TMS EEG fMRI (weeks 1, 7-9)

Question 1

What is EEG,

Answer 1

it is a method of measuring brain activity through placing electrodes on the scalp which pick up small fluctuations in electrical activity. These signals are usually noisy, but systematically related to cognitive processes

Question 2

benefits of EEG

Answer 2

non-invasive and relatively inexpensive,

Question 3

Hanz berger electrodes

Answer 3

recorded potential between front and back of head on his wife.

Question 4

how are the eeg signals manipulated for analyses

Answer 4

high and low pass filtered (brain cannot make certain frequencies), and filtered out for electricity line noise (50 or 60 Hz)

Question 5

what are some possibly confounding cariables for eeg

Answer 5

eye blinking and muscle movements can create great effects which can confound the research

Question 6

where does the EEG signal come from

Answer 6

post-synaptic potentials, which are the voltages which arise when neurotransmitters bind to the membrane of the post synaptic neuron (flow of ions from post synaptic neuron). Neuron acts as a dipole.

Question 7

why are signals hard to pick up in EEG

Answer 7

they rely on a specific alignment of the brain area, with the dendrites positioned close to the scalp, where the specific charges are detectable on the scalp, but often, the brain will not be detectable. Signal is often from the gyrus (peak of the folds in brain that look like waves). Also, it is hard to know where the signal is coming from.

Question 8

how are signals analysed

Answer 8

EEG picks up a complex superimposed wave which mathematical models separate into their individual waves (which correlate to frequencies of different brain areas). We can plot the dominant frequencies on a spectrogram to see the effects of variables.

Question 9

ERP’s (event relating potentials can be obtained by)

Answer 9

time locking the signals to the events studied, so we can analyse signals at certain timepoints, then analysing the amplitudes at those timepoints.

Question 10

the ERP approach can only work if:

Answer 10

the event is defined in time, the event consistently produces the signal, the timing is consistent, and the signal and noise are not correlated (the noise should be zero). However, it is usually far too noisy to do a single trial ERP.

Question 11

how can we process ERP trials to make them useful

Answer 11

averaging all trials increases likelihood that the noise is completely uniform, and therefore the signal is clear. (averaging amplitude data)

Question 12

how are ERP’s described

Answer 12

by their polarity and order (turn is either p or n (think which way theire turning to))

Question 13

how do we determine what an ERP reflects

Answer 13

reverse inferences are made where we look at previous data for inferences about what is shown by your data. We can look at baseline to peak (70% of data), peak to peak, area under.

Question 14

What is fMRI

Answer 14

A patient is placed into a very high magnetic field, where gradient coils modify the magnetic field for short periods of time, and a RF coil which emits RF pulses and also picks up the brain’s signals.

Question 15

how does MRI work

Answer 15

On the atomic level, each hydrogen atom is precessing, like small bar magnets with their own direction of magnetism which overall has no pole for an object (all the magnetic vectors add up to 0). All of these “spins” are alligned under the strong magnetic field of the MRI (aligned is called the B0 Field, and is aligned on the Z axis). A RF pulse is sent in a slice perpendicular to the magnetic field, resulting in the protons recessing in phase (same point of the spin at the same time). Now that they are in phase, the magnetisation vector is tilted from the Z axis, making it measurable. Then the RF is turned off, and repeated a lot to find T1, the time it takes to recover from the longitudinal magnetisation. Also it finds T2, the time it takes for the transversal relaxation (spin to get out of phase). Different types of matter take different T1 and T2, meaning we can create an image of the brain if we only knew where the signal was coming from. Then the gradient coil varies the gradient field across the Z axis such that different slices are on different field strengths. Then the RF pulse matches each field strength in order, mapping where signals come from in the Z dimension (this is called the slice selecting gradient). Then the gradient coil changes the phase for different slices on the Y dimension, meaning that you can tell the phase of the slices, and therefore where they are on the Y dimension (this is called the phase encoding gradient). Lastly, the gradient coil changes what frequency the magnetic field is within the selected slice (x dimension), meaning that we can decode where it is on the x dimension (called frequency encoding gradient). Then all the slices can be put together to make a 3D model.

Question 16

MRI functioning (aligning everything before learning where)

Answer 16

Magnetic field aligns magnetic “spins” of hydrogen atoms. An RF pulse, perpendicular to the magnetic field, makes the protons spin in phase, resulting in a tilting of the magnetisation vector from the Z axis, making it measurable. Then the RF is turned off, repeated finds T1 (recovery longitudinal mag) and T2 (transversal relaxation (spin to get out of phase)). Different types of matter take different T1 and T2, meaning image of the brain if we knew where signal from.

