Electrical brain activity in BCIs is acquired in 3 different ways:
○ EEG (electroencephalography)
○ ECoG (electrocorticography)
○ Intracranial microarrays
Two main types of BCIs, based on:
- Spontaneous oscillations (e.g., the visual alpha rhythm - eyes closed - active)
- Event-related potentials (ERPs; e.g., attention-modulated P300)
What does it mean that most rhythms are idle rhythms? Examples.
They are attenuated during activation. Examples:
○ Alpha-rhythm (around 10 Hz) in the visual cortex - activates when eyes are closed and chills when they’re open
○ Sensory-motor rhythm (SMR; around 14 Hz) in motor and sensory cortex - activates when arm is at rest, chills when arm moves
BCIs based on ERPs examples (4)
Oddball paradigm, The Matrix Speller, Hex-o-Spell, Spelling with Spatial Auditory Attention
Oddball paradigm:
- A sequence of symbols is presented with the task to attend the occurrence of a target stimulus, while ignoring the others
- The EEG shows a specific pattern (P300) after attended targets
- The trick: This allows the BCI to conclude which symbol the user wants to select.
The Matrix Speller:
- Selecting letters in one step
- The letters (or rows and columns) are flashed in random sequence.
- The user attends, e.g., the letter B
- The flashing of the target letter elicits the P300 which can be recognized by the BCI. Other letters elicits a much weaker P300
- Gaze control necessary!
Hex-o-Spell:
- Selecting letters in two steps
- Letters are placed into groups
- The user first selects the group, then specific the letter
Spelling with Spatial Auditory Attention:
- Similar principle to the Hex-o-Spell
- User learns which tones are linked to which letters
- BCI speller is based on spatial hearing, and no visual processing
Challenges in EEG (4):
Volume conduction in EEG, Subject-to-subject variability, Session-to-session variability, Trial-to-trial variability
Explain the Volume conduction in EEG challenge
- The signal arrives with almost equal intensity at different scalp locations due to the different tissue conductivities
- Spatial smearing: raw EEG scalp potentials are known to be associated with a large spatial scale owing to volume conduction
- In this typical example data set, most of the channels are highly correlated
- The map shows the correlation 0.5 coefficient of each channel with channel Cz in the center
Explain the Subject-to-subject variability problem
- The human brain is very plastic, in particular during early development (e.g., hemispherectomy during childhood)
- BCI illiteracy:
□ Large dataset of over 150 participants using a SMR-based BCI
□ An estimated 20 - 30% of the population are not able to modulate their SMR
□ Reasons remain unclear, but potentially due to tissue conductivity differences
Independent Component Analysis (ICA):
Extracts components that are maximally independent to find likely sources in the brain (or be identified as artifacts)
Linear Discriminant Analysis (LDA)
○ Well known classifier used to separate two or more classes of subjects
○ Uses means of the two classes and the noise (covariance of two classes), to find the separating line (between the two classes), the direction of noise and separate two classes
○ Given two Gaussian distributions 𝒩 (𝜇1 , Σ) and 𝒩 (𝜇2 , Σ), LDA is defined by the normal vector
Can a spontaneous voluntary movement be cancelled after the onset of the readiness potential?
Spontaneous, voluntary movements can be cancelled after onset of the readiness potential, however only until a point of no return at around 200 ms before movement onset
