The EEG (electroencephalogram) activity in infants evolves rapidly during the first year of life, reflecting both the maturation of the brain and the increasing organization of neural networks. Here's a detailed breakdown of the EEG patterns at various developmental stages:

1. First Year of Life: Key Characteristics

  • Slow, High Amplitude Delta Activity: In the first months of life, the EEG is dominated by slow, high-amplitude delta waves, which are typically associated with deep sleep and immaturity of the central nervous system. Delta waves are generally in the 1-2 Hz range and have a large amplitude, reflecting the developmental stage of the brain.
  • Symmetry and Reactivity: EEGs should be continuous and symmetric, meaning there should be no significant differences between the left and right hemispheres. During the first few months, the EEG is not highly reactive to environmental stimuli like eye opening, and this reactivity begins to improve as the child matures (typically around 2-4 months of age).

2. Developmental Milestones (2-12 Months)

2-4 Months

  • Posterior Dominant Rhythm (PDR): Starting at around 2-4 months, a Posterior Dominant Rhythm (PDR) starts to emerge. The PDR is a rhythm of 4-5 Hz and becomes more defined as the child approaches 6 months. This rhythm represents the transition from a predominantly immature, diffuse pattern to more organized cortical activity.

6 Months

  • PDR Strengthening: The PDR becomes a more stable 4-5 Hz rhythm during wakefulness.
  • Theta Activity: As the brain matures further, theta rhythms (4-8 Hz) begin to emerge and are more mixed with the remaining delta activity. This shift marks a transition in brain function towards more complex patterns of brain activity and increased awareness and processing of stimuli.

12 Months

  • Mature PDR: By 12 months, the PDR reaches 6 Hz, reflecting further maturation. The brain activity becomes more organized, and there is a clearer anterior-posterior gradient in the EEG, with the posterior regions (back of the head) showing the strongest PDR.
  • Theta and Delta: As the child approaches one year, theta activity becomes more prominent, replacing some of the delta activity. This represents a shift towards more complex cognitive and sensorimotor processing.

3. Drowsiness and Sleep Patterns

  • Drowsiness: During drowsiness, the EEG typically retains slow, high-amplitude activity, with delta and low-frequency theta waves being the dominant features. These waves can have amplitudes up to 200 µV.
  • Sleep Stages: The infant EEG also reflects the development of different sleep stages, especially in the first year of life:
    • Stage II Sleep Spindles: These are brief bursts of rhythmic brain activity that develop by 2 months. Early in life, these spindles can be very long (up to 15 seconds) and may remain asynchronous (not coordinated across both hemispheres) for a period of time, typically up to 2 years.
    • Vertex Waves: Sharp, high-amplitude waves that appear in stage I sleep. These are prominent in infants and are different from the adult version of vertex waves. They can come in long, repetitive runs and are not to be confused with abnormal brain activity like seizures.
    • K Complexes: These are large waves that occur during stage II sleep and are part of the sleep spindle complexes. They can be prominent in children.

4. REM Sleep in Infants

  • Proportion of REM Sleep: In the first few months, REM sleep can make up about 50% of the infant's total sleep time, reflecting the ongoing development of the brain. However, by 1-2 years, this proportion decreases to about 30-40%. REM sleep is associated with brain development, memory consolidation, and other cognitive processes.

5. Activation Techniques in Infants

  • Photic Stimulation: In infants, photic (light) stimulation is usually not effective before 6 months. After this time, a slow, 1-3 Hz driving response (where the EEG rhythm synchronizes with the frequency of the flashing light) can be seen. This reflects the brain's increasing ability to respond to external stimuli.
  • Hyperventilation Response: Prolonged crying or other stress responses can cause a hyperventilation-induced slowing of the EEG. This is a normal phenomenon and can be observed in infants as a part of their immature regulation of breathing and nervous system responses.

Summary of EEG Patterns by Age

  • 2 months: High-amplitude delta activity, symmetrical, and low reactivity to external stimuli.
  • 4 months: Onset of PDR at 4-5 Hz, with more theta mixed into the delta activity.
  • 6 months: Defined PDR of 4-5 Hz, emergence of theta rhythms.
  • 12 months: PDR reaches 6 Hz, clear anterior-posterior gradient, more prominent theta activity.