Neuroimaging Flashcards
Localization of function:
the idea that certain brain areas correspond to specific functions
Is function localized? - JOSEF GALL
Phrenology assumed:
* Different parts of the brain = Different functions
* Brains areas can be overdeveloped = Skull bumps (can literally feel the area that someone is particularly keen in)
* Bumps indicate the faculties of an individual
Is function localized? - KARL LASHLEY
- Law of Mass Action
- Trained rats on a task > lesion > look at task performance
- Looking for memories (“the engram”) in the cortex
Is function localized? - KARL LASHLEY: FINDINGS
- FINDINGS: large lesion = greater impairment, regardless of exact location
- Proposed “equipotentiality” – all other regions of cortex take over functions following damage (all other regions have equal potentiality in taking over any function/job)
- Too strong/overachieving, but related to modern understanding of neuroplasticity
Is function localized? - PAUL BROCA: CASE STUDY
- Case study: following a stroke, M. Leborgne could only say the word “Tan” but had intact language comprehension (e.g., pointing) > a specific impairment of speech production
- After his death, Broca discovered a left frontal lobe lesion = Broca’s area
Modern constraints on case studies
Paul Broca
Relying on naturally-occurring case studies is a limited way to map the brain (we have different opportunities)
- Strokes and injuries are rarely “clean”
- Not only affect one specific area - crosses boundaries
- Hard to establish causality
- What about brain areas necessary for life?
Is function localized? - WILDER PENFIELD
- Developed a method to treat epilepsy by directly stimulating the cortex of awake patients to make surgical decisions
- Looking to see what this feels like to the patient and if this stimulates a seizure
Is function localized? - WILDER PENFIELD: MOTOR EXAMPLE
- “Extreme flexion of write, elbow, and hand”
- “Patient states that he could not help closing his right eye but he actually closed both.”
- “Made a little noise; vocalization. This was repeated twice. Patient says he could not help it. It was associated with movement of the upper and lower lips, equal on the two sides.”
Penfield’s homunculi
visual representation of how the body is represented in the brain’s motor and sensory cortexes
Modern neuroimaging methods
- Transcranial Magnetic Stimulation (TMS)
- Single-neuron recording
Transcranial Magnetic Stimulation (TMS)
Modern neuroimaging methods
- A non-invasive brain stimulation therapy that uses magnetic pulses
- Depending on protocol, TMS can either stimulate or suppress cortical activity
Single-neuron recording
Modern neuroimaging methods
- e.g., Hippocampus-entorhinal cortex circuit
- Patients with implanted electrodes (prior to epilepsy surgery)
- Microelectrodes or needle electrodes are used to record the electrical activity of individual neurons
Structural neuroimaging
Why? What types?
- clinically important to guide interventions
- scientifically important to link injuries/dysfunction to outcomes
- X-rays
- Cerebral angiography
- Computed tomography (CT)
- Magnetic resonance imaging (MRI)
X-rays (electromagnetic radiation + film)
Structural neuroimaging
- allow us to image inside a living body
- First clinical x-ray image taken 1898
- Visually - good for skull fractures but not soft tissue
Cerebral angiography
Structural neuroimaging
- A contrast x-ray technique
- Uses a radio-opaque dye (usually iodine) into the cerebral artery (makes it visible)
Cerebral aniography - Used to locate:
Structural neuroimaging
Used to locate:
- vascular damage
- large tumours
- Arteriosclerosis
- aneurisms
Computed tomography (CT)
Structural neuroimaging
- Also a version of x-ray scanning
- Rotates x-ray source and detector to reconstruct image based on density of tissue (fat vs tissue vs bone)
Computed tomography (CT) - USED FOR:
Structural neuroimaging
skull fracture, intracranial bleeds, tumours
CT is only as good as its algorithms - PROS/CONS:
Structural neuroimaging
- Pros: quick, inexpensive
- Cons: radiation exposure (after multiple)
What if, instead of introducing a foreign contrast agent, we use an existing property of different brain structures to image them?
