EEG instrumentation, montage, polarity, and localization

Question 1

Apical vs basal dendrites

Answer 1

Each pyramidal neuron has an apical dendrite and multiple basal dendrites (Fig.2.1). Excitation of the postsynaptic membrane at the apical dendrite leads to depolarization with an intracellular shift of positive ions (Na+). Subsequently the extracellular space nearby becomes relatively negatively charged. This is coupled by an inhibitory potential at the basal dendrites with a relatively positive charge nearby.

Question 2

At which cortical layers are neurons aligned in a perpendicular fashion with the cortex allowing for a summation of the small potentials generated by each neuron which they fire synchronously?

Answer 2

At cortical layers III, V, and VI, neurons are aligned in a perpendicular fashion with the cortex.

Question 3

What determines whether a positive or negative potential at an electrode

Answer 3

Whether a positive or negative potential is recorded on the scalp electrode depends on the location of the recording electrode with respect to these dipoles.

Question 4

Epileptiform discharges

Answer 4

Are typically surface negative

Question 5

Electrode contacts

Answer 5

Commonly used electrodes for scalp EEG have a contact surface made of non-depolarizing chloride-treated silver. International standards specify that electrode resistance should be between 100 and 5000Ω. Properly applied electrodes show a resistance of a few hundred ohms.

Question 6

What minimum number of electrodes are recommended for scalp EEG?

Answer 6

21

Question 7

10-20 system

Answer 7

The 10-20 system has 21 electrodes. With this system, inter-electrode distances average from 4 to 6cm, as the “10” and “20” mean that the distances between adjacent electrodes are either 10% or 20% of the total nasion-inion or right ear–left ear distance of the skull. In addition, only the superior lateral temporal region is covered.

Question 8

10-20 system vs 10-10

Answer 8

Only the superior lateral temporal region is covered in the 10-20 system. In the 10-10 system includes sub temporal coverage.

Question 9

What does each channel represent?

Answer 9

Each channel represents the difference in potential between two electrodes, including between a referential/ground electrode and average.

Question 10

Channels vs chains

Answer 10

Channels make up chains - temporal chain, parasaggital chain, etc.

Question 11

Bipolar vs referential

Answer 11

In a bipolar montage, external noise can easily be canceled out as it measures the difference in potential between contiguous electrodes, hence amplifying local potentials. Visual detection of differences in local potentials is easier on a bipolar montage particularly when “phase reversal” is seen, signifying a negative event taking place in the region of the electrode that is common to the two channels where polarity changes.

A referential montage on the other hand would be highly susceptible to external noise but it would be able to detect both local (near field) and distant (far field) potentials. The amplitude of the deflection on a referential montage would be a closer representation of the absolute potential at an electrode.

Question 12

Electrocereberal potentials are in the …

Answer 12

Microvolt range

Question 13

Differential amplifiers and common mode rejection

Answer 13

Each electrode records potentials generated by both the brain and the environment. Filtering out the surrounding noise is done with a differential amplifier, which excludes the signals recorded by both electrodes in a channel and amplifies the differences in between. This function is also known as common mode rejection.

Question 14

Filtering the EEG signal

Answer 14

Conventional EEG interpretation requires the exclusion of very low frequencies using a high-pass (or low frequency) filter, very high frequencies using a low-pass (or high-frequency filter) filter, or a specific band of frequencies using a high-pass filter.

Question 15

A signal filtering device

Answer 15

A signal-filtering device is made from a circuit containing a capacitor and a resistor. A capacitor contains two conducting surfaces separated by non-conducting material. When placed as a part of a circuit, opposing charges will accumulate on each plate until each plate is “crowded” and the current stops (Fig.2.9). If this is a part of a circuit with a direct current (DC), then no further current may pass once the capacitor is saturated.

If, however, the circuit has an alternating current (AC) source, then once the polarity of the source is reversed a new current may pass in the circuit until the plates of the capacitor are once more saturated, though with opposite polarity.

Increasing the frequency of the AC current above the limit of the saturation of a capacitor will allow for a current to pass continuously through the circuit.

In the past, EEGs were obtained using analog recorders. Frequency filtering in these machines was done with devices that utilize resistor/capacitor circuits. Such filters are characterized by their time constant, which determines what frequencies will pass through.

The time constant is determined by the amount of resistance and capacitance in the circuit. It is defined as the time needed to discharge the capacitor in the circuit to 36.8% of its initial full charge. Its value is inversely related to the frequency that will pass through the filter.

For example, using a filter with a higher time constant will allow the lower frequencies to pass through. (It takes more time for current to move through the capacitor, so you do not need to have as high a frequency of the AC to continue charging is how I think of it.)

Question 16

Low pass filter

Answer 16

A low-pass filter (also known as high-frequency filter) allows frequencies lower than a certain value to pass. The low-pass filter is set to 70Hz in the usual scalp EEG reading settings. Changing this to 35Hz will allow only frequencies lower than 35Hz to pass through. This will filter out a lot of the faster myogenic artifact and will also slightly reduce the amplitude of signals with a steep rise time, such as epileptiform spikes and sharp waves.

Question 17

High pass filter

Answer 17

A high-pass filter (also known as low-frequency filter) allows higher frequencies to pass, usually set to about 1Hz (corresponds to a time constant of about 0.16s) for routine scalp EEG reading. Raising this to 2Hz, for example, will filter out some of the lower frequencies giving a more flat look to the EEG. This will also reduce the amplitude of the slower waveforms.

