Source: A Brain-Mind Odyssey, Ch. 17
Tags: brain imaging, brain lesion, stroke, tumour, traumatic injury, Phineas Gage, Parkinson's, Alzheimer's, X-ray, CT, MRI, NMR, nuclear spin, Tesla, EEG, Hans Berger, Wilder Penfield, structural imaging, functional imaging, invasive, noninvasive
The connection between brain and mind has historically been revealed through brain damage: damage the brain, damage the mind. Modern imaging divides into structural (what the brain looks like) and functional (what it is doing). Structural tools range from X-ray and CT to MRI; functional tools include EEG and direct cortical recording. Each has trade-offs in spatial resolution, temporal resolution, and invasiveness.
Lesion
A general term for any injury to, or abnormality in, the brain. Causes include stroke, tumour, physical trauma, and degenerative disease.
Stroke
The stopping of blood flow to a brain region, caused by either vessel blockage (clot) or vessel breakage (haemorrhage). Results in cell death in the affected area.
Aneurysm
A weak spot in a blood vessel that causes it to bulge. If it ruptures, it produces a haemorrhage, potentially leading to stroke.
CT (computed axial tomography)
A sophisticated X-ray imaging process that generates 3D representations of internal brain structure from many images taken at different angles. Still widely used.
MRI (magnetic resonance imaging)
A noninvasive imaging technique that produces 3D reconstruction of brain structure based on the physical property of nuclear spin. Does not use ionising radiation.
Nuclear spin
A quantum mechanical property of atomic nuclei (especially hydrogen protons) that causes them to behave like tiny magnets in the presence of an external magnetic field. The basis of NMR and MRI.
EEG (electroencephalography)
A method of measuring electrical activity over large regions of the cerebral cortex via scalp electrodes. Excellent temporal resolution, poor spatial resolution.
Tesla
The SI unit of magnetic field strength. 1 Tesla = 10,000 Gauss. Earth's magnetic field is roughly 50 microtesla.
Historically, the primary evidence for brain-mind connections came from observing the effects of brain damage on behaviour, thoughts, feelings, and perceptions
Damage the brain → damage the mind (actions, thoughts, feelings, perceptions)
Blood flow to a brain region stops
Two causes: blockage (clot of blood cells) or breakage (haemorrhage)
Aneurysm: a weak spot in a blood vessel that bulges; if it breaks, the region downstream loses blood supply and dies
Abnormal growth of cells that disrupts normal cellular activity
Usually of glial origin
Can be benign or malignant
Most famous case: Phineas Gage (1848)
Railroad worker who survived a metal rod driven through his frontal lobe
Lost vision in one eye; underwent major personality and social behaviour changes (became more cold and disinhibited)
Frontal lobe damage was the key finding
Parkinson's disease: deterioration of the substantia nigra, the region that produces dopamine
Alzheimer's disease: massive deterioration of brain tissue; visible as enlarged sulci and generalised brain shrinkage
Goal: produce a high-resolution 3D view of brain anatomy
Original method: autopsy (Andreas Vesalius) or open surgery (peel back the skull and dura)
X-rays are electromagnetic radiation of much higher energy than visible light
Can penetrate solid matter; bones and some lesions can be detected
Limitations: poor ability to localise lesions precisely within the brain
Hazards: X-ray radiation is ionising (causes gene mutations and molecular damage)
Wilhelm Röntgen first described X-radiation in the late 19th century; received the very first Nobel Prize in Physics in 1901
Historical note: X-ray shoe fitting was a popular (and dangerous) fad from the 1920s to 1950s
A more sophisticated X-ray technique
Takes many images from different angles, then a computer reconstructs a 3D representation
Can visualise any internal body structure, not just the brain
Still involves ionising radiation
Still widely used in clinical settings
Developed in the 1940s
Uses a strong magnetic field plus electromagnetic radiation to perturb the alignment of nuclear spins
Can identify different molecules by the energy needed to flip their spins
Generates large amounts of data requiring computer processing
Based on the physical property of nuclear spin
Protons (hydrogen nuclei) in the body behave like tiny magnets
In a strong external magnetic field, protons align either with or against the field (two energy states)
A pulse of electromagnetic energy can flip protons between states
