Topic 3: Imaging the Brain Flashcards
(35 cards)
Electroencephalography (EEG)
A complex electroencephalographic waveform related in time to a specific sensory event.
An Electroencephalography (EEG) is a test that measures the electrical activity in the brain. The test uses small sensors (electrodes) that are placed on the scalp to detect the electrical signals in the brain.
- These signals are then recorded and displayed as a series of waves on a computer screen.
- By measuring the electrical activity in the brain, an EEG can provide information about the function of the brain, including which parts of the brain are active and which parts are not.
- Scalp recorded (summation): big signals made by many neurons, presumably relevant to the question at hand
- First described by Berger (1929)
- Quantified in two domains: time domain and voltage
Electroencephalography (EEG) signals are quantified in two domains:
The time domain and the frequency domain.
The time domain represents the EEG signal as a series of voltage measurements taken at regular intervals over time. This domain provides information about the temporal evolution of the EEG signal and can be used to identify specific events in the EEG such as the onset and offset of a particular brain wave.
- VOLTAGE
The frequency domain represents the EEG signal as a series of amplitudes and frequencies. This domain provides information about the spectral content of the EEG signal and can be used to identify specific frequency bands, such as delta, theta, alpha, beta, and gamma, that are associated with different cognitive states. By analyzing the EEG signal in the frequency domain, researchers can gain insights into the underlying neural processes that are generating the EEG signal.
- POWER (Fourier transform)
Event-related Potentials
- Deflections in the EGG, not action potentials.
- ERPs are slower signals that reflect the coordinated activity of large populations of neurons.
- ERP signals can be recorded by placing electrodes on the scalp.
- An ERP reflects the sum of many action potentials that are occurring over a larger population of neurons, and it provides a more global measure of neural activity.
- time-locked activity
- neurotransmitter binding
ERPs are largely the excitatory and inhibitory graded potentials, the EPSPs and IPSPs that a sensory stimulus triggers on dendrites.
What does “time-locked activity” refer to when speaking about Event-related potentials?
In the context of event-related potentials (ERPs), “time-locked activity” refers to the electrical activity in the brain that is synchronized with a specific event or stimulus. ERPs are considered time-locked because the electrical signals they measure are in synchrony with a particular event or stimulus. For example, the event might be a visual stimulus, such as a flash of light, or an auditory stimulus, such as a tone. The ERP measurement is taken after the event and shows the electrical activity that occurs in the brain in response to the event.
What does “neurotransmitter binding” refer to when speaking about Event-related potentials?
“Neurotransmitter binding” refers to the process by which neurotransmitters bind to specific receptors on a neuron. This binding leads to the activation of the receptor, which in turn triggers changes in the electrical properties of the neuron, including the flow of ions into or out of the cell. When many neurons are activated in a coordinated manner in response to a specific stimulus or event, this coordinated activity can be reflected in the EEG as an event-related potential (ERP). The changes in the electrical properties of the neurons that are part of the ERP are thought to be related to the release and binding of specific neurotransmitters, so studying neurotransmitter binding can provide insights into the underlying mechanisms that give rise to ERPs.
Exogenous EEG
Looking at time-locked events stimulated by something external to the body.
An exogenous EEG refers to electrical activity that is externally triggered, such as by a stimulus, sound, or movement. This type of EEG activity is used to study sensory processing and perception, as well as attention and memory processes.
Endogenous EEG
Looking at time-locked events stimulated by something internally.
An endogenous EEG, on the other hand, refers to spontaneous electrical activity that is not related to an external stimulus. This type of EEG activity is used to study brain activity and function when the individual is at rest, or when they are not performing any specific task. Endogenous EEG activity is believed to reflect intrinsic brain processes, such as rhythmic activity associated with different stages of sleep.
10-20 System
The 10-20 system is a standardized method for placement of electrodes on the scalp for the recording of electroencephalography (EEG) signals.
- The system is called the 10-20 system because the electrodes are placed at specific locations on the scalp that are approximately 10% or 20% of the distance from one landmark to another.
The landmarks used are:
- the nasion (the point at the bridge of the nose where the frontal and nasal bones meet),
- the inion (the bony prominence at the base of the skull),
- and the preauricular points (the points in front of the ears).
