Introduction to Brain-Scanning Techniques

In the past, if psychologists wanted to see how the brain worked, they often had to wait for someone to pass away to perform an autopsy, or study people who had suffered specific brain injuries. Thanks to modern technology, we now have brain-scanning techniques that allow us to look inside a living, thinking brain without any surgery!

In this chapter, we will explore the three main techniques required for your Pearson Edexcel International A Level: CAT, PET, and fMRI. These tools are essential in Biological Psychology because they help us understand the link between brain activity and human behaviour, specifically aggression.

1. CAT Scans (Computerised Axial Tomography)

A CAT scan (sometimes called a CT scan) is essentially a very sophisticated, 3D X-ray. It involves taking many X-ray "slices" from different angles around the head and using a computer to join them together into a complete image.

How it works:

The patient lies on a table that slides into a large, doughnut-shaped machine. An X-ray beam rotates around the head. Denser tissue (like bone) absorbs more radiation, while less dense tissue (like brain matter) absorbs less. The computer uses this data to create a detailed map of the brain’s structure.

Strengths of CAT Scans:

  • Physical Health: They are excellent at detecting physical changes like tumours, blood clots, or structural damage caused by injury.
  • Availability: They are much quicker and cheaper than fMRI scans.
  • Safety: Unlike MRI, CAT scans can be used on patients with medical implants like pacemakers.

Weaknesses of CAT Scans:

  • Radiation: They use X-rays, which involves a small amount of ionising radiation. This can be a health risk if used too frequently.
  • Structure only: A CAT scan only shows us what the brain looks like; it cannot tell us which parts of the brain are active at a specific time.

Quick Review: Think of a CAT scan as a photograph of the brain’s hardware. It tells you if the parts are there and in the right place, but not if they are "turned on."

2. PET Scans (Positron Emission Tomography)

A PET scan is a functional scan. This means it doesn't just show the structure; it shows the brain in action. This technique was famously used in the Raine et al. (1997) classic study to compare the brains of murderers to a control group.

How it works:

The patient is injected with a radioactive tracer (usually a form of glucose/sugar). The brain uses glucose for energy. When a specific part of the brain becomes active, it "eats" more glucose. The PET scanner detects the radiation emitted by this tracer and creates a colour-coded map: red and orange usually show high activity, while blue and violet show low activity.

Strengths of PET Scans:

  • Measuring Function: They allow researchers to see which specific areas of the brain are working during different tasks (e.g., when a person feels angry or solves a puzzle).
  • Validity: They provide objective evidence of brain activity rather than relying on self-report.

Weaknesses of PET Scans:

  • Invasive: It involves injecting a radioactive substance into the bloodstream, which some patients may find stressful or have ethical concerns about.
  • Low Resolution: The images are often "blurry" compared to fMRI, making it harder to pinpoint the exact location of activity.
  • Time-consuming: The scans can take a long time, and the radioactive tracer decays quickly.

Key Takeaway: PET scans show the metabolic activity of the brain. They are the primary tool used in your classic study by Raine et al. (1997) to look at the pre-frontal cortex and limbic system in relation to aggression.

3. fMRI Scans (Functional Magnetic Resonance Imaging)

The fMRI is currently the "gold standard" in brain imaging. It provides high-quality images of both the brain's structure and its function simultaneously.

How it works:

The fMRI uses a powerful magnetic field and radio waves. It measures blood-oxygen-level-dependent (BOLD) signals. When a brain area is active, it requires more oxygen, which is delivered by the blood. Oxygenated blood and deoxygenated blood have different magnetic qualities. The fMRI detects these changes and creates a high-resolution 3D image of the activity.

Strengths of fMRI Scans:

  • Non-invasive: There are no injections or radiation involved, making it much safer for repeated use.
  • Spatial Resolution: It provides very clear, detailed images, allowing researchers to see activity in very small, specific areas of the brain.
  • Dual Purpose: It shows both what the brain looks like and what it is doing at the same time.

Weaknesses of fMRI Scans:

  • Expensive: The machines are very costly to buy and maintain.
  • Environment: The patient must stay completely still in a noisy, cramped tube. This can lead to claustrophobia or head-movement artifacts (blurry data).
  • No Metal: Because of the powerful magnets, anyone with metal implants or fragments (like certain heart valves or shrapnel) cannot have an fMRI.

Did you know? The magnetic field of an fMRI is so strong it could pull a heavy metal oxygen tank across the room! This is why safety checks are so strict.

Applying Scanning to Aggression

In Topic C (Biological Psychology), we use these scans to investigate why some people are more aggressive than others. Here is how we apply our knowledge:

  • Pre-frontal Cortex (PFC): Scans (like PET or fMRI) often show reduced activity in the PFC of impulsive or aggressive individuals. The PFC is like the brain's "brakes"—it helps control impulses.
  • Limbic System (Amygdala): Scans may show increased activity in the amygdala, which processes emotions like fear and anger.
  • Brain Injury: CAT scans can show if a person has a physical injury to these areas that might explain a change in their personality or aggression levels.

Cross-reference: For more detail on how these areas function, see the chapter on "Brain structure and aggression."

Summary Table for Revision

Technique What does it measure? Main Advantage Main Disadvantage
CAT Structure (X-rays) Quick; good for bone/clots Involves radiation; no function
PET Function (Glucose use) Shows activity in real-time Invasive (injection); radioactive
fMRI Structure & Function (Oxygen) Very detailed; no radiation Very expensive; must stay still

Common Mistakes to Avoid

  • Confusing CAT and fMRI: Remember, CAT uses X-rays and shows structure only. fMRI uses magnets and shows both structure and function.
  • Thinking PET shows "thoughts": PET scans show biological activity (glucose metabolism), not exactly what a person is thinking!
  • Forgetting Raine et al.: Always link PET scans to the study of murderers when answering exam questions about how scanning is used in Biological Psychology.

Quick Review Question: If a researcher wanted to see which part of the brain is active when a person is provoked to anger, but they didn't want to use radiation, which scan should they choose?
(Answer: The fMRI, because it shows activity/function and does not use radiation.)