Different tissues have different densities of hydrogen, producing contrast in the image
Produces a 3D reconstruction of internal brain (or body) structure in a living person
Noninvasive: strong magnetic fields have not been shown to be toxic (no ionising radiation)
Tesla: SI unit of magnetic field strength
Gauss: older unit; 1 Tesla = 10,000 Gauss
Earth's magnetic field: ~0.5 Gauss (~50 microtesla)
Clinical MRI scanners typically use fields of 1.5 to 3 Tesla
Invasive: X-ray, CT (ionising radiation is harmful), autopsy, surgery
Noninvasive: MRI (no ionising radiation; strong magnetic fields are not known to be toxic)
Goal: measure neural activity in a living brain and see how it varies over time and across tasks
Tells you which brain regions are more or less active during various conditions
Measures electrical activity over large regions of cerebral cortex
Typically uses many electrodes placed on the scalp
Strengths: excellent temporal resolution (milliseconds)
Weaknesses: poor spatial resolution; cannot precisely localise where activity originates
Hans Berger (1920s)
Recorded the first EEG from a human brain
Named the technique
Personal motivation: while in the German army, he was injured and his sister reported "feeling" what happened; this experience drove him to study the brain
1940s-1950s
Recorded electrical activity directly from the cerebral cortex during brain surgery (patients were awake)
Much higher spatial resolution than scalp EEG
Used stimulation to map somatosensory and motor cortex
This technique developed into electrocorticography (ECoG)
⚠️ Distinguish structural imaging (X-ray, CT, MRI: what the brain looks like) from functional imaging (EEG, ECoG: what the brain is doing).
⚠️ MRI is noninvasive (no ionising radiation); X-ray and CT are invasive (ionising radiation damages DNA).
⚠️ EEG has excellent temporal resolution but poor spatial resolution. Direct cortical recording (Penfield/ECoG) has much better spatial resolution but requires surgery.
⚠️ Phineas Gage is the classic traumatic brain injury case: frontal lobe damage leading to personality and social behaviour changes.
⚠️ Know the distinction between stroke causes: blockage (clot) vs. breakage (haemorrhage/aneurysm).
⚠️ MRI is based on nuclear spin, a quantum property of protons. This is conceptually different from X-rays (electromagnetic penetration) and EEG (electrical recording).
Q: What is the difference between structural and functional brain imaging?
A: Structural imaging (X-ray, CT, MRI) produces images of the brain's anatomy, showing what the brain looks like. Functional imaging (EEG, ECoG) measures neural activity over time, showing which brain regions are active during specific tasks or states.
Q: Why is MRI considered noninvasive while CT is considered invasive?
A: MRI uses strong magnetic fields and radiofrequency pulses, which have not been shown to be toxic to the body. CT uses X-rays, which are a form of ionising radiation that can cause gene mutations and molecular damage.
Q: What was Phineas Gage's injury, and what did it reveal about the brain?
A: In 1848, a metal rod was driven through Gage's frontal lobe during a railroad accident. He survived but experienced major personality changes, becoming more cold and socially disinhibited. This provided early evidence that the frontal lobe plays a critical role in personality, social behaviour, and judgement.
Q: What are the strengths and limitations of EEG?
A: EEG has excellent temporal resolution, capturing neural activity on a millisecond timescale. Its limitation is poor spatial resolution: it measures electrical activity over large regions of cortex and cannot precisely localise the source of that activity.
Q: What physical property underlies MRI, and how does it produce an image?
A: MRI is based on nuclear spin. Hydrogen protons in the body behave like tiny magnets and align in an external magnetic field. Electromagnetic pulses flip protons between energy states. Different tissues contain different densities of hydrogen, producing contrast. A computer synthesises this information into a 3D image.
brain imaging, brain lesion, stroke clot haemorrhage, aneurysm, tumour glial benign malignant, traumatic brain injury Phineas Gage frontal lobe, Parkinson's substantia nigra dopamine, Alzheimer's brain atrophy, structural imaging, X-ray Röntgen Nobel, CT computed axial tomography, NMR nuclear magnetic resonance, MRI, nuclear spin proton, Tesla Gauss magnetic field strength, invasive noninvasive, functional imaging, EEG electroencephalography Hans Berger, Wilder Penfield cortical stimulation, electrocorticography ECoG, temporal resolution spatial resolution