fNIRS Flashcards
What is near-infrared spectroscopy?
Neuroimaging technique that uses light to determine brain activity
Device is physically similar to an EEG setup (cables attached to the head and sending the signal somewhere else)
• fNIRS is a neuroimaging technique that uses light
• fNIRS is sensitive to outer cortical region of the brain
• fNIRS is based on similar principles to MRI
• Most devices measure change in haemoglobin, from which neural activity is inferred.
what are the principles of fNIRS?
Shine near infrared light (not fully infrared, travels well through body), some passes through skin and bone and into brain and we can measure it
Oxy and deoxy blood absorb red light diff so we can infer [oxy vs deoxy blood]
Source optode emits light that goes through skull, some goes through brain and back out through skull and to a detector
Measures brain activity in a small region
Not all the light will go through the path from the light source to the light detector, some will be lost => the signal we’re looking for is actually quite small
• Light sources emit at two wavelengths.
• Based on light received at detector we can infer changes in oxy- and deoxy-haemoglobin
How can fNIRS go beyond the lab?
Trying to commercialise this technology (user friendly)
Whole head FNIRS device that wear all day and
measure brain activity
Moving towards whole head but modular design that is more comfy
What are different devices used for fNIRS?
Variety of devices that go from lab based, some that are aimed to scan specific parts of the brain or targeting very specific and small regions (the ear; everyday type devices)
What are cochlear implants and their importance in fNIRS?
Practicle considerations: cochlear implant = device that restores hearing by internally (under skin), electrodes in cochlea that prod electric current that excites the nerve to prod perception of sound, external part is what capts the sound
Cant use these with MRI or MEG, and the elect signals by the cchlear implants corrupt the EEG signals
So we use FNIRS
What are 6 benefits of fNIRS?
• Easy to set up and child friendly
• More accessible than MRI & MEG
• More robust than EEG to movement
• Only probes outer brain (feature and a bug)
• Can be used at the same time as other methods (EEG)
• No electrical artifacts from cochlear implants
What are the spatial and temporal resolutions of fNIRS?
Seconds range for temporal res (poor compared to
EEG and MEG)
Poor spatial res
Non invasive
What are the biological principles behind fNIRS?
Similar to MRI, theres a typical amount of oxy and deoxy blood at rest, when theres a stim, theres an aug of é demand to that region so more blood goes there (BOLD response) and thats what we measure
• Your brain needs blood in order to function. Blood carries oxygen and glucose to the brain as fuel for your neurons.
To fuel the neurons that activate, Hb is pumped to that region
• Haemoglobin is the specific protein that oxygen binds to on your red blood cells. As your brain uses up the oxygen, the haemoglobin on your red blood cells goes from being oxygenated to deoxygenated.
• By measuring the relative amounts of oxygenated and de-oxygenated blood we have a proxy measurement for brain activity.
2 wavelengths are absorbed dif by oxy and deoxy Hb and thats how we measure diffs in the 2 [c]
• Light source emits light at two specific wavelengths
• Photons bounce off in all directions and some photons make it back to the detector
• The two wavelengths are absorbed differently depending on the concentration of oxy- and deoxy-haemoglobin
• Can estimate the concentration of oxy- and deoxyhaemoglobin between source and detector
What are the 3 types of fNIRS?
- Continuous waves NIRS
- Frequency domain NIRS
- Time domain NIRS
What is continuous waves NIRS?
Emit light, detector measures how much light comes back
Most common
Put detectors at diff depths to measure diff distances
What is frequency domain NIRS?
Modulate light, light goes through tissue, is attenuated
and slowed down (shifted in the medium) so get f info
about the tissue that the light went through
What is time domain NIRS?
Send pulse of light (not continuous), some ligth will go a short distance and come back fast, other light will goo deeper and take longer to come back so can know the depth that we’re measureing but the deeper the light goes, the more signal loss there is
What is the Haemoglobin Absorption Spectra?
CONVERSION TO HAEMOGLOBIN CONCENTRATION ESTIMATES
Shine light at 2 diff wave lengths (measure at 2 wavelengths),
Absorption coefficients of oxy and deoxy Hb
Info we have is the amount of light at the 2 f and calculate from that the amount of oxy and deoxy blood at diff t points
What is the typical pipeline for fNIRS experiments?
1) Experimental design
2) fNIRS data acquisition
3) fNIRS data pre-processing: intensity to OD conversion, OD to concentration conversion, motion artefact correction, filtering
4) Statistical inference
What does a practical fNIRS set up look like?
Each optode (source that has 2 LEDs each with one wave length and detectors) Signal is converted to electrical signal or digital signal Up to 16 sources and 16 detectors, can be uncomfortable bc of all the cables (practical considerations to get a clean signal) If optodes press on head to much and make it red and inflamed, could get confused on what is brain signal and what is inflammation Optode is placed in grommets and have to make sure that theres good physical contact with head (vs EEG that has gel). if someone has a lot of hair its harder to get a good signal, have to separate hair to get good contact 1 source and detector 3 cm apart to make sure light has room to go in brain and come back
