Lecture 27b Flashcards
List the imaging techniques learned with good spatial resolution vs good temporal resolution
spatial
- MRI
- fMRI
- CT
- PET
- NIRS
temporal
- MEG
- EEG
Magnetic Resonance Imaging
- Strengths
- Weaknesses
strengths
- non-invasive
- high spatial resolution - good for visualizing anatomy
- versatility in tissue differentiation (T1 vs T2)
- can choose any anatomical plane for MRIs
weaknesses
- poor temporal resolution. This is because it relies on blood oxygenation to indirectly measure neuronal activity
When would you use T1 vs T2 MRIs?
T1: for looking at structures (makes fatty structures appear hyperintense)
T2: to visualize pathology (makes water hyperintense)
What is a deformation matrix?
average of people’s brain anatomy so you can compare. Standardizes anatomical space. Stretches everyone’s brain into the same space so you can make inferences about neuroanatomy of some disease states
People experiencing psychosis has more or less brain anatomy in these areas
What is diffusion tensor imaging?
How does it work?
What is its purpose, strengths, and weaknesses?
Give examples of what you could use it for
measures restriction of water diffusion at each ‘voxel’ to map direction of axons and thus can tell you about white matter pathways
- T2 weighted
Strengths: Good for looking at white matter (integrity of it and its tracts). The membranes of those axons limits water diffusion allowing you to visualize the tracts
Weaknesses: like MRI - poor temporal resolution
- visualize corona radiata
- Kids with autism - check if their white matter tracts or projection pathways are different from “normal” kids
- Or how tracts are affected after injury or disease
fMRI
- how it works
- strengths
- weaknesses
- example application
Increased brain activity in certain areas –> increased metabolic demand in these areas –> increased oxygenated blood flow –> fMRI signal increases
strengths
- non-invasive
- high spatial resolution
- Doesn’t care what directions cells are pointed in or if they’re deep
weakness
- indirect measurement of neuronal activity using BOLD signal (blood oxygen level dependent)
- poor temporal resolution
Example hypothesis you can test with this: do individuals with autism process facial expression the same?
Draw and explain the BOLD response. Explain the time course of response
blood oxygenation level dependent response
initially BOLD response decreases as the stimulus is perceived, this is due to the neurons in the area firing and using up the blood
bold response sharply increases as blood rushes to area, there is a slight overshoot, and then a plateau as neuronal activity continues. the overshoot and the plateau are considered a positive bold response
as stimulus is no longer perceived, neuronal activity winds down, and the bold response decreases below baseline. this is the post stimulus undershoot stage. eventually, bold response will return to baseline
Graph is in seconds - relatively slow
What is fMRI’s weakness and how can researchers get around it via experimental design?
To get around the poor temporal spatial resolution and understand the timing of brain activation, researchers do block design
- Lump together the conditions in long blocks (condition 1 for 5 minutes; condition 2 for 5 minutes)
Example experiment: seeing neuronal activation in kids with autism in response to different facial expressions (happy vs sad face)
Compare the fMRI signal in the region of interest (ROI) across conditions (experimental - task of interest vs control condition - baseline/rest/control task)
- Short block lengths can limit return to baseline of hemodynamic response function
- Long blocks maximize difference in signal between conditions
What are resting state networks? How were they discovered?
During some tasks, a certain set of brain regions are found to be deactivated. However, during resting state (person conducting no tasks; maybe mind is wandering) these regions are really active and correlated with one another (talking to one another)
- Noticed certain areas on the left and right sides of the brain that are functionally similar have the same level of metabolic activity at the same time (based on blood oxygenation level measures)
- one well known network: default mode networks
Discovered using fMRI
functional near-infrared spectroscopy (fNIRS)
- how does it work
- strengths
- weaknesses
- shine light through skull, and measure the diffraction level. the diffraction level is impacted by blood oxygenation and thus can be used to measure blood oxygenation and infer neuronal activity
Strengths
- Cheap (poor man fMRI)
- Portable
Weaknesses
- Limited in depth, spatial, and temporal resolution
- Indirect measure of neuronal activity
PET scan
- acronym
- what is it
- strengths
- weaknesses
positron emission tomography (PET)
- radioactively labelled water is injected into the subject and active areas of the brain use more blood and thus have more radioactive labels
- positrons from the radioactivity are released, they collide with electrons in the brain, and photons are produced, exit the head, and are detected
strengths
- good spatial resolution
- can map aspects such as metabolism at dopamine binding sites
weakness
- involves injecting people with contrast agent (invasive)
- poor temporal resolution
- spatial resolution is not as great as an fMRI
Utility
- used for when you need to understand brain function and chemistry
- chemistry: What neurotransmitters or molecules are involved, when they’re binding to receptors,
What are the strengths and weaknesses of EEG and MEG
Strengths
- non-invasive technique
- directly measures neuronal activity
- strong temporal precision (ms)
- scalp EEG is cheap, simple, and portable
Weakness
- depth and orientation limits
- poor spatial resolution (excl. MEG - But, MEG is immobile and expensive)
spell out EEG and MEG
electroencephalography
magnetoencephalography
Explain the neurophysiology behind EEGs
Synaptic transmission creates post-synaptic potentials
Synchronized potentials in dendrites of pyramidal cells that are aligned perpendicular to the cortical surface summate to produce EEG potentials
These can be recorded by iEEG or scalp EEG
Utility of scalp EEG
- clinical monitoring or diagnosis
- experimentally study event related potentials (ERPs) or neural oscillations (rhythms) in brain function/dysfunction
When would you observe alpha waves on EEG?
person is relaxed, maybe with eyes closed
When would you observe beta waves on EEG?
awake and excited
When would you observe delta waves on EEG?
person is asleep
What are event related potentials (ERP)? How can we discover them?
brain responses, measured using EEG that are time-locked to specific sensory, cognitive, or motor events
derived by averaging many EEG trials in response to the same stimulus
Discuss what the following ERPs are associated with:
- N1
- P1, P2, P3… etc.
- P300
- N400
- N1 – early sensory processing (and 1st negative peak)
- P1 = 1st positive peak…
- P300 – higher cognition, inhibition, executive function
- N400 – language comprehension, semantics
intracranial EEG (iEEG)
- subdural vs stereotaxic
- strengths
- weaknesses
- utility
- Subdural EEG involves placing an electrode grid (surface array) over the cortex.
- Stereotaxic (depth array) EEG involves inserting electrodes deep into the brain.
- strengths: directly records from human brain, bypasses limitations of conventional EEGs. good spatial resolution
- weakness: highly invasive and you can’t record from a lot of areas
iEEG is typically done in patients with drug-resistant focal epilepsy
Figure out which part of the brain is responsible for seizures and which areas are eloquent areas in order to not have deficits to important things (speech production and comprehension)
Do a craniotomy, insert electrodes, see which areas cause seizures, then another day remove the areas responsible for seizures out
magnetoencephalography (MEG)
- what is it
- strengths
- weakenesses
- measures magnetic fields instead of electrical potentials. measures currents generated by groups of locally synchronous neurons
Strengths:
- has the best combination of spatial resolution and temporal accuracy
- Combines neurophysiology with functional anatomy
weakness
- Complicated, expensive, magnetically shielded room needed
- non-portable
