Lectures 10 & 11 EEG Flashcards
Neural Origins of EEG
EEG measures the electrical activity from large groups of similarly oriented neurons, primarily pyramidal cells in the cortex. When neurons fire in unison, their electrical activity generates detectable voltage changes that propagate to the scalp(L10&11 An_Introduction_…).
ERPs, specifically, represent brain responses to specific sensory, cognitive, or motor events by averaging EEG signals across multiple instances to isolate relevant neural processes from noise(L10&11 An_Introduction_…).
Advantages and Disadvantages of EEG
Advantages:
EEG has excellent temporal resolution (millisecond level), allowing researchers to track the timing of neural processes precisely
It is non-invasive and relatively affordable compared to fMRI or MEG
Disadvantages:
Low spatial resolution: EEG is not very precise at determining where in the brain the signals originate from, due to the blurring effect of the skull on electrical activity
EEG is sensitive to artifacts, such as eye movements or blinks, which can contaminate data
EEG Frequency Bands
Delta (0.5–4 Hz): Often associated with deep sleep.
Theta (4–8 Hz): Linked to memory and navigation, often seen during drowsiness or light sleep.
Alpha (8–13 Hz): Prominent when a person is awake but relaxed, especially with closed eyes
Beta (13–30 Hz): Related to active thinking, attention, and movement.
Gamma (30+ Hz): Thought to play a role in higher cognitive functions, such as perception and consciousness
Major ERP Components
Major ERP Components
P1, N1: Early sensory components, typically around 100 ms after stimulus onset, reflecting initial sensory processing.
P3 (P300): A well-known component that peaks around 300 ms and is related to attention and the processing of unexpected or novel stimuli
N170: A component associated with face perception, seen as a negative peak around 170 ms after stimulus presentation
These components help researchers understand specific cognitive processes, such as attention and visual recognition.
EEG Data Preprocessing and Outcome Variables
Preprocessing steps:
Filtering: Removing noise and irrelevant frequencies (e.g., low-pass or high-pass filters).
Artifact rejection/correction: Removing contamination from eye movements, muscle artifacts, or electrical interference
Averaging: To extract the ERP by averaging across many trials to cancel out unrelated brain activity
Outcome variables:
Amplitude: Reflects the strength of the neural response.
Latency: Time taken for a component to appear after stimulus onset, which provides insights into the timing of neural processes
Evaluating EEG Experiments and Scientific Studies
Key points to evaluate:
Data quality: Is there sufficient data, and are artifacts appropriately handled?
Component isolation: Have the researchers successfully isolated the ERP components of interest (e.g., P3, N170)?(L10&11 ERP Studies_Read…)(L10&11 An_Introduction_…).
Statistical rigor: Are the time windows and electrode sites chosen a priori, and are statistical analyses sound? The choice of analysis windows can significantly affect the results(L10&11 ERP Studies_Read…).
EEG for Cognitive Processes / Individual Differences
EEG can be used to examine how individuals differ in cognitive abilities, such as attention, memory, and perceptual processing.
For example, expertise in recognizing faces or birds can modify the N170 component, showing that EEG can reveal how specific experiences shape brain responses(L10&11 An_Introduction_…).
It can also be used to track development or neurodevelopmental disorders like autism by observing abnormal ERP components like N170(L10&11 An_Introduction_…).
Designing an EEG Experiment
To design an EEG experiment related to cognitive processes, follow these steps:
Define a clear hypothesis: For example, investigating the impact of attention on face perception using the N170 component.
Choose appropriate stimuli: Ensure the task elicits the desired neural responses (e.g., faces for N170 or oddball tasks for P3).
Set up recording and preprocessing protocols: Use proper electrode placement, ensure noise is minimized, and preprocess the data (filtering, artifact rejection) appropriately(L10&11 ERP Studies_Read…)(L10&11 An_Introductio
What are ERPs + their types
ERPs (Event-Related Potentials) are specific brain responses to stimuli, detected through EEG.
Major ERP components refer to distinct patterns in these responses, which are labeled based on their polarity (positive or negative) and timing (in milliseconds). Each component reflects a particular cognitive or sensory process.
P1 and N1; P3 (P300); N170; N400
P1 and N1 - description, function, detection
P1 (positive): Peaks around 60–100 ms after stimulus onset. It occurs earlier than N1, usually related to initial visual or auditory processing.
