Neuroimaging Flashcards by Ava Cervas

Localization of function:

the idea that certain brain areas correspond to specific functions

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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?

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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.”

How well did you know this?

Not at all

Perfectly

Penfield’s homunculi

visual representation of how the body is represented in the brain’s motor and sensory cortexes

How well did you know this?

Not at all

Perfectly

Modern neuroimaging methods

Transcranial Magnetic Stimulation (TMS)
Single-neuron recording

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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)

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

Cerebral angiography

Structural neuroimaging

A contrast x-ray technique
Uses a radio-opaque dye (usually iodine) into the cerebral artery (makes it visible)

How well did you know this?

Not at all

Perfectly

Cerebral aniography - Used to locate:

Structural neuroimaging

Used to locate:
- vascular damage
- large tumours
- Arteriosclerosis
- aneurisms

How well did you know this?

Not at all

Perfectly

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)

How well did you know this?

Not at all

Perfectly

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

1. studying mental states without requiring a response e.g., mind-wandering, lying 2. understanding mechanisms of brain dysfunction 3. understanding altered states of consciousness

Functional Neuroimaging - TYPES:

1. Electroencephalography (EEG) 2. Positron Emission Tomography (PET) 3. Functional MRI (fMRI)

Electroencephalography (EEG) ## Footnote 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 ## Footnote Functional Neuroimaging

* Pros: quick, inexpensive, high temporal resolution * Cons: hard to measure deep brain structures, low spatial resolution

Positron Emission Tomography (PET) ## Footnote 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 ## Footnote 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) ## Footnote Functional Neuroimaging

* Dominates cognitive neuroscience * BOLD Response: Blood Oxygen Level Dependent response

#1 ## Footnote 3 BOLD Response events

* 1. Neural activity triggers increase in blood flow to brain region (functional hyperaemia) * Functional hyperaemia: increase of blood flow to region at work

2 ## Footnote 3 BOLD Response events

2. Increased ratio of high-oxygen blood : low oxygen blood (oxyhaemoglobin:deoxyhaemoglobin) in brain region

3 ## Footnote 3 BOLD Response events

3. 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

* 1. Spatial averaging * 2. Temporal resolution * 3. Doesn’t tell us about causality * 4. Focus on increases in activity * 5. Testing environment * 6. Replicability and statistic flexibility

1. Spatial averaging ## Footnote fMRI Challenges

Over trials & over subjects > can produce epiphenomena

2. Temporal resolution ## Footnote fMRI Challenges

* Blood changes slower than electrical activity * May miss brief but important events

3. Doesn’t tell us about causality ## Footnote fMRI Challenges

* Sometimes mismatches lesion studies, e.g., RH activity during language tasks * Sites can be activated simply by connections

4. Focus on increases in activity ## Footnote fMRI Challenges

Important but tonic activity would be subtracted out

5. Testing environment ## Footnote fMRI Challenges

* Anxiety, children, movement * Immobilized, lying down

6. Replicability and statistic flexibility ## Footnote 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”