Measurement Techniques Flashcards
what does EEG measure
direct measure of electrical neuronal activity at the scalp level
why is EEG trickier to use between 3-4 years
very sensitive to movement and making sure toddlers dont move is hard. can lead to artefacts
can you remove artefacts from brain scans
yes but it requires powerful tools and there can still be interference left if the data was really noisy. some EEG studies even ask no blinking
what is the process of producing a postsynaptic potential
1) action potential arrives at end of transmitting neuron
2) vesicle fuses with plasma membrane
3) neurotransmitter is released into the synaptic cleft
4) neurotransmitter binds to receptor on receiving neurons membrane
5) postsynaptic potential released as neurotransmitters bind to postsynaptic cell membrane
what scale are the neurons measured on in EEG and why
groups of neurons because action of one is not enough to be picked up. Postsynaptic potential lasts a long time it is long enough for post synaptic potentials to summate and be picked up
what is an equivalent current dipole (ECD) in relation to EEG
the postsynaptic potential creates an ECD which is perpendicular to the cortex surface (not scalp due to folds) and this can be recorded through electrodes placed all over the scalp
what are the conditions that need to be met for the dipoles from postsynaptic transmissions to summate
numerous (thousands/millions) and simultaneous postsynaptic potentials
are spatially aligned and
they are all only excitatory
or all inhibitory neurotransmitters
what measure is commonly derived from an EEG
event related potential - brain activity locked to a specific event e.g give ppt a task and segment the signal around the event of interest e.g presentation of a target
what features are of interest in the event related potential peak
magnitude of the peak and the latency (delay between presentation and emergence of the peak)
how do we check the ERP graph (figuring out if it is EEG)
if it is in microvolts it is probably EEG and we need to check the scale to see if it goes up or down
what is the 10-20 system
The numbers “10” and “20” refer to the distances between adjacent electrodes, which are either 10% or 20% of the total distance (front-back or right-left) of the skull. The total distance is based on the anatomical locations on the scalp: nasion and inion (front-back direction) and the two preauricular points (right-left direction) as seen in Figure 1. Using these anatomical landmarks, the placement of the electrodes can be determined along with these directions with the pre-specified proportions: 10% is used from the anatomical landmarks and the first electrode in that direction, and 20% is used between the other electrodes. For example, the Fp1 is placed at 10% of the total distance from the nasion, and Fz is placed at 20% of the total distance from Fp1.
what is time frequency analysis
oscillations in different frequencies, how quickly it oscillates. it is time locked to a specific event. the different frequencies are grouped with names e.g gamma = over 30Hz
what is the most common measure for the time frequency analysis
power (amplitude squared) it is an indication of how strong the signals are
EEG advantages
direct measure of neuronal activity
high temporal resolution (milliseconds)
reasonably comfortable - not too scary for kids
appropriate throughout the lifespan
relatively cheap and portable
EEG disadvantages
low spatial resolution - inverse problem in source localisation
the scalp blurs the signal - could originate from other sources so we have to estimate this which gives an inverse problem of there being mroe than one possibility
requires a lot of trials
sensitive to motion artefacts
age related differences in cognitive vs anatomical differences e.g skull thickness
20-30 mins to set up for each ppt (long ish time)
what does MEG measure
the magnetic fields createed by the electrical activity of the brain. direct measure of neuronal activity
how is the magnetic field created in the brain for MEG
neurons fire = postsynaptic potential = summate = dipole. the magnetic field is perpendicular to the current dipole. orientation of the dipole in the skull it affects how easy it is to pick it up
what is a specific adv of MEG over EEG
measuring the magnetic field instead is that it wont be blurred by the skull. This solves the problem of the EEG’s poor spatial resolution. this means if you want to see where things happen in the brain this is a good measure.
what are the measures commonly derived from MEG
Similar to EEG, called ERMFs look at it in terms of amplitude and frequency but not measured in microvolts. You also have the waveform and the topography again.