N1 (negative): Peaks between 100–150 ms after stimulus onset, slightly later than P1 and reflects early sensory attention, particularly in visual and auditory tasks.
Function: These components indicate basic sensory processing of stimuli, such as visual or auditory inputs. For instance, they help in determining whether attention has been captured by a stimulus.
Detection: Detected in early ERP waves, often from occipital (vision-related) and temporal (hearing-related) electrodes.
P3 - description, function, detection
P3 (P300):
P3 is a positive component that peaks around 300 ms after a stimulus, particularly when a task involves attention or decision-making.
Function: It reflects the processing of unexpected or significant events, often in tasks where the brain recognizes something as novel or requiring attention.
Detection: Detected mainly over parietal and central electrode sites, especially in “oddball” tasks where participants respond to rare stimuli.
Relation to cognitive processes: P3 amplitude is larger when stimuli are more surprising or important, linking it to decision-making and working memory processes.
N170 - description, function, detection
A negative component peaking at about 170 ms, typically involved in face perception.
Function: It is particularly responsive to facial recognition, signaling the brain’s ability to distinguish faces from non-face objects.
Detection: Detected over occipito-temporal areas (at electrodes like P7 and P8) when faces or similar objects are viewed.
N400
A negative wave peaking around 400 ms, associated with language processing.
Function: Reflects semantic processing, such as the brain’s reaction to words or sentences that do not fit expected meanings.
Detection: Commonly detected over central and parietal sites during language comprehension tasks.
Lateralized Readiness Potential (LRP)
Function: Reflects the preparation for motor responses, especially in tasks where participants must prepare a movement (e.g., pressing a button).
Detection: It is found by comparing activity from electrodes over both sides of the motor cortex, indicating motor planning and execution.
Summary of EEG Electrode Placement
Ground Electrode: Placed on the top of the head.
Letter A: Refers to reference electrodes.
Odd numbers: Represent the left hemisphere of the brain.
Even numbers: Represent the right hemisphere of the brain.
Electrode locations and names
O (Occipital): Placed on the back of the head (occipital region).
P (Parietal): Located on the crown of the head (parietal region).
C (Central): Positioned at the top center of the head (central region).
F (Frontal): Located on the forehead (frontal region).
T (Temporal): Placed on the sides of the head (temporal region).
CNV – Contingent Negative Variation
CNV (Contingent Negative Variation) is an ERP component that reflects anticipation and preparation for an upcoming event. It is a slow negative wave that develops between two stimuli when a person expects to respond to the second stimulus.
Neurons Produce Two Types of Electrical Activity:
Action Potentials: These are rapid electrical signals sent by the axon of a neuron when it “fires” or communicates with another cell.
One cell fires/talks: The axon sends the message.
The other cell listens: The dendrite of the receiving cell takes in the signal.
Postsynaptic Potentials: These occur when a neuron receives a signal from another neuron. These are slower and occur at the dendrites and cell body of the neuron.
Can action potentials be recorded from the scalp?
Action Potentials Cannot Be Recorded from the Scalp:
Action potentials are too fast and localized to be detected by scalp EEG recordings. They are typically confined to the immediate areas of neuron-to-neuron communication.
An exception is brainstem auditory evoked responses, which involve special types of action potentials detectable at a deeper level of the brain.
Postsynaptic Potentials
These last longer (10–100 milliseconds) and occur in the dendrites and cell body of neurons.
EEG measures these postsynaptic potentials because they are more sustained and can summate across many neurons, creating a measurable signal at the scalp.
What does EEG measure?
EEG Measures Synchronized Postsynaptic Potentials in Cortical Pyramidal Cells:
EEG captures the combined electrical activity of large groups of pyramidal cells—a type of neuron found in the cortex.
Pyramidal cells have long dendrites and are aligned perpendicularly to the surface of the cortex, which makes them ideal for generating electrical fields detectable by EEG.
Pyramidal Cells are found in the cerebral cortex, amygdala, and hippocampus—regions involved in high-level brain functions like memory, emotion, and cognition.
P2
Positivity at 150-275ms
▪ attention, larger for target stimuli, especially when infrequent (oddball)
▪ Presumably simpler feature processing
▪ Overlaps with N1, N2, P3
Differences Between EEG and fMRI
EEG provides excellent temporal resolution (milliseconds), while fMRI has better spatial resolution, pinpointing the location of brain activity but is slower (seconds).
