Methods: Neurophysiology

Why is the Brain myth: “we only use 10% of our brains” a myth?

The brain uses different regions as needed, but activating 100% of all neurons simultaneously would result in a seizure.

Temporal resolution

• How "fine-grained" in time a method can measure; it refers to how precisely a technique tracks changes in brain activity over time.

Spatial resolution

How "fine-grained" in space a method can measure; it refers to the ability to distinguish between neural events or structures that are close together physically.

What does "high" vs. "low" temporal resolution mean?

• High/Fine: Measures very small time points (e.g., distinguishing what you ate for lunch vs. breakfast using exact time).

• Low/Coarse: Measures across large time blocks (e.g., grouping events by days).

What does "high" vs. "low" spatial resolution mean?

• High/Fine: Enables you to distinguish structures/locations close together (e.g., distinguishing specific individual homes in Charlottesville).

• Low/Coarse: Measures broad regions (e.g., identifying the whole city).

How to perform single unit recordings? 

• An electrode is placed in the brain of an animal (non human)

• Brain is connected to a signal amplifier and is projected to a display 

• This electrode listens to one or a few neurons 

• While this recording is happening, the animal performs a task 

What kind of signals do single unit recordings measure? 

• Action potentials/firing of neurons, commonly called “spikes” 

• Dependent variables in single unit recordings 

• Spike count and firing rate

Spike count

• the total number of times a neuron produces an action potential 

firing rate

• how many times a neuron produces an action potential within a given time window 

Firing rate is typically illustrated in ……

      -typically illustrated in raster plots, where each bar, dot, or mark is an action potential 

how is this an example of cognitive subtraction

single-unit recordings use cognitive subtraction by comparing the dependent variable (spike count or firing rate) across two tightly matched experimental conditions

WHY perform single unit recordings? 

1. Direct measure of neural activity

2. Precise temporal and spatial resolution, recording neurons exactly how they are responding

Why not perform single unit recordings? 

1. Typically only measuring a few neurons, difficult to assess how large scale brain networks operate 

2. Limited opportunity for human application 

Scalp electroencephalography / scalp EEG measures….

•  the brain’s electrical signals, where the recording is taken at the scalp 

how does Scalp electroencephalography / scalp EEG work

• Several electrodes are placed on the scalp of a human 

• Each electrode listens to many thousands of neurons 

• Electrodes do not “hear” action potentials or spikes 

• Human performs a task while recording takes place 

What kinds of signals do scalp EEG measure? 

• Each neuron acts as a dipole, creating positive vs. negative voltage differences along its dendrites.

• For scalp EEG to detect these electrical signals, neurons must be oriented perpendicular to the scalp.

• When enough neighboring neurons fire together, their dipoles summate, generating a signal strong enough to reach the scalp.

The skull acts as a physical barrier that prevents…..

weak or deep signals from reaching the surface electrodes.

Voltage

 the difference in electrical signal between the electrode’s location and a reference or ground electrode (reference electrode location depends) 

• Voltage in microvolts across time (ms) 

Q: What are EEG electrodes connected to?

A: An amplifier, which strengthens the small electrical signals so they can be recorded and analyzed.

Q: Why can’t researchers easily identify cognitive processes from raw EEG data?

A: Raw EEG contains continuous, noisy electrical activity from many different sources, making responses to a specific event difficult to identify.

How is scalp EEG data analyzed? 

event-related potential (ERP)

Q: What is an event-related potential (ERP)?

A: An averaged EEG response that is time-locked to a specific event, such as seeing an image or hearing a sound.

Q: How is an ERP created?

A: Researchers divide the continuous EEG recording into short time periods around each stimulus onset, align them, and average them together.

Q: Why is a stimulus repeated many times during an ERP experiment?

A: Repetition—such as 100 trials—allows researchers to average the trials, reduce random noise, and reveal the consistent brain response to the stimulus.

Q: What does an ERP graph show?

A: It shows how average electrical voltage changes over time in response to a particular event.

Q: What does amplitude mean in an ERP graph?

A: Amplitude is how much the voltage differs from zero, usually measured in microvolts (µV).

can you tell cognitive processes from raw EEG data

no

Q: What does “stimulus onset” mean?

A: The exact time when the stimulus begins, usually marked as time zero on an ERP graph.

Q: What is a voltage deflection in an ERP graph?

• A: A change in electrical voltage away from the baseline (zero).

What do voltage deflections mean? 

