What you'll learn
- How the main regions of the human brain are organised and what they do.
- The difference between grey matter and white matter.
- How CT, MRI, fMRI and PET scans produce images of the brain.
- How to interpret brain imaging evidence carefully in exam answers.
The brain as part of the nervous system
Your nervous system coordinates rapid responses to stimuli using electrical impulses in neurones. The central nervous system, often shortened to CNS, consists of the brain and spinal cord.
The brain receives sensory information, processes it, and coordinates responses. It also has roles in memory, emotion, learning, homeostasis and voluntary movement.
Central nervous system
The central nervous system is the part of the nervous system made up of the brain and spinal cord. It processes information and coordinates responses.
The brain is protected by the skull, three membranes called the meninges, and cerebrospinal fluid, a liquid that cushions the brain and spinal cord.
Grey matter and white matter
Brain tissue is often described as grey matter or white matter.
Grey matter and white matter
Grey matter contains many neurone cell bodies, dendrites and synapses. White matter contains many myelinated axons; it looks paler because myelin is rich in lipid.
Grey matter is especially important for processing information because it contains many synapses, where neurones communicate. White matter is important for fast communication between different brain regions because myelinated axons conduct impulses quickly.
Processing and communication
A useful summary is: grey matter processes information, while white matter connects regions so information can travel between them.
The main brain regions
The brain is highly specialised. Different regions are associated with different functions, but they also work together as networks.
Cerebrum and cerebral cortex
The cerebrum is the largest part of the human brain. It is divided into left and right cerebral hemispheres. The outer layer of the cerebrum is the cerebral cortex, a folded layer rich in grey matter.
Cerebral cortex
The cerebral cortex is the outer layer of the cerebrum. It is involved in higher functions such as conscious thought, sensory perception, language, memory and voluntary movement.
The folds in the cerebral cortex increase its surface area, allowing more neurones and synapses to fit into the skull.
The two hemispheres communicate through the corpus callosum, a band of white matter containing axons that pass information between the left and right sides of the brain.
Cerebellum
The cerebellum is found at the back of the brain. It helps coordinate muscle activity, balance and posture.
It does not usually initiate movement by itself. Instead, it fine-tunes movements so they are smooth and accurate. This is why damage to the cerebellum can cause jerky movements, poor balance or difficulty with precise motor tasks.
Medulla oblongata and brainstem
The brainstem connects the brain to the spinal cord. The medulla oblongata is part of the brainstem and controls many involuntary processes.
These include regulation of heart rate, ventilation rate and blood pressure. Because these are essential for survival, damage to the medulla oblongata can be life-threatening.
Hypothalamus and pituitary gland
The hypothalamus is a small region below the cerebrum. It helps maintain homeostasis, which means keeping internal conditions within narrow limits.
It is closely linked to the pituitary gland, an endocrine gland that releases hormones into the blood. The hypothalamus can influence pituitary hormone release, linking the nervous system and endocrine system.
Examples of hypothalamus-related functions include temperature regulation, water balance, hunger and circadian rhythms.
Predicting effects of brain damage
A patient has damage to one brain region and shows poor balance, but normal breathing and heart rate.
- Compare the symptoms with the functions of key regions. Poor balance and inaccurate movement are strongly linked to the cerebellum.
- Check whether the symptoms fit other regions. Normal breathing and heart rate make serious damage to the medulla oblongata less likely.
- Conclude that the symptoms are most consistent with damage to the cerebellum, because this region coordinates movement and balance.
Localisation of function
Localisation of function means that particular functions are associated with particular areas of the brain.
For example, visual information is processed mainly in the visual cortex at the back of the brain, while voluntary movement is controlled by motor areas of the cerebral cortex.
Localisation of function
Localisation of function is the idea that different areas of the brain are specialised for different roles.
However, you should not think of the brain as a set of completely separate boxes. Many behaviours involve networks of regions working together. For example, speaking involves areas for language, hearing, memory, muscle control and emotional context.
Over-localising behaviour
Avoid writing that a complex behaviour is controlled by only one tiny brain area. It is better to say that a region is associated with or has an important role in that function.
Brain imaging: looking inside the living brain
Brain imaging refers to techniques used to produce images of the brain. Some techniques show structure, while others show activity.
Structural and functional imaging
Structural imaging shows anatomy, such as brain shape, tumours or bleeding. Functional imaging shows changes linked to activity, such as blood flow or glucose use.
CT scans
A CT scan, or computed tomography scan, uses X-rays taken from different angles. A computer combines the data to produce cross-sectional images.
CT is useful in emergencies because it is quick and can detect bleeding, swelling or skull fractures. However, X-rays are ionising radiation, meaning they can damage DNA and slightly increase cancer risk.
