Physiological Basis Of The

The EEG represents a set of field potentials recorded by multiple electrodes on the surface of the scalp. The electrical activity of the EEG is an attenuated measure of the extracellular current flow from the summated activity of many neurons. The surface EEG predominately reflects the activity of cortical neurons close to the EEG electrodes. Deeper structures, such as the hippocampus, thalamus, or brainstem, do not contribute directly to the surface EEG. However, transmission of electrical impulses from distant sites has substantial effects on the surface EEG. For example, thalamocortical connections are critical in the synchronization of electrical activity, such as sleep spindles. Oscillatory EEG patterns arise because of pacemaker cells, in which membrane voltage fluctuates spontaneously, or because of the reciprocal interaction of excitatory and inhibitory neurons in circuit loops. The human EEG shows activity over the range of 1 to 30 Hz, with amplitudes in the range of 20 to 300 ^V.

The waveforms recorded by the surface electrodes depend on the orientation and distance of the electrical source with respect to the recording electrode. To understand how the EEG is obtained, it is useful to examine a single pyramidal neuron situated in layer 5 of the cortex, with its apical dendritic arbor above, although it is clear that EEG activity derives from thousands of such neurons functioning within networks.

Figure 5 shows such a single neuron with current flowing into the dendrite at the site of generation of an EPSP, creating a current sink. Current flow must complete a loop and, therefore, this generates a source somewhere along the dendrites or cell body. The size of the voltage change created by the EPSP is predicted by Ohm's law, V = IR. The Rm (membrane resistance)

Neurons Source Sink
Fig. 5. Current flow in a cortical neuron. See text. Modified from Kandel et al., 2000 with permission.

is much larger than the extracellular fluid and, therefore, the corresponding voltage recorded by an intracellular electrode is larger, and opposite in polarity to an extracellular electrode positioned near the current sink. At the site of generation of an EPSP, the extracellular electrode detects current (positive ions) flowing away from the electrode into the cytoplasm as a negative voltage change, whereas the intracellular electrode detects a positive change in voltage caused by the influx of Na+ ions. An extracellular electrode near the source has an opposite deflection. The direction of voltage change is determined by location in regards to the sink and source. In Fig. 5, note the differences in pen deflection depending on whether the extracellular electrode is near the source or sink.

If instead of an apical (layer 2/3) EPSP, we examine the instance of a basal IPSP, close to the soma of such a pyramidal neuron, the current flow loops and derived extracellular and intracellular recordings turn out to be identical to those described above for the superficial EPSP. In both cases (a superficial EPSP or a basal IPSP), a dipole is created with separation of charge oriented vertically in the cortex, with extracellular negativity in more superficial laminae, and extracellular positivity in deeper laminae.

Now consider how these extracellular field potentials will behave when recorded from the scalp. It is fortunate that the cortex is organized as a sheet just under the scalp, with an

Polarity Eeg
Fig. 6. Polarity of the surface EEG depends on the location of the synaptic activity within the cortex. See text. Modified from Kandel et al., 2000 with permission.

intrinsic columnar organization. It is these pivotal features that afford our ability to obtain a useful EEG signal. The highly organized columns of pyramidal neurons are arranged just so, with cell bodies in deeper laminae, and dendritic arbors extending upward to laminae that are more superficial. Excitatory afferent fibers innervate the superficial layer 2/3 dendrites, whereas inhibitory contacts favor the deeper cell bodies below. When such EPSPs are generated because of the coordinated excitation of numerous afferent inputs into layer 2/3 dendrites, broad regions of the cortex coordinately generate transient dipoles that lead to measurable extracellular voltage negativity, as recorded by scalp electrodes.

Figure 6 illustrates these anatomic and physiological properties. In Fig. 6, there are afferent inputs into either the apical dendrites (A) or cell body (B). In both cases, the afferent stimuli lead to depolarization (sink) with current flow into the cell body. This results in negativity extracellularly. The current flow in A results in a source in the apical dendrites, whereas, in B, the source is located at the soma. The examples thereby lead to two vertically oriented dipoles of opposing polarity. Surface EEG electrodes will detect the extracellular electrical fields generated closer to the cortical surface (i.e., the superficial laminae), and there will be less influence from activity occurring at the cell body. Therefore, the deflection of the pen is opposite in the two conditions. Again, as stated above, a deep IPSP will generate a similar scalp electrical field as will a superficial EPSP—both lead to scalp negativity.

Because of these geometric reasons, only vertically oriented dipoles are detectable with scalp electrodes. Thus, only those portions of the underlying cortical sheet that are parallel to the scalp are detectable with EEG electrodes. Portions of the cortex that run down the sulci, oriented orthogonal to the scalp, generate radially oriented dipoles, which are not well seen by scalp EEG technique. Magnetoencephalography may be able to detect such radial dipoles with much greater facility, however.

It follows that EEG detects the summed extracellular electrical field potentials from a swath of underlying cortex. It has been estimated that each EEG electrode "sees" the summed activity of roughly 6 cm2 of underlying cortex. We obtain cogent signals because there is a significant amount of synchrony underlying the behavior of thousands of cortical neurons. This synchrony may be physiological, as seen in the alpha rhythm over the posterior channels. However, when the cortex becomes excessively synchronized, we may detect pathological EEG morphologies, termed spikes and sharp waves.

Such epileptiform features represent the summed activity of numerous rapidly firing neurons, which have been depolarized to threshold in a coordinated and excessively synchronized fashion. A wave of cortical excitation, termed the paroxysmal depolarization shift (PDS) is thought to be responsible for this broad and synchronous cortical excitation. The PDS is sufficiently strong to bring a large collection of neurons synchronously to threshold, generating a rapid and synchronized bursting of action potentials that "ride" atop the more sustained PDS potential. It is thought that PDS waves are followed by strong waves of hyperpolarization, perhaps recruited by inhibitory interneurons in the involved circuits. These pathological waves of cortical excitation and subsequent inhibition affecting broad regions of excessively synchronized cortex are thought to underlie the spikes, sharp waves, and sharp and slow wave complexes we recognize as pathological correlates in routine EEG interpretation.

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  • cathy freeman
    What are the physiological basis of EEG?
    8 months ago

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