Neuroimaging (oct 9/11)

Question 1

Describe the major contributions of broca, lashley, and penfield to our thinking about localization of function.

Answer 1

Broca - Found Brocas area (for production of speech) in patient via autopsy and found lesion in left frontal lobe. Found localization.

Lashley - Trained rats and found that it didnt matter where the lesion was but rather how big it is, his findings were similar to how neuroplasticity works now. Showed us it is not all aout localization

Penfield - Used electrcal currents to find parts of the brain in epileptic patients that was electrically irregular to remove the part. showed us some things are to do with localization. Discovered the sensorimotor cortex and how it is tonotopic, named it Penfields homunculi.

Question 2

Describe and compare 3 different methods of structural brain imaging

Answer 2

x-ray:
-normal electromagnetic radiation + film.(bone rather than tissue)
- cerebral angiography, insertion of dye (vascular damage, large tumours, artiosclerosis, aneurisms)

CT (computed tomography):
- a version of x-ray but rotates to reconstruct image based on density. (skull fracture, intracranial bleeds, tumours)
- with or without dye.

MRI (magnetic resonance imaging):
- used water instead of dyes
- used for small lesions, conditions that affect white matter
- uses magnet not radiation.
- see the difference between gray vs white, brain vs. CSF
- DTF (diffusion tensor imaging) is to see how water moves in the brain.

Question 3

Describe and compare 3 different methods of functional brain imaging

Answer 3

1. EEG
   - electrodes on the scalp surfce detect electrical activity in cerebral cortex
   - used for epilepsy, delirium, encephalitis
2. PET
   - a radioactive labelled substance is injected and imaged to see the parts of the brain that are working (need more fuel) or that act upon a certain substance (dopamine system)
3. fMRI
   - uses the bold response to image where the blood flows to the brain as blood flows at a delay to the electrical activity.

Question 4

Provide an explanation of how the PET and fMRI work

Answer 4

1. PET
   - a radioactive labelled substance is injected and imaged to see the parts of the brain that are working (need more fuel) or that act upon a certain substance (dopamine system)
2. fMRI
   - uses the bold response to image where the blood flows to the brain as blood flows at a delay to the electrical activity.

Question 5

Recount the events of the BOLD response

Answer 5

BOLD response (Blood oxygen level dependent)
1. neural activity (electrical activity) triggers an increase in blood flow to brain region. At the synapse glutamate is released do to the activity, the astrocyte detects the glutamate and signals the blood vessels to dilate to get more oxygenated blood to the brain region.
2. increased ratio of high-oxygenated blood:low oxygenated blood in the brain region.
3. this changes the magnetic properties of the brain region, this is visible on the fMRI image (due to the change of how the brain material is interacting with the MRI machine.
4. Following the stimulus 500ms later we see the change and it lasts for 3-5 seconds.

Question 6

Identify challenges in collecting and interpreting fMRI data

Answer 6

1. Spatial Averaging
   - looking at multiple trails to avergae instead of ust one subject in one trial. This average location doesnt mean it is the exact location on every individual.
2. Temporal resolution
   - blood changes slower than the electrical activity itself which may lead to missing brief causality events.
3. Doesn’t tell us about causality
   - sometimes mismatches in lesion studies
   -sites can be activated simply by connections.
4. Focus on increases in activity
   - important but tonic activity would be subtracted out.
5. Testing environment
   - Anxiety, children, movement
   - immobilized, lying down.
6. Replicability and statistic flexibility
   - need to make many pipeline choices, correcting for different anatomy, filtering noise, correcting for multiple comparisons, etc.

Question 7

Explain what the default mode network is

Answer 7

Some regions are more active during “rest” than during goal- oriented tasks. May be inwardly-focused attentional processes; construction of the “sense of self”

Question 8

What is localization of function

Answer 8

the idea that certain brain areas correspond to specific functions

Question 9

Franz Josef Gall believed what?

Answer 9

he assumed phrenology, every part of the brain had a different funtion, if you were good at something you could even see the growth through the skull. We do not follow this now.

Question 10

What did Karl Lashley believe and how?

Answer 10

Law of Mass action, he trained rats to do a task and then created lesions, did not matter much about area but rather the size of the lesion. He believe other parts of the brain took over the part that couldn’t work anymore. This idea is too big but similar the our modern day understanding of neuroplasticity.

Question 11

Who is Paul Broca and what did he discover?

Answer 11

Paul Broca was a French physician that had a patient M. Leborgne who could only say the word tan but had full speech comprehension. After his dealth Broca found a lesion in the left frontal lobe and named is Broca’s area - part of the brain for speech production.

Question 12

What do we understand about the Broca’s area now and how does that relate the whether or not function is localized.