Question 18

Band pass aka band stop filter

Answer 18

A band pass or a band stop filter is also used. Commonly used such filters are notch filters, which stop a very narrow band around the 50 or 60Hz noise generated from alternating current sources such as city power lines.

Question 19

Filtering in intracranial recordings

Answer 19

Subdural and depth electrodes allow the recording of frequencies that fall outside the usual range of the scalp EEG. These include high frequencies ranging from 80 to 500Hz known as high-frequency oscillations (HFOs) and very slow frequencies appearing as slow baseline shifts. Specialized systems are needed to acquire these activities. High-pass and low-pass filters are manipulated to facilitate viewing the required range.

Question 20

Machine reference

Answer 20

In digital EEG machines, an additional electrode used as the machine reference is also needed. The signal from each electrode is recorded as the difference in potential between that electrode and the machine reference, which is then stored as digital data (bits).

Question 21

Sampling rate

Answer 21

The signal from each channel is recorded and stored at regular intervals. This is reflected in the sampling rate of the EEG machine. Most current commercially available machines have a sampling rate that ranges between 256 and 1024Hz. Higher sampling rates allow more accurate recording of brain signals and smoother appearance of the waveforms, but require higher data storage capacity.

Question 22

Aliasing and sampling rate

Answer 22

Sampling rates determine what EEG frequencies can accurately be represented. If the sampling rate falls below a certain point, then the resulting waveform would no longer represent the original one. This erroneous representation is called aliasing.

Question 23

Nyquist theorem

Answer 23

The Nyquist sampling theorem determines that the sampling rate should be at least twice the frequency of the original signal to avoid aliasing, or distortion of waveforms.

Question 24

ACNS sampling rate guidelines

Answer 24

The ACNS guidelines recommend a sampling rate at 3times or more the frequency of the original signal.

Question 25

Anti-aliasing filter

Answer 25

A filter is used on the signal prior to digitizing to exclude all frequencies above a certain frequency determined by the Nyquist theorem (anti-aliasing filter).

Question 26

Monitor display

Answer 26

Most LCD monitors can display 1920dots (pixels) horizontally. The sampling rate on the EEG machine may actually exceed the capacity of the monitor display. Low-definition monitors will give a “grainy” tracing, and this could be a particular concern with high-frequency activity.

Question 27

EEG and electrical outlet for earth grounding

Answer 27

Proper grounding during EEG is an important patient safety issue. The EEG machine should be connected to a three-pronged hospital grade outlet. The third prong ensures shunting of excess current from the EEG machine to the earth ground. All electrical devices in the EEG room should be connected to a common earth ground.

Question 28

Patient ground electrode

Answer 28

A single ground electrode is placed anywhere on the patient and connects to the appropriate jack in the input jackbox of the EEG machine. The patient should not be connected to the earth ground.

In ICU setting, a patient may be connected to another electrical device with a ground connection. Double grounding should be avoided in these situations.

Question 29

What type of artifact is this?

Answer 29

a Electrode pop artifact. Poor contact at the P3 and C4 electrodes resulted in an isolated potential at these two contacts (60-Hz notch filter on). b The notch filter is removed and the 60-Hz artifact is now seen at the P3 and C4 electrodes, which have higher impedance due to poor contact with the scalp. The difference in impedance compared with the other electrodes interferes with the ability of the differential amplifier to reject the 60cycle noise which actually gets amplified.

Question 30

What type of artifact is this?

Answer 30

Movement artifact. The disorganized EEG potentials do not have the typical field seen in brain-generated waveforms.

Question 31

What type of artifact is this?

Answer 31

Phone-ringing artifact

Question 32

What type of artifact is this?

Answer 32

Sweat artifact. Slow undulation (less than 1Hz) of the EEG tracing is seen in a diaphoretic patient.

Question 33

What type of artifact is this?

Answer 33

Myogenic artifact. These high-frequency activities are generated by the frontalis and temporalis muscles; therefore, these are seen maximally in the anterior midline and temporal chains.

Question 34

What type of artifact is this?

Answer 34

Rhythmic myogenic artifact is seen during chewing.

Question 35

What type of artifact is this?

Answer 35

Blink artifact. The cornea has a slightly positive potential compared to the retina. During a blink, the eyelid makes contact with cornea allowing for that positive potential to be recorded from the anterior frontal electrodes. This is represented as a down-going waveform, which falls in amplitude exponentially from the front to the back.

Question 36

What type of artifact is this?

Answer 36

Lateral eye movement artifact and lateral rectus spike. In this example, the patient is reading a book, the rapid saccade to the left brings the positive potential of the cornea closer to the left frontal channels producing a positive phase reversal at F7 and away from the right with a resulting negative phase reversal at F8 (arrows). This is followed by a slow rightward movement of the eyes with the potentials slowly shifting to the opposite direction (ovals). A small spike preceding the saccade is noted which is generated from the left lateral rectus muscle (arrowhead).

Question 37

What type of artifact is this?

Answer 37

Glossokinetic artifact. The difference in potential between the tip and the base of the tongue produces diffuse, slow waves with a frontal maximum.

Question 38

What type of artifact is this?

Answer 38

ECG artifact. Small sharp transient can be seen time locked to the ECG QRS potentials. The lower tracing shows this ECG contamination during an EEG performed for the evaluation of electrocerebral inactivity.