EEG Derived Neuronal Dynamics during Meditation: Progress and Challenges

Table 2

EEG brainwave frequency bands [4, 70, 78–81].

Frequency bands with subbands	Brain regions	Characteristics	Pathologies
Delta (<4 Hz)	(i) Frontal in adults; (ii) Posterior in children	(i) Tendency to be of the highest amplitude; (ii) During deep sleep stages in adults, appear in infants up to one year; (iii) Slowest brain rhythms; (iv) Minimal conscious brain involved; (v) FIRDA (Frontal Intermittent Rhythmic Delta): 2 to 3 Hz having amplitude 50–100 mV generated frontally, in wake adults needing differentiation from artifacts like slow eye blinks; (vi) OIRDA (Occipital Intermittent Rhythmic Delta): linked largely with seizures in children	(1) Focal delta activity: abnormal indicating lesions; (2) Lower delta power: complex depression, continual migraine, closed posterior head/neck damage; (3) Excess delta activity: learning disability, Alzheimer’s disease, edema
Theta (4–8 Hz); (i) Theta1 (4–6 Hz); (ii) Theta2 (6–8 Hz)	Medial prefrontal and anterior cingulate cortex	(i) Amplitude 30–60 µV (FmTheta); (ii) In normal children and infants, in adults during drowsiness, sleep, and meditation relaxed state; (iii) Neural indicator of inner processes requiring self-management; (iv) FmTheta: theta rhythm of 6-7 Hz centered around 6.5 Hz, a good indicator of continuous attention, linked with less anxiety, less phobic activity	In normal awaken adults, very small theta activity but a high value accounts for pathological conditions
Alpha (8–13 Hz); (i) Alpha1 (8–8.9 Hz); (ii) Alpha2 (9–10.9 Hz); (iii) Alpha3 (11–12.9 Hz)	(i) Posterior on each side of the head with higher amplitude on leading side; (ii) Faster at posterior, slower at anterior recording positions	(i) Amplitude < 50 μV; (ii) Dominant frequency in adults; (iii) Tasks requiring attention, semantic memory; (iv) Easily seen with closed eyes and under mental inactive conditions; (v) Lower alpha band (alpha1, alpha2) is an index of internalized attention; (vi) High alpha band (alpha3) is an index of engagement in task demands; (vii) ERD: during bigger task demands, alpha is desynchronized and theta is synchronized; (viii) Alpha blocking occurs when eyes are opened in a well lit room. The waves are dominated by fast low amplitude beta rhythms; (ix) Mu rhythm: alpha activity in sensorimotor cortex	Alpha coma, a diffused alpha in EEG occurs in coma which does not respond to external stimuli
Beta (13–30 Hz); (i) Beta1 (13–18 Hz); (ii) Beta2 (18–22 Hz); (iii) Beta3 (22–30 Hz)	(i) Frontocentral; (ii) Posterior-occipital in infants which shifts to frontal areas as we grow up	(i) Usually amplitude ≤ 30 μV; (ii) In all age groups in alert and anxious subjects enhanced by expectation states; (iii) Beta1 consists of corticothalamic feedback loop that changes attention level, beta bursts shift the system into attention state that allow gamma synchronization; (iv) Beta2 during focused managerial processing	(1) Decreased beta activity: focal lesions, stroke, or tumor diffused encephalopathy such as anoxia; (2) Excess beta activity during alcoholism along with decreased alpha and theta
Gamma (>30 Hz)	Different brain regions	(i) Extremely fast frequency with lowest amplitude; (ii) Provides synaptic plasticity for long-term memory by closely tracking the local change in blood flow; (iii) Higher in focused stimuli	Sometimes of no clinical interest and filtered out in EEG recordings

Although most researchers use these universally accepted frequency ranges, since frequency varies with age, neurological diseases, brain volume, task requirements, and memory performance, some researchers use their own range of boundaries.
Also, in some studies, decimal values have been used for defining frequency bands instead of whole numbers.
Increased delta activity considered with normal adults performing calculations, reaction time tests in some studies.