Brain Imaging Techniques

Introduction

Brain imaging techniques allow researchers to observe the structure and function of the living human Brain without surgery. These techniques have revolutionised our understanding of brain-behaviour Relationships and are essential tools in biological psychology, cognitive neuroscience, and clinical Practice. Each technique has distinct strengths and limitations, and the choice of technique depends On the research question being addressed.

Structural Imaging Techniques

MRI (Magnetic Resonance Imaging)

MRI uses strong magnetic fields and radio waves to generate detailed images of brain structure. Hydrogen atoms in the body align with the magnetic field; when a radiofrequency pulse is applied, These atoms emit signals as they return to their original alignment. Different tissues produce Different signal intensities, allowing the construction of high-resolution anatomical images.

What it measures: Brain anatomy, including the size and shape of brain structures, the presence Of lesions or tumours, and grey matter/white matter differentiation.

Spatial resolution: Approximately 1—2 mm.

Temporal resolution: Minutes (it is not suitable for tracking rapid changes in brain activity).

Strengths:

• Non-invasive and does not involve ionising radiation.
• Provides extremely high-resolution structural images.
• Can be used repeatedly on the same individual, allowing longitudinal studies.
• Can measure specific tissue properties through techniques such as diffusion tensor imaging (DTI), which maps white matter tracts.

Limitations:

• Cannot directly measure brain activity.
• Expensive to operate and maintain.
• Cannot be used with individuals who have metal implants (pacemakers, cochlear implants, certain aneurysm clips).
• Claustrophobia can be a problem for some individuals.

CT (Computed Tomography)

CT scanning uses X-rays to create cross-sectional images of the brain. Multiple X-ray beams are Passed through the brain from different angles, and a computer reconstructs a three-dimensional Image from the resulting data.

What it measures: Brain structure, particularly useful for detecting bleeding, tumours, and Skull fractures.

Strengths: Fast, widely available, good for detecting acute brain injuries (particularly Haemorrhage).

Limitations: Involves ionising radiation (limiting repeated use), lower spatial resolution than MRI for soft tissue.

Functional Imaging Techniques

fMRI (Functional Magnetic Resonance Imaging)

FMRI measures brain activity by detecting changes in blood oxygenation and blood flow (the Blood-oxygen-level-dependent, or BOLD, signal). When a brain region becomes more active, it consumes More oxygen, leading to an increase in local blood flow (a phenomenon known as the haemodynamic Response). Because the increase in blood flow exceeds the increase in oxygen consumption, the Concentration of oxygenated haemoglobin increases relative to deoxygenated haemoglobin. FMRI detects This difference, as oxygenated and deoxygenated haemoglobin have different magnetic properties.

What it measures: Changes in neural activity (indirectly, through the BOLD signal).

Spatial resolution: Approximately 1—3 mm.

Temporal resolution: Approximately 1—2 seconds (limited by the speed of the haemodynamic Response).

Strengths:

• Non-invasive and does not involve ionising radiation.
• Good spatial resolution, allowing localisation of activity to specific brain regions.
• Can be used to study both cognitive and emotional processes.
• Allows within-subject designs (each participant serves as their own control).

Limitations:

• Indirect measure of neural activity: the BOLD signal reflects changes in blood flow, not electrical activity directly. The temporal lag between neural activity and the haemodynamic response (approximately 5—6 seconds) limits temporal resolution.
• Susceptible to motion artefacts (head movement during scanning).
• Expensive and requires specialised facilities.
• Statistical issues: the massive number of comparisons (each voxel in the brain is tested separately) increases the risk of false positives (the “voodoo correlations” problem; Vul et al., 2009).
• The assumption that the BOLD signal is linearly related to neural activity has been questioned.

Haxby et al. (2001): Distributed Neural Representation of Faces and Objects

Haxby and colleagues used fMRI to investigate how the brain represents different categories of Visual objects. Participants viewed images of faces, cats, houses, chairs, scissors, shoes, and Bottles while undergoing fMRI scanning.

