Definition and Overview
Quantitative electroencephalography (QEEG) is a brain function assessment method that digitally processes electroencephalographic (EEG) signals recorded from multiple scalp electrodes to quantitatively analyze frequency components, power spectra, inter-electrode phase relationships (coherence), and asymmetry [1].
Following the standard international 10-20 electrode placement system, electrical signals are simultaneously recorded from multiple brain regions using 19 to 256 channel electrodes. The analysis results are visualized as brain maps, allowing the functional status of each brain region to be assessed at a glance.
While standard EEG primarily provides qualitative evaluation of epileptic discharge waveforms and focal abnormalities, QEEG objectifies functional problems by quantitatively comparing functional connectivity, activation patterns, and deviations from normative reference databases [1].
Frequency Bands and Clinical Significance
Brain waves are classified according to their oscillation frequency as follows.
Delta Waves (Delta, 1–4 Hz)
Delta waves predominantly appear during normal deep sleep (slow-wave sleep). An increase in delta activity during wakefulness may suggest brain injury, stroke, or severe metabolic disturbances. Focal increases in delta waves reflect stroke or tumor sites [1].
Theta Waves (Theta, 4–8 Hz)
Frontal theta waves are associated with attention and working memory. Excessive frontal theta activity is linked to ADHD, depression, and cognitive decline [3]. The alpha-theta transitional state is a period during which relaxation and creative insight occur.
Alpha Waves (Alpha, 8–13 Hz)
Alpha waves are the hallmark of a relaxed, wakeful state. They increase in the occipital region when the eyes are closed (alpha synchronization) and decrease upon eye opening or initiation of cognitive tasks (alpha desynchronization). Frontal alpha asymmetry is related to emotional processing, and a pattern of increased left frontal alpha has been observed in depression [3].
Beta Waves (Beta, 13–30 Hz)
Beta activity increases during active thinking, concentration, and problem-solving. Excessive beta waves (particularly high-frequency beta) are associated with anxiety, hyperarousal, and increased muscle tension. Low beta activity may reflect decreased concentration and reduced cognitive function.
Gamma Waves (Gamma, 30–100 Hz)
Gamma waves are involved in higher-order cognitive processes, perceptual binding, and focused attention. Reduced gamma activity has been reported in Alzheimer's disease.
Clinical Applications
ADHD
Increased theta and decreased beta activity in the frontal-central region (elevated theta/beta ratio) are characteristically observed in ADHD [5]. QEEG is utilized for differentiating ADHD subtypes and monitoring neurofeedback treatment outcomes. According to meta-analyses, neurofeedback treatment in children with ADHD produced significant improvements in attention and hyperactivity [5].
Depression
Excessive left frontal alpha (frontal alpha asymmetry) and increased frontal theta activity are associated with depression [3]. Research is ongoing into the use of QEEG patterns for predicting antidepressant treatment response. In QEEG-guided TMS therapy, protocols targeting asymmetric regions are employed.
Anxiety Disorders and Post-Traumatic Stress Disorder (PTSD)
Excessive frontal beta activity and reduced frontal alpha reflect anxiety and hyperarousal states. Neurofeedback training aimed at alpha enhancement is used for anxiety reduction [4].
Autonomic Nervous System Function Assessment
The insula and anterior cingulate cortex are key cortical structures in autonomic regulation. Evaluating the functional activity and connectivity of these regions with QEEG helps elucidate the central mechanisms underlying autonomic dysfunction. Combining HRV analysis with QEEG enables simultaneous assessment of peripheral autonomic status and central regulatory function.
Post-Stroke Rehabilitation
Delta-theta wave distribution around stroke lesions correlates with sites of neurological impairment, and changes in brain wave patterns can be tracked throughout the recovery process. QEEG changes before and after TMS treatment are used to monitor neuroplastic responses.
Traumatic Brain Injury (TBI)
Following head trauma, asymmetric increases in delta-theta activity and decreased alpha activity may be observed in diffuse axonal injury. QEEG serves as a complementary tool for evaluating functional damage not visible on MRI.
QEEG Examination Process
Preparation
- Wash your hair on the day of the test and avoid using hair products (gel, spray).
- Avoid caffeine intake for 12 hours prior to the examination.
- Ensure adequate sleep before the test (sleep deprivation affects brain wave patterns).
Test Procedure
1. An EEG electrode cap with conductive gel is placed on the scalp according to the international 10-20 system.
2. The impedance (resistance) of each electrode is maintained below 5 kΩ.
3. Brain waves are recorded for 3–5 minutes with eyes open and 3–5 minutes with eyes closed.
4. Additional recordings are made during cognitive tasks (reading, arithmetic, etc.).
5. After artifact removal (eye movements, muscle noise), frequency analysis is performed.
Interpretation of Results
Deviations are calculated as z-scores by comparison with a normative reference database [2]. Color-coded brain maps visualize excess (red) or deficit (blue) states for each frequency band across brain regions.
QEEG-Based Neurofeedback
Individualized neurofeedback protocols are designed based on QEEG findings [4]. Patients receive real-time brain wave feedback (visual and auditory signals) and train to modify their brain wave patterns in the desired direction. For example, protocols targeting reduction of frontal theta and enhancement of beta activity are used to improve concentration [5].