Event-Related Potential Recording

Event-Related Potential Recording

Sam Kaminsky

University of New Brunswick

        Introduction

        This semester, my fellow students and I got together with Dr. Harker to learn about ERPs.  An ERP is an event related potential, which is the electrical potential that is created by the neurons in your brain in response to an event (Luck, 2005). To learn the methodology of performing an ERP experiment, we participated in an “experiment” where we looked at faces of people we knew and faces of people we did not know. Had this been a real experiment, we would have compared our brains responses to a familiar face to its response to a new face by comparing the electrical response elicited by each type.  However, our goal this semester was simply to learn how to set up and run an experiment like this.  First and foremost we had to have an understanding on how ERPs are created.  In order to understand this, we had to first have a general understanding of the basic principles of electricity.  

Principles of Electricity

        Ohm’s Law states that current (I) is equal to voltage (V) over resistance (R),       (I = V/R) (Sullivan, n.d.).  Current is the flow of electrons and voltage is the difference in electric potential between two points (Sullivan, n.d.).  When two points have different electricity, electrons are attracted to the more positive point, so they move from the negative to the positive end creating a current (Lewis, 1998). Voltage basically acts as a force that pushes the electrons from negative to positive.  Going back to Ohm’s Law you can see that current is directly proportional to voltage, so the more voltage means more current (Sullivan, n.d.).  Resistance is the force that opposes the flow of electrons through a substance (Resistance, 1998).  In terms of Ohm’s Law, resistance is inversely proportional to both current and voltage, so a higher resistance will reduce the flow of electrons and the voltage (Sullivan, n.d.).  So, electrons want to flow (current) from negative to positive (voltage), but the substance through which they are flowing has properties that can slow down this flow (resistance).  

How ERP Recording Works

        This relates to the activity of neurons because neurons release excitatory or inhibitory neurotransmitters that cause either a positive or negative electrical change in the cell.  This change creates two points that have a different electricity (outside the cell vs. inside the cell), meaning there is a voltage, or an electric potential across the cell membrane (Luck, 2005).  This happens in your brain in two ways, through action potentials and through postsynaptic potentials (PSPs).  An action potential occurs when the voltage in the neurons spikes, and this spike in voltage travels along the axon causing neurotransmitters to be released. This voltage spike occurs because electrons travel outside the cell, creating a greater positive charge inside and a greater negative charge outside leading to an even greater electric potential (Luck, 2005).  A postsynaptic potential occurs when these neurotransmitters land on the dendrites of another neuron and cause a voltage change on this postsynaptic neuron by opening ion channels and allowing electrons to flow in and out.  When an excitatory PSP occurs at the dendrites of a neuron, the cell becomes more negative because electrons flow in.  As a result, current will also flow out of the cell body creating a positive and a negative end of the neuron.  This is called a dipole, when there is a net negative and net positive charge separated by a small distance (Luck, 2005).  When this happens across many neurons that are oriented in the same way and at the same time, the dipoles summate.  However, this summation is much rarer than one would think because dipoles can be easily cancelled out by another dipole in the opposite direction or of opposite charge.  Inhibitory PSPs cancel out excitatory PSPs, and PSPs of opposite orientation also cancel each other out.  It is rare that two neurons are of the same orientation because the cortex has many folds (Luck, 2005). The cortex is a good conductor though, so when a dipole does summate, the signal travels through the cortex until it reaches the skull, where we can use electrodes to pick up this signal.  As it travels through the brain, the signal spreads out and finds the path of least resistance towards the skull.  Once it reaches the high resistance of the skull, it spreads out further laterally.  Due to all this spreading, it is very difficult to determine the location of generation (Luck, 2005).

         Action potentials are much harder to pick up at the scalp since they often cancel each other out. This happens because action potentials are so swift that they rarely ever occur at the same time or place in order to summate and reach the scalp.  However, there is the rare time that this could happen, so it is said the ERP recordings are of both PSPs and action potentials, but mostly just PSPs (Luck, 2005).

Data Collection

        Each of us participated in a paradigm where we had to determine if the face shown on the screen was a familiar or non-familiar face, while our brain activity was being measured by a 20 or 21 channel electrode cap.  Remember that a voltage is measured between two points, the cap measures the voltage changes between active electrodes and reference electrodes.  These variations are called EEG (Conduit, n.d.).