Cortical Origins of EEG Signals

Hello! Welcome to the fifth module of our course. As we discussed at the end of our last lesson on cardiac output, we are now shifting our focus from the cardiovascular system to the nervous system. This module is dedicated to the principles of Electroencephalography (EEG), a topic that lies at the core of your work with Neuraease.

Today's lesson addresses a fundamental question: What exactly is an EEG signal? We'll fulfill the learning outcome: Describe the neurophysiological origin of EEG signals from cortical neuron populations.

Understanding this is not just academic; it's the bedrock for everything that follows. When you design a wearable device, filter out noise, or apply machine learning to interpret EEG data, a clear mental model of the signal's origin is invaluable. It helps you understand the signal's characteristics, its limitations, and the physiological meaning behind its patterns.

1. The Big Picture: What an EEG Signal Is (and Isn't)

At its simplest, an EEG measures electrical voltage fluctuations on the scalp. It's a common misconception that EEG directly measures the "spikes" of neuronal firing, known as action potentials. This is not the case for two main reasons:

1. Duration: Action potentials are extremely brief events (1-2 milliseconds). Their electrical fields are too short-lived to summate effectively and be detected by distant scalp electrodes.
2. Geometry: The electrical fields generated by action potentials tend to be closed loops that cancel each other out over a distance, making them difficult to detect from the scalp.

So, if not action potentials, what are we measuring?

Scalp EEG detects the summed electrical activity of large populations of neurons firing in synchrony. One key resource, "Basic EEG Electrophysiology," notes that it requires at least 6 square centimeters of synchronized cortical activity for a signal to be detectable on the scalp. This is a crucial point: EEG reflects the coordinated activity of vast neural networks, not individual cells.

2. The Fundamental Building Block: The Postsynaptic Potential

The electrical activity that does summate and become detectable is the postsynaptic potential (PSP). This is the small, slow voltage change that occurs in a neuron when it receives a chemical message from another neuron across a junction called a synapse.

To understand how these PSPs are generated, let's watch a short video that provides a clear review.

As the video explained, there are two types of PSPs:

• Excitatory Postsynaptic Potential (EPSP): Occurs when neurotransmitters cause positive ions (like Na⁺) to flow into the neuron. This makes the inside of the neuron more positive (a process called depolarization) and brings it closer to firing an action potential.
• Inhibitory Postsynaptic Potential (IPSP): Occurs when neurotransmitters cause negative ions (like Cl⁻) to flow in, or positive ions (like K⁺) to flow out. This makes the inside of the neuron more negative (hyperpolarization) and makes it less likely to fire an action potential.

From your ECE background, you can think of these PSPs as slow-changing analog input signals to the neuron. Many of these small inputs must sum up in space and time to reach a threshold, which then triggers the fast, digital, all-or-nothing output pulse—the action potential. EEG is concerned with measuring the sum of the inputs (PSPs), not the outputs (action potentials).

3. From a Single PSP to a Measurable Signal

An individual PSP is far too tiny to be detected on the scalp. The magic of EEG relies on three key ingredients: a special type of neuron, its organization, and synchronized activity.

3.1 The Pyramidal Neuron: The Star Player

The primary source of the EEG signal comes from pyramidal neurons located in the cerebral cortex (the outer layer of the brain). These neurons are special for two reasons critical to EEG:

1. Structure: They have a long, straight trunk-like dendrite (the apical dendrite) that extends towards the surface of the cortex.
2. Organization: They are arranged in dense columns, all aligned parallel to each other and perpendicular to the cortical surface.

This parallel alignment is key. Imagine thousands of tiny batteries. If they are oriented randomly, their voltages cancel out. But if you line them all up in the same direction, their voltages add up. The pyramidal neurons are organized just like this.

3.2 The Electrical Dipole

When a PSP occurs on a pyramidal neuron, it creates a separation of charge along the cell's long axis. For example, an EPSP at the apical dendrite (near the surface) causes an inflow of positive ions.

