EEG Parameters

Question 1

What is sensitivity?

Answer 1

Sensitivity in EEG refers to how much the machine amplifies the brain’s electrical signals so they can be seen clearly on the screen.

Question 2

Why does signal sensitivity matter?

Answer 2

EEG signals from the brain are very tiny, usually in the range of microvolts (µV). That’s millionths of a volt — far too small to see directly.

So, the EEG machine uses an amplifier to make these signals bigger. Sensitivity controls how much the amplifier stretches the voltage signals and how tall they look on the EEG screen.

Question 3

How is sensitivity measured?

Answer 3

S = microvolts per millimeter (µV/mm)

It tells you how many microvolts (input voltage) from the brain cause a 1 mm deflection (output height) on the EEG display.

Lower sensitivity value (e.g., 7 µV/mm) = higher amplification (a small signal looks big)

Higher sensitivity value (e.g., 15 µV/mm) = lower amplification (the same signal looks smaller)

Question 4

Give an example of sensitivity

Answer 4

Let’s say a brain wave is 70 µV in amplitude.

If sensitivity is set to 7 µV/mm, the wave will be shown as:

70÷7=10mmtall

If you change sensitivity to 14 µV/mm (less sensitive), the same wave now appears:
70÷14=5mmtall
👉 Same brain signal, different appearance depending on sensitivity.

🔍 Clinical importance:
Too high sensitivity = waves look small; you might miss subtle abnormalities.

Too low sensitivity = waves look too big; you might overinterpret normal variation as abnormal.

In practice, we adjust sensitivity to get the clearest picture of what the brain is doing, without distorting the signals.

Question 5

When sensitivity setting is high (for example, 30 uV/mm), the signal amplitudue of the posterior alpha background (on P4-O2) is low, this shows up as a _____________ morphology

Answer 5

Sinusoidal

Question 6

Which leads do you look at to assess posterior dominant rhythm/background alpha activity?

Answer 6

To evaluate the posterior dominant rhythm (PDR) — also known as the alpha rhythm — you typically focus on the posterior head regions, especially the occipital electrodes.

🔍 Key EEG channels to examine:
In a longitudinal bipolar montage, these channels commonly show the PDR:
O1–P3
O2–P4
P3–C3
P4–C4
In some systems, you may also see:
O1–O2 (in a referential montage)
Pz–Oz (in additional midline recordings)

✅ Characteristics of PDR:
Appears in awake, relaxed individuals with eyes closed
Frequency: 8–13 Hz (alpha range)
Best seen over occipital lobes
Disappears or attenuates with eye opening or mental activity

Question 7

Background alpha activity

Answer 7

Clinical significance of alpha activity:
1. Normal finding in awake individuals
- Seen in healthy children (usually >3 years) and adults.
- Frequency increases with age and stabilizes around 8–13 Hz in adolescence.
- Best seen in occipital regions when eyes are closed and the person is relaxed.
- It attenuates (blocks) with eye opening or attention — this is called alpha blocking or desynchronization, and it’s normal.

2. Used to assess cerebral function
   Presence of a well-formed alpha rhythm suggests:
   - Intact cortical function
   - Normal thalamocortical connectivity
   - Absence of diffuse encephalopathy
3. Abnormal findings may indicate pathology
   Absent or slow alpha rhythm may suggest:
   - Diffuse cerebral dysfunction (e.g., encephalopathy, delirium)
   - Sedation or coma
   - Developmental delay in children
   - Asymmetry between hemispheres (e.g., alpha only on one side) could suggest:
   - Focal lesion (e.g., stroke, tumor)
   - Structural abnormality

🔍 Summary:
Background alpha activity is a key marker of normal, alert brain function. Changes in its frequency, symmetry, or reactivity can point to neurological problems, making it a vital feature to assess in routine EEGs.