1. Awake State:

PDR (Posterior Dominant Rhythm): In a relaxed, awake state with eyes closed, the EEG shows a rhythmic activity called the alpha wave (usually 8-13 Hz), which is most prominent in the posterior regions of the brain (occipital lobe). This rhythm is symmetrically present on both sides of the brain, hence the term "posterior dominant rhythm." When the eyes are open, alpha activity decreases and is replaced by beta waves (14-30 Hz), which are faster and appear more prominent in the frontal regions.

Absence of Delta Activity: Delta waves (0.5-4 Hz) are associated with deep sleep, and their presence in an awake adult EEG is abnormal.

Artifacts: Various external factors or body movements cause artifacts in the EEG:

  • Eye Blinks: These produce a sharp, transient deflection in the EEG due to the movement of the eyelids and changes in the electrical potentials near the eyes.
  • Movement Artifacts: These are caused by body movements or muscle contractions, creating chaotic, high-amplitude waves that can obscure the true brain signals.
  • Myogenic Artifacts: These are low-frequency, high-amplitude signals caused by muscle activity, particularly from the forehead muscles, which manifest as high-frequency, low-amplitude activity, most noticeable in the frontal regions.
  • Chewing Artifacts: When chewing, muscles in the face and jaw contract, generating electrical signals that appear as periodic, low-frequency signals on the EEG.

2. Drowsy State:

Mild Diffuse Slowing: As the person begins to fall asleep, the alpha rhythm gradually fades, and the background EEG slows down. There’s a general reduction in the frequency of brain activity, but it’s not as pronounced as in deep sleep.

Decreased Frequency of Eye Blinks: As drowsiness sets in, the frequency of eye blinks decreases, and the individual may start to have periods of prolonged eye closure.

Roving Eye Movements: These are slow, rolling movements of the eyes, which become more noticeable during drowsiness. The frontal regions (F7, F8) will show the typical slow waves due to the shift in electrical potential when the eyes move back and forth. This is because the corneas are positively charged, and the different charges between the two eyes during movement create opposing waveforms in the frontal electrodes.

Slow Undulating Waves in the Frontal Region: The movement of the eyes back and forth during drowsiness generates slow waves in the frontal region. These undulating waves are a hallmark of the drowsy state, and their slow, rhythmic appearance is often seen in the early stages of sleep transition.

3. Transition to Stage I Sleep (Stage I Sleep):

POSTS (Posterior Occipital Sharp Transients of Sleep): These are sharp, brief waves (typically 1-2 Hz) that appear in the occipital region of the brain during the transition from drowsiness to sleep. They are the hallmark feature of stage I sleep and represent the beginning of true sleep.

Vertex Waves: These are sharp, high-amplitude waves that are often seen over the central part of the brain (vertex region, top of the head) and mark the initiation of sleep. They appear in stage I sleep and are also seen during the transition from awake to sleep.

Summary of Key Features in Each State:

  • Awake: Dominated by alpha and beta waves with various artifacts (eye blinks, movement, myogenic, and chewing).
  • Drowsy: Mild slowing, decreased eye blinks, roving eye movements, and slow, undulating waves in the frontal regions.
  • Transition to Stage I Sleep: Appearance of POSTS and vertex waves indicating the onset of true sleep.