What does a normal electroencephalogram (EEG) look like for a resting patient?

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Normal EEG Patterns in a Resting Patient

A normal electroencephalogram (EEG) in a resting patient is characterized primarily by dominant posterior alpha rhythms (8-12 Hz) that are most prominent in occipital leads during eyes-closed conditions. 1

Key Characteristics of Normal Resting EEG

  • Posterior (dominant) alpha rhythms (8-12 Hz) are the most prominent oscillations during the resting state with eyes closed 2
  • Alpha rhythms show characteristic amplitude reduction when transitioning from eyes-closed to eyes-open condition due to activation of visual-spatial cortical systems 2, 1
  • In artifact-free recordings, delta (1-4 Hz) and theta (4-7 Hz) rhythms typically show small amplitude in healthy adults 2
  • Beta rhythms (13-30 Hz) are present but at lower amplitude than alpha during resting conditions 2

Physiological Significance of Resting EEG Patterns

  • Low-frequency alpha rhythms (8-10 Hz) reflect low levels of general brain arousal, attention, and readiness during quiet vigilance 2, 1
  • High-frequency alpha (10-13 Hz) and low-frequency beta (12-20 Hz) rhythms may reflect low levels of perceptual, somatomotor, and memory processes 2, 1
  • During sensorimotor and cognitive events, alpha rhythms are replaced by faster cortical oscillations, namely beta (20-30 Hz) and gamma (30-70 Hz) rhythms 2
  • Abnormally prominent theta or delta rhythms in the resting state are considered signs of brain dysfunction 2

Technical Aspects of Resting EEG Recording

  • Standard recording conditions include 5-15 minutes of eyes-closed recording with the subject instructed to:
    • Sit quietly
    • Stay relaxed in a state of mind wandering (no goal-oriented mental activity)
    • Keep eyes closed 2
  • EEG is typically recorded from 19-25 to >80 scalp electrodes placed according to the 10-10 montage system 2
  • Artifact-free resting state EEG data of 20-60 seconds duration is typically sufficient for reliable quantitative analysis 2
  • Fast Fourier Transform (FFT) is commonly used to analyze the power density of EEG frequencies 2, 1

Regional Distribution of EEG Activity

  • Alpha rhythms are most prominent in posterior (occipital) regions 1, 3
  • Increased alpha power correlates with decreased MRI signal in multiple regions of occipital, superior temporal, inferior frontal, and cingulate cortex 3
  • Spatial resolution of EEG is low to moderate (few centimeters) compared to other neuroimaging techniques 2

Common Pitfalls in Interpretation

  • Normal variants can be mistaken for pathological patterns if not properly identified 4
  • Age-related differences must be considered - children normally have greater low frequency power (<8 Hz) compared to adults 5
  • Drowsiness can significantly alter the EEG pattern, with alpha rhythm attenuation and increased theta activity 2
  • Residual blinking and saccades during eyes-open condition can affect reproducibility of spectral EEG measures 2

Clinical Applications

  • The test-retest reliability of resting EEG measures is generally high, with correlation coefficients of 0.8-0.9 at 4 weeks 2
  • Relative power density is slightly more repeatable than absolute power density 2
  • Changes in normal patterns, particularly decreased alpha and increased theta power, can be indicators of neurological conditions such as Alzheimer's disease 6

References

1
Guideline

EEG Indicators of Resting State

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

2
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

4
Research

Epileptiform electroencephalographic patterns.

Mayo Clinic proceedings, 1996

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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