• Specific components are thought to reflect specific functions, depending on the stimuli, tasks, and electrode location 

Q: How are ERP components commonly named?

A: By their polarity and latency.

Q: What does polarity mean?

A: Whether the voltage deflection is positive or negative.

Q: What does latency mean?

A: The amount of time after the starting event or stimulus onset when the deflection occurs.

Q: What does “N” mean in an ERP component’s name?

A: A negative voltage deflection.

Q: What does “P” mean in an ERP component’s name?

A: A positive voltage deflection.

Q: What does the number in an ERP component’s name mean?

A: The approximate latency of the voltage deflection in milliseconds after stimulus onset.

Q: What does N100 mean?

A: A negative voltage deflection that occurs approximately 100 milliseconds after stimulus onset.

Q: What does P200 mean?

A: A positive voltage deflection that occurs approximately 200 milliseconds after stimulus onset.

Q: What do early ERP components typically reflect?

A: Early components typically reflect sensory perception and the brain’s initial processing of a stimulus.

Q: What ERP response may follow an auditory stimulus?

A: A negative voltage deflection may be recorded over the auditory cortex.

Q: How could researchers test whether faces and objects are processed differently?

A: They could record and average the EEG responses to faces separately from the responses to objects, then compare the two ERP waveforms.

Q: Why can’t researchers identify face and object processing differences from raw EEG voltage?

A: Raw EEG is noisy and contains activity from many processes, so a clear response to each stimulus type cannot easily be identified.

Q: What happens to the EEG data for the face/object condition?

A: EEG trials recorded after different faces/objects are aligned to stimulus onset and averaged to create a face/object ERP.

Q: What is cognitive subtraction in an ERP experiment?

A: It is comparing the ERP waveforms from two experimental conditions to isolate brain activity associated with the process that differs between them.

Q: What is spectral analysis?

A: A method that breaks an EEG voltage signal into sine waves of different frequencies and measures the strength, or power, of each frequency.

Q: What is power in spectral analysis?

A: The strength or amount of activity at a particular frequency.

Q: Why do researchers examine the power of different frequencies?

A: Different frequency bands may be associated with different sensory or cognitive functions.

Q: What is the dependent variable in spectral analysis?

A: Power, or the strength of activity at different frequencies.

Q: What is a spectrogram?

A: A graph showing how frequency power changes across time.

Q: What do the axes of a spectrogram represent?

A: Time is on the x-axis, frequency is on the y-axis, and color represents power.

Q: How can spectral analysis compare face and object processing?

A: Researchers calculate and average frequency power across all face trials and separately across all object trials, then compare the conditions.

Q: What is cognitive subtraction in spectral analysis?

A: Comparing conditions to identify differences in the power of particular frequencies over time.

Q: What frequency difference was found in the theta range?

A: Faces produced greater power than objects at approximately 6–8 Hz across the stimulus interval.

Q: What frequency difference was found in the alpha range?

A: Objects produced greater power than faces at approximately 8–16 Hz during the first half of the stimulus interval.

Q: How does spectral analysis differ from ERP analysis?

A: ERP analysis compares voltage changes over time, while spectral analysis compares the power of different frequencies over time.

Why perform scalp EEG recordings? 3 reasons

1. Direct measure of neural activity

2. Precise temporal resolution 

3. Easy to collect data from humans 

Why not perform scalp EEG recordings? 

1. Poor spatial resolution -> can't control where the signals originated in the brain 

• Many possible places where the signals may originate

What is ECoG / Intracranial electroencephalography

• The measurement of the brain’s electrical signals, where the recording is taken “intracranially” or from within the skull 

how does ECoG work?

• Several electrodes are placed either on the brain’s surface or deep within the brain of a patient 

• Human performs task while recording takes place 

• Surface electrodes “listen” to ______ neurons

• Deep electrodes “listen” to _________ neurons

• many neurons 

• one or a few neurons 

What kind of signals do they measure? 

• The same as signal unit and scalp EEG (depending on the type of electrodes used) 

They measure _____________ from single neurons, if a _________ electrode is implanted within the brain

firing rate / spikes / action potentials

micro

They also measure _________ from many neurons, if a _________ electrode is placed on the brain’s surface. These are commonly called ______________.

Voltage signals

macro

local field potentials

Why perform intracranial EEG recordings? 

• Direct measure of neural activity -> accessing electrical activity of neurons

•  Precise temporal and spatial resolution

Why not? 

Limited population of people who want to do this -> recordings come into specific patients who have electrodes in their brain (patients with epilepsy)