Brain Imaging Techniques Flashcards
(23 cards)
EEG
-Detecting neural activity by placing electrodes on the scalp
-Electrodes pick up small fluctuations of electrical signals, originating from activity of neurons
-We are interested in the current flow from the scalp to the ground, and the EEG measures these voltage signals
Neurophysiology of the EEG signal
-EEG activity originated mostly from post-synaptic potentials (voltages arise when neurotransmitters bind to receptors on the membrane of the post-synaptic cell
-With these electrical changes, the neuron acts as a small ‘dipole’
-If many neurons spatially align, then their summed potentials add up and create the signals we can record
Inverse problem
If the sources are known, the resulting scalp configuration of signals can have multiple dipole solutions
H/W, one given scalp configuration of signals can have multiple dipole solutions
The event-related potential method (ERPs)
-obtained by time-locking the signal to the events we want to study, so we can analyse the signal amplitude at specific channels
Studying conditions using ERPs
-Subtract the waves from one condition from the waves from a control condition
-Measured the error-related negativity (ERN), a negative deflection of up to 10 μV in amplitude observed at central electrodes ~80-100 ms after an erroneous response
MEG
-Detects neural activity by placing the head within a sensor array
-Sensors pick up small fluctuations of magnetic field signals
MEG signal is not identical to the EEG signal
o Magnetic field permeates brain tissue, cerebrospinal fluid (CSF), skull, scalp, and air with little distortion
o High permeability allows us to measure the magnetic field activity without contacting the scalp
-MEG requires (1) a super high-fidelity measurement device and (2) a magnetically shielded room to measure such a weak signal
Superconducting QUantum Interference Devices (SQUIDs)
-As the magnetic flux due to the postsynaptic current passes through the coil, a current is induced with a strength proportional to the magnetic field strength
-At room temperature, the current disappears quickly because of the resistance of the coil itself
-As the current runs through multiple turns of the input coil, the flux is intensified and rectified before passing to the SQUID
SQUIDs essentially act as amplifiers for the magnetic signal, with virtually no noise and high gain
Magnetometer
-Picks up fluctuations of the magnetic field strength, which reflect dynamic changes in cortical activity/ also picks up magnetic flux noise
-Magnetic flux from the brain activity goes through the coil and induces current
o strength of the current measures the strength of the magnetic field
o direction of the induced current indicates the direction of the magnetic flux - coming out of or sinking into the brain
Optically pumped magnetometers (OPMs)
-Magnetic fields are larger, increasing sensitivity/ closer proximity allows denser sampling of focal field patterns, increases spatial resolution
fMRI
-Placing participant into a strong, static magnetic field, generated by large superconducting electromagnet, cooled by liquid helium
-Head coil used to send radio frequency pulses and also functions as a receiver for the incoming signal
Reconstructing brain images
-Use gradients created by gradient coils
-An RF pulse of a specific frequency will now only excite one slice of the brain- precisely the slice where the resonance frequency of the protons matches the frequency of the RF pulse
Phase encoding gradient
it changes the precession (or, “spin resonance”) frequency of the excited protons depending on their location in the gradient
frequency encoding gradient
changes the magnetic field within the selected slice
fMRI BOLD signal
-Slightly delayed increase of glucose and oxygen consumption
-Triggers an increase in cerebral blood flow local increase in blood oxygenation
-Increase in blood oxygenation much larger than initial dip oversupply of oxygen in the blood
-Causes Blood Oxygen-Level Dependent (BOLD) signal
fNIRS
-Hemoglobin and deoxyhemoglobin are strong absorbers of light in this part of the spectrum
-When concentrations of two molecules changes, so will the reflectance of light in different parts of the NIR range
Measuring brain activity in NIRS with optodes
-Light is directed to the skull by a set of small photo transmitters (participants wear a cap containing multiple photo transmitters and photo receivers)
-Photons travel in crescent shaped paths
Limitations of fNIRS
-Bone scatters the transmitted and reflected light, preventing fine spatial localisation of the source of neural activity
-Can only measure superficial activity
-Very sensitive to highly vascularised extracerebral structures
Understanding face processing
Found region in fusiform gyrus responding more strongly to faces than objects
Reverse Inference and task specificity
-The probability that we can really learn from our fMRI results that cognitive process X is involved depends on:
o The specificity of the task to measure this particular cognitive process
o The specificity of the brain region to reflect only this cognitive process
Spatial resolution
-fMRI not sensitive enough to detect differences between conditions
-The reason is that in each voxels, there are many neurons coding for all possible line orientations
temporal resolution
-Because it takes 1-2 seconds to measure the entire brain once, we cannot see any changes that take place within this time period
=poor temporal resolution of fMRI
-If we want to measure neural activity changes for fast processes, we need a different method such as MEG or EEG