Question 17

MRI functioning (finding where signal from)

Answer 17

The gradient coil varies the field strength for different Z slices (slice selecting gradient), varies the phase for different y slices (phase encoding gradient), and varies frequency for different x slices (frequency encoding gradient, meaning that we can decode where it is on the x dimension (called frequency encoding gradient). Then all the slices can be put together to make a 3D model.

Question 18

what does fMRI measure

Answer 18

the fMRI is able to measure the dimagnetic nature of oxyhemoglobin (oxygenated blood). After a stimulus, particular parts of the brain fire, using oxygen for energy metabolism, resulting in a higher concentration of deoxyhemoglobin blood, increasing the cerebral blood flow, increasing the blood oxygenation above initial value, before dipping below baseline and slowly going back to baseline. The oversupply of oxygen creates a stronger signal for the fMRI, which is the BOLD (blood oxygen level dependent) signal we measure.

Question 19

what is an issue with timing of results for fMRI

Answer 19

the process is slow and needs time in between each image, means that there needs to be an estimation of the HRF (hemodynamic response function) curve, as we already know the general shape it takes

Question 20

the change in fMRI signal is called

Answer 20

the Hemodynamic Response Function. Not all brain areas will follow the same HRF.

Question 21

Kanwisher 1997 (fMRI) story.

Answer 21

compared processing when looking at faces and objects. An area was found in the fusiform gyrus. They then created another condition where they scrambled up faces (same visual info, just in another orientation), and another condition where they tried using houses because they have a structure, and then hands because it could just be related to body. In all conditions, only faces were more activated. They even named the areas of the brain they were looking at (FFA). However, people werent 100% convinced, and thought that the FFA could be anything activating that people are experts at seeing. They did an experiment for making people experts at determining these things called greebles. they did not initially show FFA activation for greebles, but after a couple weeks of learning, the FFA is highly activated. Therefore, maybe the FFA is expert activated. Later more researchers came along and said it could be for stuff that is often in our centre of view, so maybe the FFA could be what is in our foveal view often. They showed objects in the peripheral, middle ground, and fovea areas, mapped it onto the brain, and compared to FFA, and it highly correlated. This all shows how hard it is to pick what the fMRI results show.

Question 22

how are results combined to show an effect

Answer 22

many participants’ results are averaged to make visual representations, where t tests are used to determine wether the brain areas are significantly involved.

Question 23

what are the difficulties of connecting BOLD with brain activity

Answer 23

BOLD has seen to more reflect the local field potentials (LFPs, which reflect synaptic inputs from neurons) rather than action potentials, however this is still debated. Therefore, BOLD could be involved in both excitation and inhibition, of neuron firing, meaning that a BOLD signal may not nessecarily show causation of behaviour or correlation either.

Question 24

what is the fMRI problem of reverse inference

Answer 24

problem is we make an inference off what an area does by what it’s activated for. This means, when we find activation, we infer that as the role for the area, but the area can do many things including this thing. Also, this means if the area is activated by many different tasks, we will learn very little from any fMRI studies. Areas can be activated at different amounts depending on how demanding they are.

Question 25

what is the fMRI problem of statistical testing

Answer 25

because of the statistical testing we use, the final observed image we look at neglects alot of the brain areas used which werent significantly different. Also, we run the risk of results being slipped through (we use p < .01, but that means that 1% are false, of the 50000 areas viewed, there will be an average of 500 false positives, and ifthey all clump together we could think we found a new area (but this is stupid because the chances of that are low and it would never be able to be replicated.)). To cater for this, they divide the T test by the number of area subunits (50,000) and therefore have a p < 0.0000002, with a risk of overall 1% of having a false positive

Question 26

what is the fMRI problem of task specificity

Answer 26

task A to produce cognitive process X may not be the best task. therefore if your task does not correctly manipulate the cognitive process of interest, it cannot provide useful information about that process. If we see activation in Z, how do we know its because of X

Question 27

what is the fMRI problem of null results

Answer 27

Also, interpreting null results is tough, because you cant say i didnt see activation, therefore this brain region isnt activated.

Question 28

what is the fMRI problem of spatial resolution

Answer 28

the smallest measurement unit is a voxel, which is usually 3 x 3 x 3mm. This means we will know nothing about what is going on inside a voxel (about 100,000 neurons). In other words, the fMRI might not be sensitive enough to determine the changes that are occurring, like with colour processing. No results arent no activity, theyre just not able to be commented on.

Question 29

what is the fMRI problem of temporal resolution

Answer 29

it takes 1-2 s to measure the entire brain. Therefore if a process happens once every 100ms, we miss it over and over and over again. therefore fMRI has poor temporal resolution (as a tradeoff for spatial resolution).

Question 30

what is the fMRI problem (????) of adding value by doing neuroimaging

Answer 30

If we can never rule out brain sections involvement in tasks, then what is the value of MRI and fMRI? it does not always add value to your experiment, especially for the cost.