task 7 - fNIRS and BCIs Flashcards
Functional principle fNIRS
- photons introduced at scalp pass through most tissue -> absorbed, scattered or reflected back from oxy-Hb and deoxy-Hb
- Predictable quantities of photons follow a banana-shape path back to the surface of the skin -> measure by photodetectors
- Measuring absorbance/reflectance changes at 2 wavelengths (one more sensitive to oxy-Hb, other to deoxy-Hb) -> changes in relative concentration can be calculated
Optodes
light source is coupled to participant’s head via LEDs or though fiber-optical bundles (optode, or source)
-gives light
(de-)oxygenated haemaglobim in fNIRS
- Oxygenated and deoxygenated hemoglobin have characteristic optical properties in visible and near-infrared light range
- > change in concentration of molecules can be measured using optical methods
- > oxy-Hb and deoxy-Hb reflect specific wavelengths in this range
- > we mainly measure indirect brain response -> measure change in blood flow
- we measure how light intensity changes
fNIRS apparatus:
- light source is coupled to participant’s head via LEDs or though fiber-optical bundles (optode -> thing that measures light), or source)
- light detector that receives light after it has been reflected from the tissue
- light is scattered after entering the tissue -> photodetector placed 2-7 cm away from the optode can collect light after it has passed through the tissue
fNIRS resolution
Spatial:
- better than EEG
- lower than fMRI
- can’t go as deep -> approx. 1 cm
- light source and detector can only be 4cm apart from each other
Temporal:
-temporal resolution on the order of seconds bc limited by hemodynamic response
fNIRS pros
- safe = non invasive
- portable
- low costs
- more ecologically valid -> translating laboratory work into more realistic everyday settings and clinical environments
- can be integrated with other technologies (EEG and TMS) to provide multiple indices of neurological and physiological function
Application examples fNIRS
- diagnostic imaging of solid tumors (breast)
- on-site clinical assessment
- studying neonatal cerebral hemodynamics
- Neurodegenerative aspects of diseases studied over extended periods of time
- valuable as a portable bedside tool for elderly, multimorbid, and epileptic patients
- Functional brain rehabilitation through optimum neurometabolic timing and immediacy of feedback
fNIRS limitations
limited spatial resolution
- limitations in use of cranial reference points (if you put optode and detector too close / too far from each other -> signal is distorted)
- attenuation of light signal by extracerebral matter
- comparisons of fNIRS data between subjects
- impact of skin pigmentation on signal detection
- difficulties obtaining absolute baseline concentrations of oxy-HB and deoxy-Hb
- signal to noise ratio in EROS
BCI
also referred to as direct neutral interface
- system that provides direct communication link between neural activity of the brain and computer hardware/software
- > without involvement of peripheral nerves and muscles
- first introduced by using EEG to generate control commands for electronic devices
BCI purpose
primary goal: helping individuals with severe motor-disability as a result of amyotrophic lateral sclerosis (ALS), stroke, accidental injuries and other neuromuscular disorders
Yes/no communication paradigm
-relies on mental drawing (most reliable results compared to mental singing or mental talking)
- mental drawing: yes -> oxy-HB/deoxy-Hb response will be high compared with rest
- > we have certain cut off value
- no change in mental state: no
- answers were decoded offline (=after the measurement) using univariate and multivariate statistics
- useful communication means & diagnostic tool for detecting preserved conscious awareness in non-responsive patients
In mental imagery procedure, pp performed 2 localizer runs
one at beginning, one at end
-we need to create reference points
->
Purpose:
1)
-in univariate approach, used to localize the optode pair for which the activity patterns evoked by mental drawing could be best discriminated
-> analysis of the main data was focused exclusively on this best optode pair/channel
2)
-localizer data used to train a classifier decoding participants answers in multivariate approach
fMRI based BCI approach
- costly
- tied to clinical or research institutions
- unsuitable for everyday-life usage
fNIRS based BCI
- easy to apply
- inexpensive
- safe
- portable
- in this study: limited number of optodes was chosen to ensure clinical applicability ( reasonable optode placement time allowing rapid bedside measurements of patients)
- samplig rate of optical signals was 12.5 Hz -> fNIRS signal is slow
Preprocessing of data
- linear rend removal, temporal low-pass filtering and high-pass filtering
- Serves to get rid of frequencies of no interest in the measured signal
- > improves signal to noise ratio and corrects for artifacts
