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Signal Characteristics - Coggle Diagram

- - - - source and detector optodes usually spaced 3cm apart
        
        light can penetrate cortex without becoming too diffuse
        
        spatial resolution limited to cm scale
        
        light can only reach cortical surface (~1.5cm from scalp)
        
        records the HRF
  - - - data may reflect distortions due to participant physiology
      - physiological monitoring recommended to separate neuronal changes from non-neuronal distortions
    - - can be avoided with careful setup
  - - - Physiological Noise
        
        Mayer waves
        
        low-frequency rhythmic oscillations in blood pressure
        
        around 0.09Hz
        
        Breathing
        
        around 0.2Hz
        
        Heart rate
        
        around 1.3Hz
        
        important that task frequency does not overlap with physiological components
      - Head Motion
        
        ⬆️ tolerant of head motion than fMRI
        
        not immune to motion
        
        optodes may shift relative to head
        
        altered coupling between scalp and source/detector
        
        changed distance between source and detector
        
        artefact that changes with motion (spikes with amplitude much larger than true signal)
        
        biased/false conclusions when motion-induced signal change falsely attributed to neuronal event
        
        Solutions
        
        CBSI
        
        Premises
        
        motion leads HbO and HbR to become more positively correlated
        
        HbO and HbR strongly negatively correlated
        
        Solution
        
        HbO :arrow_right: \(x_0 =\frac{1}{2}(x-\alpha y)\)
        
        HbR :arrow_right: \(y_0 = -\frac{1}{\alpha}x_0\)
      - Instrument noise
- - - - around 5 to 15 cm
      - difficult to localise signals to specific origin
    - - caused by eye, face or jaw movements
      - difficult to remove
  - - - transient changes in neuronal signals from small behavioural changes detectable with EEG
      - sample rates upwards of 1kHz
    - - Delta, Theta, Alpha, Beta, Gamma