Key findings:

• Different categories of objects activated distinct but overlapping regions of the ventral temporal cortex.
• The fusiform face area (FFA) showed the strongest response to faces, but it also responded, albeit weakly, to other object categories.
• The pattern of activation across the entire ventral temporal cortex (rather than the response of any single region) contained sufficient information to discriminate between object categories.
• This finding challenged the “modular” view of object recognition (which held that specific brain regions are dedicated to specific object categories) and supported a “distributed” view, in which object categories are represented by patterns of activity across broad cortical regions.

Evaluation:

• The study was methodologically rigorous, using a large number of stimulus categories and participants.
• The finding that object representations are distributed rather than modular has important implications for understanding how the brain organises knowledge.
• However, the study used relatively simple, isolated objects presented on a grey background. Object recognition in the real world involves more complex stimuli (objects in context, overlapping objects, degraded images), and the neural mechanisms may differ.

PET (Positron Emission Tomography)

PET involves injecting a radioactive tracer (a positron-emitting isotope attached to a biologically Active molecule, most commonly fluorodeoxyglucose, or FDG) into the bloodstream. The tracer Accumulates in active brain regions (because active neurons consume more glucose). As the Radioactive tracer decays, it emits positrons, which collide with electrons, producing gamma rays That are detected by the PET scanner. A computer reconstructs a three-dimensional image of brain Activity from the gamma ray data.

What it measures: Metabolic activity (glucose consumption), blood flow, or neurotransmitter Receptor binding (depending on the tracer used).

Spatial resolution: Approximately 4—6 mm.

Temporal resolution: Minutes.

Strengths:

• Can measure specific neurochemical processes (by using different tracers, e.g., a dopamine receptor tracer).
• Can provide absolute measures of metabolic activity (unlike fMRI, which measures relative changes).
• Useful for studying neurotransmitter systems and receptor density.

Limitations:

• Involves exposure to ionising radiation, limiting the number of scans that can be performed on a single individual.
• Lower spatial resolution than fMRI.
• Lower temporal resolution than EEG/MEG.
• Requires injection of a radioactive substance, raising ethical concerns (particularly for healthy research participants).

Hariri et al. (2002): Serotonin Transporter and Amygdala Response

Hariri and colleagues used fMRI to investigate the relationship between a common genetic Polymorphism in the serotonin transporter gene (5-HTTLPR) and amygdala activation in response to Fearful faces.

Key findings:

• Participants with one or two short alleles of the 5-HTTLPR polymorphism showed significantly greater amygdala activation in response to fearful faces compared to participants with two long alleles.
• This finding provided a neural mechanism for the gene-environment interaction reported by Caspi et al. (2003): the short allele is associated with heightened amygdala reactivity to threatening stimuli, which may increase vulnerability to anxiety and depression in the context of stressful life events.

Evaluation:

• The study demonstrates how brain imaging can bridge genetics and psychology by identifying the neural circuits through which genetic variation influences behaviour.
• The sample size was small (N = 28), which limits statistical power and generalisability.
• The study measures brain activation during a laboratory task (viewing fearful faces), which may not fully capture the complexity of emotional processing in real-world settings.

Electrophysiological Techniques

EEG (Electroencephalography)

EEG measures the electrical activity of the brain using electrodes placed on the scalp. Neurons Communicate through electrical signals (action potentials and synaptic potentials), and the summed Electrical activity of large populations of synchronously firing neurons produces voltage Fluctuations that can be detected at the scalp.

What it measures: Electrical brain activity, measured as voltage fluctuations over time.

Spatial resolution: Poor (approximately 1 cm at best; cannot precisely localise the source of Activity).

Temporal resolution: Excellent (milliseconds).

Strengths:

• Excellent temporal resolution, allowing measurement of rapid changes in brain activity.
• Non-invasive, relatively inexpensive, and portable.
• Can be used with infants, children, and individuals who cannot tolerate fMRI.
• Event-related potentials (ERPs) can be extracted from the EEG signal by averaging the electrical response time-locked to specific events (e.g., stimulus onset). ERPs provide precise information about the timing of cognitive processes.

Limitations:

• Poor spatial resolution: EEG cannot precisely determine where in the brain the electrical activity originates (the “inverse problem”).
• Only measures activity from the cortical surface; deep brain structures (e.g., amygdala, hippocampus) are difficult to detect.
• Susceptible to artefacts from muscle activity (e.g., blinking, jaw clenching) and electrical interference.

