Lecture 6 Flashcards
X-Rays
It can show a lot—but at first glance, it may not be as useful
Energy (like visible light, but on a different wavelength), cross through the tissues (some absorb more or less), crossing into the other side of the person, and picked up by a sheet
Structural: it’s like a map, there is no information about time-based changes or activities
There is no time component, devoid of time-based activity
X-rays mainly show bones
Cerebral Angiography
X-ray with contrast agent
An angiogram using iodine
Still a static image but does show blood vessels in the brain
blood-related disfunction is major, this is useful for seeing blocked/ clipped arteries related to strokes
Useful since X-ray is commonly used and can run this test if needed
Computed tomography (CT or CAT scans)
X-ray but modified
Multiple weaker x-rays on different angles
The tube shot beams from different angles
Overcoming limitations of traditional X-ray
X-rays are strong and can give radiation, but CT scans can avoid this issue
“Reconstruction” putting weak, 2 dimensional images together and reconstructed as a 3D model
Shows the difference between tissue and fluid
EX. Having a stroke—the brain fills up with fluid, which is shown by lighter areas in the scan
Limitations:
Shows where there’s fluid or tissues hydrocephalus—ventricles would look,larger, but cannot show white vs. gray matter in the brain
A tumor is just tissue, so it doesn’t show up in a CT scan, because it looks the same as all the other fluids
Useful because it’s accessible everywhere—everyone has a CT scan
Mainly about availability
Magnetic Resonance Imaging (MRI)*
It’s not x-ray—it uses magnetic fields
Non-invasie; shows grey matter, white matter, fluids
Step one: align all the protons with the large magnetic flied
Step two: momentarily perturb that alignment with a second varying magnetic field
Step three: measure radiofrequency (RF) signal produced during realignment with the large magnetic field (“relaxation”)
Example of an overlay plot
With structural MRI we can overlay different lesions to study across patients
Only possible with high resolution view of the structure of the brain
Diffusion Tensor Imaging (DTI)*
Variant of MRI
Relies on how water molecules move in brain (random movement of hydrogen)
Water in your axon moves randomly but with more constraint and within the axon (looks different)
Same MRI scan but measuring random movement
Picture of lots of axon in the brain
White matter
White matter map of the brain
Electroencephalography (EEG)
Electricity—voltage
Picks up changes and fluctuations in the brain
Electrodes in the scalp goes through thick, thick layers of the brain—and are very noisy and weak signals
EEG waves look different in different states
It doesn’t tell you a lot—you just see a lot of noise (waves show interneurons)
Frequency: how many waves per second (how active your neurons are)
Higher frequency waves = more action potentials
Amplitude: how big the waves are (the synchronization of the neurons)
Positron Emission Tomography (PET)*
Participant is given a radioactive version of cocaine—in a very small dose
Cocaine acts on some parts of the brain only—dopamine
PET measures the radioactivity in the brain
The basal ganglia
You can give whatever radioactive molecule as you want (early one were water and glucose, because our brains have a lot of those)
Cyclotrons can make the radioactive molecules
Limitations: Very expensive technology, is slow and has poor resolution
The areas where there’s high levels on the heatmap is a way of seeing function
Measures indirect levels of brain activity—it doesn’t show voltage, not direct measures of activities
Can lead to some limitations
Issues with PET
PET is expensive—radioactive molecules need to be produced in a cyclotron
These molecules also need to be maintained properly because they lose
Functional MRI (fMRIs)*
Functional: means that it uses a time-based series of pictures
A neuroscientist called Seiji Ogawa discovered that not only does hydrogen (which is used in regular MRI) align with magnetic poles, but there’s also a significant difference in the magnetic properties of oxygenated blood compared to deoxygenated blood (which was used to create fMRI)
Don’t have to know name for the exam
The main idea of fMRI is that areas that are most active require the most oxygenated blood
This method indirectly measures brain activity
The main difference:
Structural MRI: focuses on hydrogen
Functional MRI: focuses on blood, specifically oxygenated and deoxygenated blood.