A simple legend for remembering the subscripts in the 10-20 system:
Fp: Frontopolar electrodes, located near the forehead
F: Frontal electrodes, located near the forehead
C: Central electrodes, located near the midline of the head
T: Temporal electrodes, located near the temples
P: Parietal electrodes, located near the top and back of the head
O: Occipital electrodes, located near the back of the head
- An electrode labelled “Fz” has a subscript of “z,” indicating that it is located on the midline of the head, at the point where the forehead meets the scalp.
- On the right of the head = subscript with an even number
- On the left of the head = subscript with an odd number
Examples:
- Pz = parietal zero
- O1 = occipital left
- T4 = Temporal right
What do Beta (15 Hz) waves indicate in EEG?
Beta waves are high-frequency (15 Hz) brain waves commonly associated with a state of alertness and focused attention. They are typically more prominent during activities that require active thought, such as problem-solving, decision-making, and planning.
- Beta waves tend to have a lower amplitude compared to alpha or delta waves
- Beta waves tend to be seen as fast, low-amplitude activity, while delta waves are seen as slow, high-amplitude activity.
What do Alpha (9-12 Hz) waves indicate in EEG?
Alpha waves are lower frequency (9-12 Hz) brain waves that are typically associated with a state of relaxed, daydreaming or meditative focus. They are often more prominent when the eyes are closed, when a person is relaxed, or when they are in a state of mental detachment from their surroundings.
What do Delta (1-4 Hz) waves indicate in EEG?
Delta waves are the slowest brain waves, with frequencies ranging from 1-4 Hz. They are typically associated with deep, dreamless sleep and are most prominent in infants and young children. In adults, delta waves are typically present in deep sleep and may be reduced in certain sleep disorders, such as insomnia.
Brain Mapping with ERP: Interpret
- The waves correspond to successive activations of synaptic connections through the auditory pathway from brainstem to cortex.
- ERP signals identified as I through VI (roman numerals) are from brainstem signal generators (neurons in the pathway)
- Those designated N0 through P1 represent activity the from primary auditory cortex regions (A1).
- Those designated N1 through P3 are from secondary and tertiary (association) regions of the cortex.
- The dotted lines indicate brain waves associated with thought processes in response to the signal.
- Negative upwards on the y-axis
For example, P3, produced 300 ms after stimulus presentation, represents decoding the meaning of the sounds.
Signal Averaging in EEGs
Signal averaging in EEG refers to a method of processing EEG data to obtain a clearer and more stable representation of the underlying brain activity.
- collect multiple EEG trials and to average their voltage signals over time to produce a single, averaged EEG trace.
- reduces noise and other random fluctuations in the data, and enhances the visibility of the underlying EEG patterns that are of interest, such as event-related potentials (ERPs).
- brain waves are unique to each individual, and there is a lot of between-subject variabilities
- all subjects typically have the same paradigm, but all look different from the average
In practice, signal averaging is performed by recording multiple trials of the same EEG response, and then superimposing these trials by taking the mean of their voltages at each sample point. The resulting averaged trace is then plotted as a function of time. This process is repeated for multiple electrodes and for multiple conditions or stimuli, to produce a series of averaged traces that can be compared to one another and to theoretical predictions.
Between-subject variability
Between-subject variability refers to the differences that exist between individuals in terms of their characteristics, behaviour, or responses to stimuli. In the context of EEG, between-subject variability refers to the differences in EEG patterns and amplitudes between individuals. These differences can be due to factors such as age, gender, genetic factors, or environmental exposure.
Interpret the phrase: “ERP can elucidate specific cognitive processes underlying overt behaviour.”
The phrase refers to the idea that Event-related Potentials (ERPs) can be used to reveal the underlying cognitive processes that give rise to observable behaviour. In other words, by examining the electrical activity of the brain in response to specific stimuli or tasks, researchers can gain insight into the neural processes that are involved in things like attention, memory, language, perception, and decision-making.