Answer 12

The brocas area is actually a lot larger than what Broca believed it to be. This is because of the limited amount of studies due to naturally occuring cases for us to map the brain. It is hard to localize causality because strokes are never clean due to their nature of following blood vessels.

Question 13

Who is Wilder Penfield and what did he do?

Answer 13

He developed a method to treat epilepsy by directly stimulating the cortex of awake patients to make surgical decisions. He found the focal point where the electrical parts of the brain was irregular.

Question 14

What part of the brain did Penfield really find? How does it relate the localization?

Answer 14

Penfield found the sensorimotor cortex located on the 2 biggest gyri coronally in the brain. But are tonotopic. This discovery is called Penfield’s homunculi (little man)

Question 15

What is the debate surrounding the nature of the motor map with higher electrical stimulation?

Answer 15

Some people think we get more meaningful (reaching/flinching) movements where others think that movements just become bigger.

Question 16

What is transcranial magnetic stimulation (TMS) and what is it used for?

Answer 16

TMS uses magnetic stimulation to either stimulate or temporarily suppress cortical activity. This is used for research to see where movements happen and where they are connected.

Question 17

What does single-neuron recording do and how?

Answer 17

Patients with implanted electrodes to show which cells are apart of the system and what they do. We can use this before an epilepsy surgery to make sure we remove the proper parts. This is justified for major medical reasons.

Question 18

What are the 2 modern methods of localizing function?

Answer 18

the TMS (transcranial magnetic stimulation), and the single-neuron recording process.

Question 19

Why are structural brain imaging important?

Answer 19

Clinically important to guide interventions and scientifically important to link injuries/dysfunction to outcomes.

Question 20

What are x-rays and what are they good for?

Answer 20

x-rays are electromagnetic radiation + film that allows us to image inside of a living body. Good for skull fractures but not good at imaging soft tissue.

Question 21

What is cerebral angiography and what is it used for?

Answer 21

it is a contrast x-ray technique that uses radio-opaque dye (iodine) injected into the cerebral arteries, we can locate vascular damage, large tumours, arteriosclerosis (thick stiff vessels) and aneurisms (bulging blood vessels)

Question 22

What is computed tomography (CT) scan and what is it used for?

Answer 22

• a verson of x-ray scanning
• rotates x-ray and film (detector) to reconstruct image based on density of the tissue (fat vs. tissue vs. bone) can be used w/ or w/o dye.
• used for skull fractures, intracranial bleeds, tumours.

Question 23

What are the pros and cons of a CT scan?

Answer 23

• Pros: quick, inexpensive, good for people allergic to dye as it can be used without.
• cons: radiation exposure

Question 24

What is an Magnetic resonance imaging maching and how does it differ from x-rays/CT.

Answer 24

It uses water in the human body rather than injected dyes. It is used for small lesions, conditions affecting white matter and this is where water content is important.

Question 25

How does an MRI work?

Answer 25

1. It aligns all the protons with the large magnetic field.
2. Momentarily perturb that alignments with a secondary magnet
3. then we measure the radiofrequency signal produced during the realignment with large magnetic field. The realignment releases rf signal. the degree of rf depends on water content. By changing the propeties of the pulse sequence, we can further enhance difference between the gray vs white matter, brain vs CSF etc.

Question 26

What are the pros and cons of an MRI

Answer 26

Pros: spatial resolution
Cons: slow and expensive; excludes patients with pacemakers or metal.

Question 27

What is the MRI diffusion tensor imaging? what are its pros and cons?

Answer 27

• still structural MRI, but about how water moves in the brain.
• Pros: good for network connectivity and white matter.
• cons: expensive; computationally complex.

Question 28

What are potential applications for functional brain imaging?

Answer 28

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

Question 29

What is an electo-encepha-lography (eeg)

Answer 29

• Electrodes but on the scalp surface can detect electrical activity in the cerebral cortex (outer wrinkly part of forebrain), it is used for epilepsy, delirium, encephalitis.
• Different frequencies can show different states like sleep or awake.
• Pros: quick, inexpensice, high temporal resolution (real time fast updates)
  Cons: Hard to measure deep brain structures, low spatial resolution (each electrode being a mm apart)

Question 30

What is a positron emission tomography (PET)

Answer 30

• a radioactively labelled substance is injected and imaged.
• active areas consume more fuel - so more radioactivity when a glucose like molecule is injected. can also see how drugs affect this and see dopamine areas etc.
• less common with the rise of fMRI’s
• Pros; useful for looking at specific systems or proteins (tau); useful for looking at lifespan of condtion changes (eg stroke or CTE)
  Cons: expensive, poor spatial resolution.

Question 31

How does the functional MRI (fMRI) work?

Answer 31

Due to the BOLD response we are able to see where blood goes to the brain when it is being used and we can connect that part of the brain to the activity. (part of brain that recognizes faces when it sees a face)