• The inside of the dendrite becomes locally positive.
• This pushes positive ions away from the outside of the dendrite, leaving the extracellular space near the dendrite with a net negative charge.
• The opposite end of the neuron (the cell body, or soma, deeper in the cortex) becomes relatively positive in the extracellular space to maintain electrical balance.

This separation of a negative charge at one end and a positive charge at the other creates an electrical dipole. The following video provides a concise explanation of this process and how these tiny dipoles summate.

3.3 Summation and Synchronization

As the video showed, the signal from one neuron is undetectable. However, when thousands or millions of adjacent pyramidal neurons receive a synchronized volley of synaptic inputs (all EPSPs or all IPSPs at the same time), their individual electrical dipoles align and sum together. This creates a much larger electrical field, strong enough to be measured at the scalp.

This image provides a wonderful summary, showing how activity scales from a single neuron's dendrites to a cortical column and is finally detected by a scalp electrode.

Test your understanding!

Imagine two scenarios:

1. A single, large action potential fires in one cortical neuron.
2. Thousands of adjacent pyramidal neurons receive small, synchronized excitatory postsynaptic potentials (EPSPs) on their apical dendrites.

Which scenario is likely to produce a detectable EEG signal, and why?

Show answer

Scenario 2 is the one that produces a detectable EEG signal.

The single action potential in Scenario 1, while a large voltage change locally, is too brief and its field is not oriented correctly to be detected by a distant scalp electrode.

In Scenario 2, even though each individual PSP is tiny, there are two crucial factors:

• Synchronization: The PSPs occur at the same time across many neurons.
• Summation: Because the pyramidal neurons are all aligned in parallel, their individual dipoles add up constructively, creating a large, composite electrical field that is strong enough to be measured on the scalp.

4. The Journey to the Electrode: Volume Conduction

The electrical field generated by the cortical neurons doesn't reach the electrode directly. It must pass through several layers: the brain tissue itself, the cerebrospinal fluid (CSF), the meninges (protective membranes), the very resistive skull, and finally the scalp. This process is called volume conduction.

These layers have two main effects on the signal:

1. Attenuation: They significantly weaken the signal. The voltage measured at the scalp (in the microvolt range, µV) is much smaller than the voltage at the cortical surface.
2. Filtering: The skull, in particular, has high electrical resistance. This acts as a low-pass filter, smearing the signal spatially and preferentially blocking higher frequency components. This is a primary reason why scalp EEG is most sensitive to signals below ~100 Hz.

The following reading provides more detail on volume conduction and another fascinating aspect: how the location of the synapse on the neuron affects the signal we see.

By convention in EEG, negative voltage is plotted as an upward deflection on the trace. As you just read, an upward deflection (negative potential at the electrode) can be caused by either:

• An EPSP at a superficial/apical dendrite (making the nearby extracellular space negative).
• An IPSP at a deep/basal dendrite (making the deep extracellular space positive, which in turn makes the superficial extracellular space relatively negative).

Understanding this complexity is key to advanced EEG interpretation, but for now, the main takeaway is that the signal we see is a complex sum of activity occurring at different depths in the cortex.

Conclusion

Today, we've peered under the hood of the EEG signal. You now have a solid neurophysiological model for the data you work with.

Key Takeaways:

• EEG does not measure individual action potentials. It measures the summed postsynaptic potentials (PSPs) of large, synchronized neuron populations.
• Pyramidal neurons in the cortex are the primary source of the EEG signal due to their specific structure and their parallel, columnar organization.
• A PSP creates a charge separation along the neuron, forming an electrical dipole.
• The summation of thousands of these tiny dipoles creates an electrical field strong enough to be detected on the scalp.
• The signal travels via volume conduction through the head, where it is attenuated and low-pass filtered, particularly by the skull.
• The polarity of the EEG signal (upward or downward deflection) depends on both the type of PSP (excitatory vs. inhibitory) and its location on the neuron (superficial vs. deep).

Preview of the Next Lesson:
Now that we understand what the EEG signal is and where it comes from, our next logical step is to learn how to measure it systematically. In the next lesson, we will cover the 10-20 international system for electrode placement and discuss different montage types, which are standard ways of configuring electrodes to view brain activity.