radiation released by the tracer and creates an image of which brain regions are using more of the tracer. This shows areas of higher activity and metabolism.

In psychology, $\mathrm{PET}$ scans have been used to study many behaviours and disorders. For example, researchers can compare brain activity when a person is resting and when they are performing a task. If a region shows greater tracer uptake during the task, that region is likely involved in the process.

Unlike $\mathrm{fMRI}$, a $\mathrm{PET}$ scan is especially useful for studying chemical processes in the brain, including neurotransmitter systems. That makes it valuable in biological psychology, especially for understanding disorders and drug effects. However, because it involves a radioactive tracer, it is less commonly used for healthy volunteers than $\mathrm{fMRI}$.

How fMRI and PET Fit the Biological Approach

The Biological Approach to Understanding Behaviour explains behaviour through physical processes such as brain structure, brain function, neurotransmitters, hormones, and genetics. $\mathrm{fMRI}$ and $\mathrm{PET}$ are important because they provide empirical evidence about the brain, rather than relying only on observation of behaviour.

These methods help psychologists test biological explanations. For example, if a theory says that a certain brain area is involved in fear, a researcher can use $\mathrm{fMRI}$ to see whether that area becomes active when participants view threatening images. If a theory says a neurotransmitter system is involved in a disorder, a $\mathrm{PET}$ scan may help show abnormal activity in that system.

This is a strong example of the scientific method in psychology. Researchers form a hypothesis, collect data from scans, and then analyze whether the brain activity matches the predicted pattern. That makes these scans useful for correlation studies and for building explanations of behaviour based on biological evidence.

Importantly, the scans do not by themselves prove cause and effect. They show association between a task or condition and brain activity. If students sees a highlighted brain area in a scan, that means the area is associated with the activity being studied, not necessarily that it causes the behaviour alone.

Comparing fMRI and PET Scans

Although both methods help psychologists study the living brain, they are not the same.

fMRI

$fMRI$ is non-invasive in the sense that it does not require surgery or radioactive tracer injection. It measures the $\mathrm{BOLD}$ signal, which is linked to oxygenated blood flow. It produces detailed images and has good spatial resolution, meaning it can show where activity is happening quite precisely.

A strength of $\mathrm{fMRI}$ is that it gives clear images of brain activity in many different tasks. It is widely used in research because it is safer than methods involving radiation. Another strength is that it can measure activity across the whole brain at once.

A limitation is that $\mathrm{fMRI}$ does not measure neural firing directly. It measures a blood-flow signal that is indirectly related to brain activity. Also, participants must stay very still inside the scanner, which can be difficult for young children or people who feel anxious in enclosed spaces.

PET

$\mathrm{PET}$ measures metabolic activity by tracking radioactive tracers, often linked to glucose use. It can provide useful information about brain chemistry and is especially helpful for studying the effects of drugs or abnormal neurotransmitter function.

A strength of $\mathrm{PET}$ is that it can reveal information about metabolic processes and chemical activity that other scans may not show as clearly. A limitation is that it exposes the participant to radiation, so repeated scanning is not ideal. It also has lower temporal resolution than $\mathrm{fMRI}$, meaning it is less precise about exactly when activity occurs.

In simple terms: $\mathrm{fMRI}$ is usually better for studying where and when brain activity changes during a task, while $\mathrm{PET}$ is often better for studying metabolism and chemical processes. Both are useful, but they answer slightly different questions.

Example of Application in IB Psychology

Imagine a researcher wants to study fear responses. They might show participants images of spiders and neutral objects while using $\mathrm{fMRI}$.

If the amygdala becomes more active when spider images are shown, the researcher could argue that the amygdala is involved in processing fear-related stimuli. This supports a biological explanation of behaviour because it links an emotion to a specific brain structure.

Now imagine another study looking at depression and glucose metabolism. A researcher might use a $\mathrm{PET}$ scan to compare brain activity in people with depression and people without it. If certain areas show reduced metabolic activity, that may suggest differences in brain functioning associated with the disorder.

For IB Psychology SL, this is the type of reasoning you should use: describe the method, explain what it measures, and connect the result to behaviour or a disorder. The key is to go beyond naming the scan and show why it matters in biological psychology.

Issues, Ethics, and Scientific Value

Both methods raise important practical and ethical questions.

With $\mathrm{PET}$, the use of a radioactive tracer means researchers must think carefully about the amount of radiation and whether the study is justified. This makes $\mathrm{PET}$ less suitable for repeated use in some situations.

With both $\mathrm{fMRI}$ and $\mathrm{PET}$, the scanner environment can be uncomfortable. Participants may feel stressed, and stress itself can influence brain activity. This is important because the scan should measure the target behaviour as accurately as possible.

There are also interpretation issues. A colourful brain image may look very convincing, but students should remember that the picture does not automatically mean the brain area alone caused the behaviour. Brain functions are often shared across networks, not isolated to one tiny spot.

Despite these limits, the scientific value is very high. These tools provide objective data, support experimental research, and help psychologists move from speculation to evidence-based explanations. That is exactly why they are central to the Biological Approach.

Conclusion

$fMRI$ and $\mathrm{PET}$ scans are two major methods for studying the brain in biological psychology. $\mathrm{fMRI}$ measures changes in blood oxygenation linked to brain activity, while $\mathrm{PET}$ measures metabolic activity using a radioactive tracer. Both help researchers connect brain function to behaviour, emotions, and mental disorders.

For the Biological Approach to Understanding Behaviour, these scans are powerful because they provide evidence from the living brain. They support research into brain areas, cognitive processes, and biological explanations of abnormal behaviour. In IB Psychology SL, you should be able to describe how they work, compare their strengths and limitations, and explain why they matter in psychological research. 🧠✨

Study Notes

$\mathrm{fMRI}$ = functional magnetic resonance imaging.
$\mathrm{PET}$ = positron emission tomography.
$\mathrm{fMRI}$ measures the $\mathrm{BOLD}$ signal, which is linked to blood oxygen changes.
$\mathrm{PET}$ measures metabolic activity using a radioactive tracer, often attached to glucose.
Both methods help psychologists study the living brain and connect brain activity to behaviour.
$\mathrm{fMRI}$ is usually stronger for spatial detail and does not require radiation.
$\mathrm{PET}$ is useful for studying metabolism and neurotransmitter-related processes, but it uses radiation.
These scans support biological explanations of behaviour by providing empirical evidence.
They show association between brain activity and behaviour, not direct proof of cause and effect.
In IB Psychology, always explain what the scan measures, why it is useful, and what its limitations are.