Structural neuroimaging
- Brain structures vary in their water (hydrogen + oxygen) content
- ANSWER: Magnetic resonance imaging (MRI)
- Used for: small/subtle lesions, conditions affecting white matter
MRI: how it works
Structural neuroimaging
We cause hydrogen to behave in a special way within a magnetic field:
* Spins in all different directions at rest
* PULSE SEQUENCE
MRI - How to introduce magnetism - pulse sequence: =>
Structural neuroimaging
- Align all the protons with the large magnetic field
- Momentarily perturb (“knock down”) that alignment with a second magnetic field
- Measure the radiofrequency (RF) (what the hydrogens are releasing) signal produced during the realignment with the large magnetic field (“relaxation”)
By changing properties of the pulse sequence, we can further enhance…
Structural neuroimaging
…differences between gray vs white matter, brain vs CSF, etc.
MRI - PROS/CONS
Structural neuroimaging
- Pros: spatial resolution
- Cons: slow and expensive; excludes patients with pacemakers, metal
Variant of MRI:
Diffusion Tensor Imaging (DTI) 28
- Variant of MRI
- Relies on how water molecules move in the brain
- Pros: good for network connectivity & white matter
- Cons: expensive; computationally complex
Functional Neuroimaging - 3 potential applications
- studying mental states without requiring a response e.g., mind-wandering, lying
- understanding mechanisms of brain dysfunction
- understanding altered states of consciousness
Functional Neuroimaging - TYPES:
- Electroencephalography (EEG)
- Positron Emission Tomography (PET)
- Functional MRI (fMRI)
Electroencephalography (EEG)
Functional Neuroimaging
- Electrodes on scalp surface detect electrical activity in cerebral cortex
- Used for: epilepsy, delirium, encephalitis
EEG: Electrical signals can be statistically separated into different frequencies
Functional Neuroimaging
- Pros: quick, inexpensive, high temporal resolution
- Cons: hard to measure deep brain structures, low spatial resolution
Positron Emission Tomography (PET)
Functional Neuroimaging
- A radioactively labelled substance is injected and imaged
- e.g., active brain areas consume more fuel > show more radioactivity when a glucose-like molecule is injected
- Can also follow metabolism of radiolabelled drugs
PET - use, PROS/CONS
Functional Neuroimaging
- Less common with rise of fMRI
- Pros: useful for looking at specific systems (e.g., DA) or proteins (tau); useful for looking at lifespan/condition changes (e.g., stroke, CTE)
- Cons: expensive, poor spatial resolution
Functional MRI (fMRI)
Functional Neuroimaging
- Dominates cognitive neuroscience
- BOLD Response: Blood Oxygen Level Dependent response
1
3 BOLD Response events
- Neural activity triggers increase in blood flow to brain region (functional hyperaemia)
- Functional hyperaemia: increase of blood flow to region at work
2
3 BOLD Response events
- Increased ratio of high-oxygen blood : low oxygen blood (oxyhaemoglobin:deoxyhaemoglobin) in brain region
3
3 BOLD Response events
- Changes in magnetic properties of the brain region > visible in fMRI image
* This lasts from ~500ms to 3-5 s
fMRI: Paired image subtraction
A) Task of interest: remembering learned words
* Cued recall
B) Stuff we want to control out: Motor components of speech, Visually reading something on-screen, hearing loud MRI sounds, etc.
* Baseline
6 fMRI Challenges
- Spatial averaging
- Temporal resolution
- Doesn’t tell us about causality
- Focus on increases in activity
- Testing environment
- Replicability and statistic flexibility
- Spatial averaging
fMRI Challenges
Over trials & over subjects > can produce epiphenomena
- Temporal resolution
fMRI Challenges
- Blood changes slower than electrical activity
- May miss brief but important events
- Doesn’t tell us about causality
fMRI Challenges
- Sometimes mismatches lesion studies, e.g., RH activity during language tasks
- Sites can be activated simply by connections
- Focus on increases in activity
fMRI Challenges
Important but tonic activity would be subtracted out
- Testing environment
fMRI Challenges
- Anxiety, children, movement
- Immobilized, lying down
- Replicability and statistic flexibility
fMRI Challenges
- Need to make many pipeline choices > correcting for different anatomy, filtering noise, correcting for multiple comparisons, etc.
Default mode network
- Some regions are more active during “rest” than during goal-oriented tasks:
- > medial prefrontal cortex, posterior cingulate cortex, angular gyrus/lateral parietal cortex
- May be for inwardly-focused attentional processes; construction of the “sense of self”