The Bold Response
Hemodynamic response:
Shows how blood flow in the brain changes when brain cells become active
As blood flow increases, there’s a rise in oxygen levels in that area of the brain
fMRI is used to measure these changes in blood flow and oxygen levels
The signal they look at in fMRI is called the BOLD (Blood Oxygen Level Dependent) response
BOLD reflects changes in the ratio of oxygenated to deoxygenated blood in a particular brain region
What Issues are there with Neuroimaging
1. Spatial Averaging
When you take averages of averages, you encounter epiphenomena, meaning you don’t have a signal that truly represents the other signals
EX. In an office filled with books, the average position of a book might be calculated to be in the center of the room (because there’s books on the walls, chairs, and desk), even when there are no books in the center specifically
An average might not accurately describe the layout of the office’s books, just like how it might not accurately describe where activity is happening in our brains
2. Spatial Resolution
Voxels: structural MRI resolution is 1mm by 1mm, while fMRI resolution is 3mm by 3mm
In a 1mm cube of brain tissue, there are about 13 million neurons
Small but still too big, an issue because you can’t tell when and where a neuron will fire, it’s too much activity
The fact that fMRI is even bigger makes it not precise enough
So when you have an active voxel, it doesn’t tell you anything about specific brain activity—just that there is activity
We can’t tell where or what specific neuron fires, only that many neurons fired in a general brain region
3.Temporal Resolution
Temporal resolution can be as fast as a few seconds (around 2 seconds).
However, this isn’t fast enough to accurately capture brain activity—our neurons fire in milliseconds, meaning that a second is too long to detect the firing of specific neurons
So, fMRI scans will capture millions of action potentials over a few seconds and because of the huge amount of neuronal activity, we can’t tell when a specific neuron is fired
4. Not Necessarily Necessity
Correlation does not equal causation
Just like this, you might see brain activity, but that doesn’t mean it’s directly related to your task
EX. In split-brain patients, language is localized in the left hemisphere
fMRI studies on language showed bilateral activity in areas like the cerebellum and the right hemisphere
Just because you observe brain activity doesn’t mean it’s relevant to your study—the cerebellum isn’t necessarily important to language just because there was activity there
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5. Focus on Increases in Activity**
Researchers believed that increased thought correlates with an increase in BOLD responses
Problematic because negative correlations—when increased brain activity in one area is associated with decreased activity in another area—are also important
(more info in doc)
6. Regional Hemodynamic responses:
Ideally, it should look the same across the entire brain, but in reality, different regions have slightly varied hemodynamic responses
This can lead to bad or inconsistent data
7. Confounds like anxiety and boredom:
Participants might react poorly to the noise in the scanner, fall asleep, or move their heads slightly during the experiment
This could distort the image and affect data quality
8. Confounds like drugs:
Drugs can introduce variability in brain activity, complicating results
But it’s difficult to find people who are completely drug-free
EX. A study couldn’t operate because it was too difficult to find undergrad participants who didn’t consume caffeinated drinks
Also medications for ADHD, depression, or asthma (like a ventilation inhaler) can be problematic drugs for fMRI studies
9. Anticipatory hemodynamics:
If participants repeat the same task multiple times, their brains may adjust and send oxygen in advance
This anticipation can shift the hemodynamic response curve to the left, causing issues for data interpretation
10. Reliability:
Brain activity can vary from day to day in a person
Test-retest reliability usually only show about 30% consistency
This means that brain activity on one day would correlate only about 30% with brain activity on another day
This a low number—it’s hard for studies on brain activity to be reliable
11. Statistics:
p-values:
A p-value isn’t an effect size—it shows how likely an observation is real, but never with 100% certainty
The threshold is 0.05, meaning there’s a 95% chance the observation is real and a 5% chance of a false positive or negative (Type 1 or Type 2 error)
Every single voxel in an fMRI scan represents a statistical test
If you’re using 1 million voxels, 5% of them (or 50,000 voxels) could result in a false positive or negative, which is a lot of room for error—not good
Lots of voxel= lots of chance for error