TMS (Transcranial Magnetic Stimulation)

TMS uses a rapidly changing magnetic field to induce electrical currents in a specific region of the Cortex, either stimulating or temporarily disrupting neural activity in that region. A coil is Placed over the scalp, and brief magnetic pulses are delivered.

What it measures: Not primarily a measurement technique; rather, TMS is a method for causally Manipulating brain activity. By temporarily disrupting activity in a specific brain region and Observing the behavioural consequences, researchers can determine whether that region is necessary For a particular cognitive function.

Strengths:

• Allows causal inferences about brain-behaviour relationships (unlike fMRI and PET, which are correlational).
• Non-invasive (does not require surgery).
• Can be used to create “virtual lesions” that temporarily mimic the effects of brain damage.

Limitations:

• Limited to cortical regions (cannot directly stimulate deep brain structures).
• Can be uncomfortable (the magnetic pulse produces a loud click and can cause scalp discomfort or muscle twitching).
• There is a risk of seizures (approximately 1 in 10,000 sessions).
• Spatial resolution is limited (approximately 1 cm), and the magnetic field can spread to adjacent regions.

Lesion Studies

Lesion studies investigate the effects of brain damage on behaviour and cognition. By studying Individuals with damage to specific brain regions (from stroke, tumour, injury, or neurosurgery), Researchers can infer the functions of those regions.

Logic: If damage to brain region X impairs function Y, then region X is necessary for function Y. This is the method of “necessity” (as opposed to fMRI, which identifies regions that are active During a task but does not demonstrate that they are necessary).

Strengths:

• Provide strong evidence for the necessity of specific brain regions for specific functions.
• Have historically been the primary source of evidence for brain localisation (e.g., Broca”s and Wernicke’s areas).
• Can reveal functions that are not apparent from imaging studies of healthy individuals (e.g., the role of the hippocampus in memory was revealed by studying patient HM).

Limitations:

• Lesions are rarely confined to a single brain region; damage often extends to surrounding areas and to white matter tracts connecting distant regions.
• No two lesions are identical, making it difficult to replicate findings.
• The brain may reorganise after injury (neuroplasticity), so the observed deficits may not fully reflect the original function of the damaged region.
• Patients with brain damage may have additional cognitive deficits (e.g., attentional problems, fatigue) that confound the assessment of specific functional impairments.
• Ethical concerns: studying individuals with brain damage requires careful attention to informed consent and the welfare of participants.

Ethical Considerations in Brain Imaging

The use of brain imaging in research raises several ethical issues:

1. Informed consent: Participants must understand the nature of the procedure, including any risks (e.g., claustrophobia in MRI, exposure to radiation in PET, discomfort in TMS).
2. Incidental findings: Brain scans may reveal unexpected abnormalities (tumours, aneurysms, signs of neurodegenerative disease). Researchers must have protocols for handling such findings, including referral to medical professionals.
3. Privacy: Brain imaging data could potentially reveal sensitive information about an individual’s cognitive abilities, emotional states, or predisposition to neurological or psychiatric conditions. Protecting the privacy and confidentiality of brain imaging data is essential.
4. Use with vulnerable populations: Extra care must be taken when using brain imaging with children, individuals with cognitive impairments, or individuals with psychiatric disorders, who may have difficulty providing fully informed consent.
5. Dual use: Brain imaging technology has potential military and commercial applications (e.g., lie detection, neuromarketing) that raise ethical concerns about the appropriate use of the technology.

For an overview of biological topics, see Biological Level of Analysis.

Common Pitfalls

1. Confusing correlation and causation in psychological research evidence.

2. Failing to discuss ethical issues (informed consent, deception, debriefing, right to withdraw) when evaluating studies.

3. Confusing the approaches (biological, cognitive, behavioural, psychodynamic, humanistic) and their key assumptions.

4. Describing a study without evaluating its methodology (e.g., sample, controls, ecological validity).

Summary

The key principles covered in this topic are linked in the sub-pages above. Focus on understanding the definitions, applying the formulas or frameworks, and evaluating strengths and limitations of each approach.

Worked Examples

Worked examples demonstrating the application of key concepts are covered in the detailed sub-pages linked above.