Event-Related Potentials

Electromagnetic activity is generated by neurons in the cerebral cortex and subcortical structures of the brain. Neurons generally differ from other cell types of the body by being specialized for the reception, integration, and conduction of excited states. The shape of the neuron is related to its specialized function. Two types of branching extension of the cell body, the dendrite and the axon, allow the neuron to receive excited states from, and transmit them to, other neurons. The dendrites are specialized to receive excited states (and inhibition) from other neurons at synapses, where the release of chemical transmitter from the presynaptic axon terminal causes postsynaptic ion channels gated by ionotropic receptors to open. Transmembrane current flow through the open ion channels ensues, driven by the electromotive force across the postsynaptic membrane (1). The electromotive forces at synaptically opened ion channels distributed over the branches of a neuron’s dendritic tree drive current flow in closed loops. Excitatory synapses create loop currents consisting of net positive charge that flows inward across the postsynaptic dendritic membrane, passes through the intracellular compartment, flows outward across passive membrane with a strength that decreases with distance from the sites of influx, and finally completes the loop through the extracellular space. The excitatory postsynaptic potential is a depolarization of the dendritic transmembrane potential due to this net inward flow of positive current across the postsynaptic membrane. Loop currents created by inhibitory synapses flow in the opposite direction (1,2). The net outward flow of positive current across the postsynaptic membrane at inhibitory synapses produces a hyperpolarization called the inhibitory postsynaptic potential. Loop currents establish a gradient of transmembrane potential that continuously varies in time and spatially along the dendrites as a function of current strength. The sum of currents contributed by all the active synapses on the dendritic tree produces a resultant transmembrane potential at the cell body and the initial segment of the axon. A critical effect of the loop currents occurs when this resultant potential exceeds the firing threshold and the initial segment responds by generating a train of pulses (action potentials). Each pulse, with a relatively fixed amplitude and duration, actively propagates along the axon, diverging into axonal branches and reaching all the branch terminals, at whose synapses chemical transmitters are released. In the extracellular space, loop currents generated by neighboring neurons summate when they flow in the same direction, and cancel otherwise. The passage of current across the resistance in this space is manifested by an extracellular electrical field of potential, or field potential. The field potential recorded by an electrode in the extracellular space represents the sum of potentials associated with the loop currents generated by a set of active neurons. The intracellular components of the same closed-loop currents that give rise to the field potential are primarily responsible for the closely related magnetic field (3). The magnitude of the field potential recorded by an extracellular electrode (with respect to a neutral reference) at any instant in time depends on multiple factors, including the number of active nearby neurons, the strength and directions of their currents, their morphology and alignment, and the position of the electrode in the field. For a population of neurons to generate a strong field potential, it is not sufficient that the neurons actively generate strong extracellular currents. The morphology and alignment of those neurons must also promote the summation of the currents in the extracellular space. For example, the field potential generated by a population of neurons in which the orientations of the dendrites are uniformly distributed in all directions is zero, on average, due to cancellation of extracellular currents, even if the individual dendrites are all maximally excited. On the other hand, parallel alignment of the dendrites promotes extracellular current summation if the same portion of each dendrite, e.g., the distal end, is excited. However, cancellation may still occur if the location of the excitation is randomly distributed along the dendrites. In general, populations in which the neurons each have a single long dendrite aligned in parallel across the population, and concurrently receive either excitation or inhibition at the same dendritic locale, e.g., distal or proximal end, tend to generate extracellular currents that maximally summate and augment the field potential. This type of population is called a dipole generator and the field it generates is a distributed dipole field, meaning that the summated loop currents which emerge from one end (pole) of the dendrites are detected by an extracellular electrode there as a current source, and the currents that enter into the other end (pole) are detected by an extracellular electrode there as a current sink (1). An important property of such a dipolar source-sink population geometry is that it generates an open field, meaning that the currents spread in the volume of the brain and can be detected at a distance from the generating population (4). A superficial cortical sink is recorded as a negative potential, and a superficial source as a positive potential, by an electrode in the superficial cortical layers, at the cortical surface, or at the scalp (5). The question of what causes field potentials to change over time is central to understanding the relation of ERPs to brain function. Although the determinants of temporal variation of the field potential are diverse, and their effects are not well understood, some basic aspects of neuronal population activity that bring about temporal variation of its generated field potential may be identified (6). One