BCI Fundamentals

Brain-Computer Interfaces (BCI) establish a direct communication channel between the brain and external devices. This technology detects central nervous system activity and translates it into output signals to replace, repair, enhance, or supplement normal physiological functinos. By analyzing neural data through sensors placed on the scalp or within the cranium, BCI systems can decode user intent, cognitive states, and neurological health conditions. This facilitates improved interaction with environments and provides rehabilitation avenues for patients with motor impairments such as paralysis or amyotrophic lateral sclerosis.

Classification of BCI Systems

BCI technologies are categorized based on the surgical invasiveness required for signal acquisition:

1. Non-Invasive: Utilizes surface electrodes (EEG) to record electrical activity without breaching the skull.
2. Semi-Invasive: Involves placing electrodes on the exposed surface of the brain during neurosurgical procedures (ECoG).
3. Invasive: Electrodes are implanted directly into the brain tissue for high-fidelity recording.

Signal Control Strategies (Non-Invasive)

Common non-invasive methodologies include:

• Steady-State Visual Evoked Potential (SSVEP): Generates neural responses by flashing stimuli at specific frequencies targeting the retina. Applications include keyboard input systems and vehicle control interfaces.
• P300 Event-Related Potential (ERP): Detects a positive voltage peak occurring approximately 300ms after a rare target stimulus is presented. Primary use cases involve text entry selection tasks.

Neuroanatomical Framework: Brodmann Areas

The cerebral cortex is divided into functional regions based on histological structure (cellular arrangement):

Electroencephalography (EEG) Standards

The International 10-20 System

This standardized method ensures consistent electrode placement regardless of head size. Distances are measured relative to anatomical landmarks (nasion, inion, preauricular points):

• Geometric Logic: 10% and 20% intervals are used along the sagittal and coronal planes.
• Nomenclature:
  • Odd Numbers: Left hemisphere.
  • Even Numbers: Right hemisphere.
  • Z: Midline coordinates (e.g., Cz).
  • Fp/Fz/Cz/Pz/Oz: Anterior-posterior midline points.
• Lobes: F=Frontal, T=Temporal, O=Occipital, P=Parietal, C=Central/Sensorimotor.
• Notation Rules:

  /   : Midpoint between two locations
  -   : Continuous span between locations
  Example: C3-T3 covers the path from left-central to left-temporal
  

Signal Acquisition Configurations

• Monopolar Montage: Measures potential difference between an active electrode and a neutral reference point (often theoretically zero).
• Bipolar Montage: Measures the differential potential between two active electrodes.

Provocative Testing Procedures

Specific actions induce electrical changes to aid diagnosis:

1. Hyperventilation: Can trigger absence seizures or spike-wave discharges.
2. Photic Stimulation: Used to diagnose photosensitive epilepsy via flashing light exposure.
3. Eye Opening/Closing: Tests for occipital alpha block or seizure generation.
4. Sleep Deprivation/Induction: Increases detection rates of paroxysmal discharges.

Electrophysiological Waveforms

Frequency Bands

Signal classification relies on oscillation rates:

{
  "Delta": "0.5 - 3 Hz",
  "Theta": "4 - 7 Hz",
  "Alpha": "8 - 13 Hz",
  "Beta": "13 - 30+ Hz",
  "Gamma": "> 30 Hz"
}

Morphology and Amplitude

• Amplitude: Vertical distance from the baseline trough to crest (typically calibrated as 1mm = 10μV).
• Morphology: Waves may appear monophasic, biphasic, or polyphasic. Composite waves (spike-and-slow-wave, polyspike) indicate pathological states.
• Polarity: Defined relative to the reference (G2). Negative polarity (G1 < G2) often deflects upward; positive polarity deflects downward.
• Phase Reversal: Occurs in bipolar montages when a common electrode shows opposing deflections across adjacent channels, localizing the focal source.

Spatial and Temporal Patterns

• Distribution: Generalized (symmetric bilateral), Focal (localized origin), or Diffuse (asymmetric/disorganized).
• Temporal Organization:
  • Random: Isolated events.
  • Paroxysmal: Sudden bursts.
  • Rhythmic: Regular repetition (>3 cycles).
  • Periodic: Discharges recurring at fixed intervals.

Sleep Architecture and EEG Correlation

Sleep stages reflect transitions in brainwave synchronization:

1. Wakefulness: Alpha rhythm dominates relaxed wakefulness; Reticular activating system maintains alertness.
2. NREM Stage I: Drowsiness; breakdown of alpha, emergence of theta.
3. NREM Stage II: Presence of sleep spindles and K-complexes.
4. NREM Stage III/IV (Slow Wave Sleep): Dominance of high-amplitude Delta waves (>50% in deep stages).
5. REM Sleep: Rapid eye movement associated with muscle atonia; EEG resembles wakeful desynchronization despite deep sleep physiology.