Brain Imaging Techniques
Brain Imaging Techniques
Brain imaging techniques allow researchers and clinicians to observe the structure and activity of the living human brain without surgery. These tools have transformed biological psychology by making it possible to study the neural correlates of behaviour, emotion, and cognition directly. The two most commonly used imaging techniques in psychology are functional magnetic resonance imaging (fMRI) and positron emission tomography (PET).
Functional Magnetic Resonance Imaging (fMRI)
fMRI measures brain activity indirectly by detecting changes in blood oxygenation. The technique is based on the BOLD signal (Blood-Oxygen-Level-Dependent): when a brain region becomes active, it requires more oxygen, which increases blood flow to that area. fMRI detects the difference in magnetic properties between oxygenated and deoxygenated haemoglobin and maps these differences onto brain anatomy with high spatial resolution.
fMRI does not require injections or radiation, making it relatively safe and suitable for repeated use. Participants can perform tasks inside the scanner, allowing researchers to observe which brain regions activate during specific cognitive or emotional activities. For example, Raine et al. (1997) used PET scans to compare the brain activity of murderers who pled not guilty by reason of insanity against matched controls. They found reduced activity in the prefrontal cortex — a region involved in impulse control and decision-making — and abnormal activity in areas linked to emotion regulation. This provided biological evidence that violent behaviour may be associated with specific neurological differences.
Positron Emission Tomography (PET)
PET scanning involves injecting a participant with a radioactive tracer that binds to specific molecules such as glucose or neurotransmitter receptors. As the tracer decays, it emits positrons that produce detectable gamma rays, allowing the scanner to map which brain areas are most metabolically active. PET has lower spatial resolution than fMRI but can measure specific neurotransmitter activity, making it particularly useful for studying dopamine and serotonin systems in addiction and mood disorders.
Applications and Evaluation
Brain imaging has provided valuable insights into the neural basis of mental disorders. Studies using fMRI have shown hippocampal volume reduction in individuals with PTSD, amygdala hyperactivity in social anxiety disorder, and altered prefrontal connectivity in depression. These findings have supported biological models of mental illness and guided pharmacological treatment development.
However, brain imaging techniques have important limitations. fMRI data is correlational — activation in a brain region during a task does not prove that region causes the behaviour. The statistical thresholds used to identify significant activation have been a source of controversy, with some studies found to report false positives. Reverse inference — concluding that a mental state caused an observed pattern of brain activation — is logically problematic because most brain regions participate in many different processes. Additionally, imaging studies often use small samples that are not representative of broader populations.