what are the advantages of MEG
direct measure of brain activity
high temporal resolution (millliseconds)
better spatial resolution than EEG (cm)
appropriate through lifespan
comfortable (sensors not attached to head)
disadvantages of MEG
still relatively low spatial resolution
high sensitivity of current flow orientation
sensitive to movement
can be a bit scarier for children
expensive and non portable
what does functional near infrared spectroscopy (fNIRS) use
near infrared light to measure changes in local concentrations of oxyhaemoglobin (HbO or HbO2) and deoxyhaemoglobin (HbR or HHb) that result from brain activity. indirect measure of the neuronal activity
what are the sensors called in fNIRS
optodes
describe the haemodynamic response
neuronal activity increases metabolic demand (greater need for oxygen)
HbO consumption leads to initial HbO decrease (HbO<Hbr concentration)
this is overcompensated by a subsequent increase in (oxygenated) blood flow (HbO>HbR concentration)
produces localised haemodynamic response (HbO up, HbR down)
characteristics of the haemodynamic response measured in fNIRS
expressed in micromoles
sluggish response it is in terms of seconds. 0 seconds = start of the event of interest and peak happens 30 seconds later indirect measures have bad temporal resolution
how is the light used in fNIRS
Shine near infrared light to estimate how much oxy or oxyhaemoglobin there is. the light travels and scatters through the skull and biological tissue and is absorbed by HbO and HbR at different wavelengths. if there is a lot of oxygen a lot of the light that is shined will be absorbed. it corresponds to the space between the two optodes.
what are the measures derived from fNIRS and what is it comprised of
Haemodynamic response function (HRF) we look for a change in the levels from a baseline, not just the levels e.g the response from normal speech to unfamiliar speech.
- HbO concentration
- HbR concentration
Functional connectivity
fNIRS advantages
greater spatial resolution than EEG
greater temporal resolution than fMRI
less sensitige to motion artefacts than other methods
requires fewer trials than EEG
even better measurements in infant/children than adults
silent reasonably comfortable not too scary to kids (but light needs to be dimmed)
relatively cheap and portable
fNIRS disadvantages
lower spatial resolution than fMRI
lower temporal resolution than EEG/MEG (sluggish)
3cm depth limit sensitive to systematic physiological interference in the superficial layers of the head
hair absorbs infrared light
wait for HbO/HbR to return to baseline
may be difficult to cover the entire scalp
what does fMRI measure
an indirect measure of neuronal activity
using changes in the concentrations of oxyhaemoglobin that result from brain activity
describe the haemodynamic response to neuronal activity for fMRI
neuronal activity increases metabolic demand (greater need for oxygen)
HbO consumption leads to initial Hbo decrease (hbO<HbR concentration)
this is compensated for by a subsequent increase in oxygenated blood flow (HbO>HbR concentration)
after an initial dip BOLD signal goes up.
how does the HbO and HbR interact with the magnetic field of the scanner
The magnetic field inside the scanner affects the magnetic nuclei of atoms. Normally atomic nuclei are randomly oriented but under the influence of a magnetic field the nuclei become aligned with the direction of the field. The stronger the field the greater the degree of alignment.
they yield different responses to radiofrequency pulses.
HbO = diamagnetic, there is a slower dephasing of the protons in hydrogen atoms
HbR = paramagnetic, faster dephasing as a weaker attraction to the magnetic field of the scanner
what is the general process for the machine in fMRI
magnetic field aligns the protons in hydrogen atoms together. radio frequency pulse dephases the aligned protons at different rates, allowing you to infer the concentrations of oxygen in the different parts of the brain
what measures are commonly derived from fMRI
BOLD signal - blood oxygen level dependant signal
shown in voxels
contrast between two conditions
Functional connectivity - temporal synchronisation of spontaneous BOLD fluctuations either while completing a task or at rest
describe the BOLD signal in detail
contrast between 2 conditions either two tasks or rest and task
brain regions are plotted are plotted based on brain activity - warm colours for areas that are more active in the dynamic cond and cold colours that are more active in the static cond for example
the differences have to be statistically significant to be plotted
describe functional connectivity fMRI in more detail
look at the extent to which there is temporal synchronisation of spontaneous BOLD fluctuations - some areas may be coupled - do they engage at the same time? can also use for quantifying structure of the brain (this is structural fMRI)
what can you map using structural fMRI
cortical surface
cortical thickness
white matter - myelin
how do you measure that white matter integrity
Diffusion Tensor Imaging (DTI)* Diffusion of water molecules. Greatermyelin integrity = more anisotropy(greater diffusion along axons)
what are the advantages of fMRI
highest spatial resolution
whole brain structure coverage (including subcortical structures)
functional and structural (quantification of brain structure) measures
what are the disadvantages of fMRI
low temporal resolution
very sensitive to movement
uncomfortable loud cramped movement restriction - diff to use fMRI with children under 7 years - structural MRI is easier than fMRI for younger children
non portable and expensive
which measures are of direct neuronal activity
MEG
EEG
what are the different bands and their corresponding hz for time frequency analysis
gamma = >30 hz
beta = 12-30 hz
alpha = 8-12 Hz
theta = 4-8 hz
delta = < 4hz
what does the haemodynamic response function look for
fNIRS
Haemodynamic response function (HRF) we look for a change in the levels from a baseline, not just the levels e.g the response from normal speech to unfamiliar speech.
