Summary Task 7 - fNIRS & BCI

TASK 7: FNIRS & BCIS
FNIRS

FUNCTIONAL PRINCIPLES

 Functional state of a tissue can influence its optical properties
 Responding to stimuli  physiological changes in the brain  affects its optic
properties
 fNIRS capitalises on this change in optic properties by using near-infrared light (700-
900nm) to measure physiological changes
 Increase in neural activity  increased glucose & oxygen consumption
 Reduction in local glucose & oxygen stimulates brain to increase cerebral blood flow
(CBF) & cerebral blood volume (CBV)
 Increased CBF carries glucose & oxygen (carried by oxygenated haemoglobin) to
the area
 Increase of deoxygenated haemoglobin as oxygen is withdrawn from
haemoglobin to use in metabolization of glucose
 Overabundance of cerebral blood oxygenation in the active area
 Oxygenated (oxy-Hb) & deoxygenated haemoglobin (deoxy-Hb) – have different characteristic
optical properties in visible & near-infrared light range  changes in concentration
can be measured using optical methods
 Optical window – oxy-Hb & deoxy-Hb reflect specific wavelengths
 Photons introduced at scalp pass through most of the tissue & are either
absorbed, scattered, reflected back from oxy-Hb & deoxy-Hb
 Can be measured at surface of the skin using photodetectors

MEASURE ABSORBANCE / REFLECTANCE CHANGES AT TWO (OR MORE) WAVELENGTHS –
ONE SENSITIVE TO OXY-HB & THE OTHER TO DEOXY-HB  CHANGES IN RELATIVE
CONCENTRATIONS CAN BE CALCULATED

THE APPARATUS

 Light source coupled to the participant’s head via source / optodes (= light-emitting
diodes (LEDs) / fibre-optical bundles)
 Photodetector – light detector, receives light after it has been reflected form the tissue
 Light takes a banana shaped pathway from source to detector
 Placed 2-7cm away from optode – because light is scattered
after entering
 Optimal distance: 3-4cm  fNIRS signal becomes sensitive to
haemodynamic changes within top 2-3mm of cortex

,  The further the distance between detector & optode, the deeper the signal can go
in the brain BUT signal gets weaker, because more volume has to be passed
 Spatial coverage depends on optode montage – you can switch source & detectors to
include blind spots
 High-density fNIRS – more sources & detectors  more information, could build 3D
activation map

DIFFERENT TYPES OF FNIRS


Time-resolved &  Provide info on phase & amplitude  necessary for more precise
frequency quantification of fNIRS signals
domain systems


Continuous wave  Apply light to tissue at constant amplitude, measuring attenuation of
systems amplitude of the incident light
 Provides less info than time / frequency domain systems
 Advantages: (1) can use LEDs rather than lasers, (2) cheaper
manufacturing, (3) can be very small


LIMITATIONS & ADVANTAGES

 Advantages: (1) non-invasive & safe, (2) noiseless, (3) portable & compact, (4) low-cost, (5)
more ecologically valid than other brain imaging techniques, (6) more individuals can
be evaluated comfortably, (7) can be integrated with other technologies, (8) high
signal-to-noise ratio, (9) high single-trial reliability, (10) no inverse problem, (11) ease of
application, (12) low sensitivity to head motion artifacts
 Limitations: (1) low to medium spatial resolution, (2) limitations in use of cranial reference
points, (3) attenuation of light signal by extracerebral matter, (4) comparisons of fNIRS
data between subjects, (5) impact of skin pigmentation on signal detection, (6) difficult
to obtain absolute baseline concentrations of oxy-Hb & deoxy-Hb, (7) indirect measure
of brain activation, (8) limited depth perception

FNIRS VS. FMRI


fNIRS fMRI


(1) Indirect measure of neuronal activity, (2) assess changes in relative concentration of
deoxy-Hb, (3) similar temporal resolution – dependent on HR, (4) safe & non-invasive, (5) can be
used repeatedly with same individuals, (6) need repeated stimulation due to SNR


Worse spatial resolution (1cm3) Better spatial resolution (1mm3)