MRI scans
MRI, or magnetic resonance imaging, uses a strong magnetic field and radio waves to produce detailed images of soft tissues.
MRI is very useful for showing brain anatomy, such as tumours, lesions or changes in brain structure. It does not use ionising radiation, but it cannot be used safely for some patients with certain metal implants or pacemakers.
fMRI scans
fMRI, or functional magnetic resonance imaging, is based on MRI but is used to detect changes associated with brain activity.
When a brain region becomes more active, neurones need more ATP. This increases local blood flow. Oxygenated and deoxygenated haemoglobin have different magnetic properties, so MRI can detect changes in blood oxygenation.
BOLD signal
The BOLD signal is the blood-oxygen-level-dependent signal measured in fMRI. It is an indirect measure of brain activity based on changes in oxygenated blood flow.
In fMRI experiments, scientists often compare brain activity during a task condition with activity during a control condition. The difference helps identify regions associated with the task.
Interpreting fMRI task data
A participant views a flashing checkerboard. The mean BOLD signal change is 2.4% in the visual cortex during the task and 0.6% during a blank-screen control. In the auditory cortex, it is 1.1% during the task and 1.0% during the control.
- Calculate the task-related change for the visual cortex: 2.4% minus 0.6% gives 1.8 percentage points.
- Calculate the task-related change for the auditory cortex: 1.1% minus 1.0% gives 0.1 percentage points.
- Compare the changes. The visual cortex shows the larger task-related increase, so the checkerboard stimulus is most strongly associated with activity in the visual cortex.
PET scans
A PET scan, or positron emission tomography scan, uses a radioactive tracer, often a labelled glucose molecule. Active brain regions take up more glucose because they have higher respiration rates.
The tracer emits radiation that can be detected by the scanner. PET can therefore show metabolic activity, but it involves ionising radiation and usually has lower spatial resolution than MRI.
Comparing imaging techniques
| Technique | What it mainly shows | Strength | Limitation |
|---|---|---|---|
| CT | Brain structure using X-rays | Fast; useful for bleeding and fractures | Uses ionising radiation |
| MRI | Detailed soft tissue structure | High-resolution anatomy; no ionising radiation | Slower; not suitable for some metal implants |
| fMRI | Activity linked to blood oxygenation | Shows active regions during tasks | Indirect measure; affected by blood flow |
| PET | Metabolic activity using radioactive tracer | Can show glucose use or receptor activity | Uses radioactive substances |
Imaging signals are indirect
Functional brain scans do not directly show thoughts or action potentials. They show measurable changes, such as blood flow or glucose uptake, that are associated with neuronal activity.
Resolution and interpreting scans
Two useful terms when comparing scans are spatial resolution and temporal resolution.
Resolution in brain imaging
Spatial resolution is how precisely a scan can locate activity in space, usually on the scale of millimetres. Temporal resolution is how precisely it can track changes over time, often in seconds or milliseconds.
MRI has good spatial resolution for anatomy. fMRI has useful spatial resolution, but its temporal resolution is limited because blood flow changes occur more slowly than nerve impulses. Neuronal action potentials happen in milliseconds, while the BOLD response changes over seconds.
Treating coloured scan areas as simple photographs
The coloured regions on many fMRI images are not literal photographs of the brain “lighting up”. They usually show statistically processed differences between conditions.
Good brain imaging studies need careful controls. For example, if you want to study regions involved in reading words, the control condition might involve looking at similar shapes that are not words. This helps separate activity due to vision from activity due specifically to reading.
Ethical and practical issues
Brain imaging can be very useful, but it has limitations.
CT and PET involve ionising radiation, so researchers and clinicians must balance benefit against risk. MRI and fMRI do not use ionising radiation, but they can be uncomfortable because the scanner is noisy and enclosed.
Participants must give informed consent where possible. Researchers should also consider what to do if they discover an unexpected abnormality, known as an incidental finding.
How to evaluate scan evidence
When evaluating a brain scan study, ask: What does the signal actually measure? What was the control condition? How many participants were used? Could movement, blood flow or statistics affect the result?
In the exam
- Link each brain region to its function precisely: cerebellum for coordination, medulla oblongata for involuntary control, hypothalamus for homeostasis, cerebral cortex for higher processing.
- For imaging questions, state whether the technique shows structure or function, then explain what physical signal is being detected.
- Use cautious wording for fMRI and PET: say the scan shows activity associated with a task, not that it directly shows thoughts or nerve impulses.
Check yourself
- What is the difference between grey matter and white matter?
- Why can fMRI show brain regions associated with a task, but not directly measure action potentials?
- Which imaging technique would be most suitable for a quick check for bleeding after a head injury?
