Electroencephalography (EEG)


           EEG uses electrodes placed directly on the scalp to measure the weak (5–100 μV) electrical potentials generated by activity in the brain. Because of the fluid, bone, and skin that separate the electrodes from the actual electrical activity, signals tend to be smoothed and rather noisy. Hence, while EEG measurements have good temporal resolution with delays in the tens of milliseconds, spatial resolution tends to be poor, ranging about 2–3 cm accuracy at best, but usually worse. Two centimeters on the cerebral cortex could be the difference between inferring that the user is listening to music when they are in fact moving their hands.
                                                                Fig. EEG signal 1 channel
           The theoretical and the application studies are based on the knowledge that the EEG signals are composed of waves inside the 0-60 Hz frequency band and that different brain activities can be identified based on the recorded oscillations. For example, signals within the delta band (below 4 Hz) correspond to a deep sleep, theta band (4-8 Hz) signals are typical for dreamlike state, alpha frequencies (8-13 Hz) correspond to relaxed state with closed eyes, beta band (13-20 Hz) are related with waking activity and gamma frequencies (20-50 Hz) are characteristics for mental activities as perception and problem solving. [1]

Delta Waves  [2]
  • Too much: Brain injuries, learning problems, inability to think, severe ADHD
  • Too little: Inability to rejuvenate body, inability to revitalize the brain, poor sleep
  • Optimal: Immune system, natural healing, restorative / deep sleep
Theta Waves  [2]
  • Too much: ADHD, depression, hyperactivity, impulsivity, inattentiveness
  • Too little: Anxiety, poor emotional awareness, stress
  • Optimal: Creativity, emotional connection, intuition, relaxation 
Alpha Waves  [2]
  • Too much: Daydreaming, inability to focus, too relaxed
  • Too little: Anxiety, high stress, insomnia, OCD
  • Optimal: Relaxation
Beta Waves  [2]
  • Too much: Adrenaline, anxiety, high arousal, inability to relax, stress
  • Too little: ADHD, daydreaming, depression, poor cognition
  • Optimal: Conscious focus, memory, problem solving

Reference:

[1]  Desney S. Tan .Anton Nijholt, “Brain Computer Interfaces Applying Our Minds To
Human Computer Interaction”,2010, page number [vi, 7, 8, 9 ]
[2]  http://neurosky.com/2015/05/greek-alphabet-soup-making-sense-of-eeg-bands/

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