Lateralized Readiness Potential (LRP)
The Lateralized Readiness Potential (LRP) is an event-related potential (ERP) that reflects the neural activity associated with the preparation of a voluntary movement. It is a slow, gradual shift in the electrical activity of the brain that occurs before a person makes a movement. The LRP is lateralized, meaning that it occurs more prominently on one side of the brain than the other, depending on the direction of the movement. The LRP is thought to reflect the activity of the motor cortex and other brain areas involved in preparing and executing voluntary movements.
How are the Lateralized Readiness Potential (LRP) and the Stroop task related?
The Lateralized Readiness Potential (LRP) is related to the Stroop task because it is often used as a measure of the preparation for action in the Stroop task.
- Initially, it was thought that our prep happens at the perceptual level (P300) but EEG revealed otherwise.
The Stroop task is a classic example of a conflict task, where there is an interference between an automatic response and a controlled response.
In the Stroop task, the LRP provides information about the degree of preparation for a response in the Stroop task, and it is often used to study the neural processes underlying attentional control and executive function. For example, the LRP might be used to investigate the difference in preparation for action when responding to congruent versus incongruent stimuli in the Stroop task.
Stroop Task
In the Stroop task, participants are asked to name the color of the ink in which words are printed, but the words themselves are color names that are either congruent or incongruent with the ink color. For example, the word “RED” might be printed in blue ink.
- the LRP might be used to investigate the difference in preparation for action when responding to congruent versus incongruent stimuli in the Stroop task.
Computerized Axial Tomography (CAT) Scan
A Computerized Axial Tomography (CAT) scan, also known as a Computed Tomography (CT) scan, is a medical imaging technique that uses X-rays to produce detailed cross-sectional images of the body.
What do the dark and light regions represent in a CT scan?
In a CT scan, the dark regions typically represent areas of low density or air-filled spaces (e.g., the ventricles), while the light regions represent areas of higher density, such as bones or tissues.
The brightness of a particular region in a CT scan is determined by the amount of X-rays that are absorbed by that region. When X-rays pass through soft tissues, more of them are absorbed, resulting in a darker image. When X-rays pass through bones or other dense tissues, fewer of them are absorbed, resulting in a brighter image.
Interleaving
Interleaving is a technique used in CT scanning to help improve the quality of the images produced by the scan. The basic idea behind interleaving is to acquire data from multiple slices of the body at the same time.
In interleaved scanning, the CT scanner takes alternating slices of the image, rather than capturing all slices in a contiguous fashion. For example, the CT scanner might scan slice 1, then skip over slice 2 and scan slice 3, then skip over slice 4 and scan slice 5, and so on. This approach can be used to reduce motion artifact in the final image or to improve the speed of the scan.
Magnetic Resonance Imaging (MRI)
Magnetic Resonance Imaging (MRI) is a non-invasive medical imaging technique that uses strong magnetic fields and radio waves to produce detailed images of the body’s internal structures. The technique works by exposing the body to a strong magnetic field, which causes the nuclei of certain atoms within the body to align in a certain direction. A radio frequency pulse is then applied, which temporarily disrupts the alignment of the nuclei. As the nuclei return to their original alignment, they emit signals that are detected by the MRI machine and used to construct images of the body. MRI can be used to visualize a wide range of tissues, including bones, muscles, organs, and blood vessels, making it a valuable tool for diagnosis and treatment planning in many medical conditions.
Rock-table Analogy (MRI)
Analogy used to describe how MRI images are formed:
- In a static field, all the hydrogen atoms in the human body will align in the same direction as the magnetic field. (putting the rock on the table)
- RFs are being sent into the static field: Radiofrequency (RF) pulses are applied to the patient, causing the hydrogen atoms to change their orientation (the hydrogen atoms absorb the energy, causing this to resonate = unaligned and spinning). These changes in orientation are then detected by the MRI machine and used to produce an image. (picking up the rock)
- Turning off the RFs: hydrogen atoms will go from resonance to realignment, this process gives off electromagnetic energy which is used by the system to create the image (placing the rock on a scale)
- In the rock-table analogy, the RF pulse is like a gentle tap on the rock, causing it to move. The movement of the hydrogen atoms in response to the RF pulse is then used to produce an image.
