in most of the studies the oxygenated hemoglobin is more pronounced (increased almost five time than deoxy) so they  limited their analysis to oxy signals. others they reported bot; oxy and deoxy in order to validate the actual oxygenation purpose. more details can be found (Tachtsidis and Scholkmann, 2016).

Dear Sir

Andrew Benjamin Gundra,

I am working on fNIR for almost 2 years. Once upon a time I also seek this question. As far I learnt from my research and the other literature based on fNIR, everyone uses oxy, de-oxy, or total oxygenation change or both oxy and deoxy data. But in all case, they used the data that gave them significant result or significant difference. By ANOVA or U test you may not get any significant differences in case of oxy data, then you must try for deoxy or total change in oxygenation or deoxygenation. Some times both results are combined to make the information meaningful. I think this is just to convince and make the research meaningful.

If I become wrong in my opinion, I beg your pardon in advance.

Thanks

Md. Asadur Rahman

BME, KUET, Khulna, Bangladesh.

Dear Andrew,

Both changes in oxy- and deoxyhemoglobin concentration are different entities, so what you decide to publish is linked to your research question. However, some researchers choose to only report the significant changes. 

In practice, oxyhemoglobin (O2Hb) is mainly linked to the oxygen inflow of the tissue, while deoxyhemoglobin (HHb) is linked to the amount of oxygen absorbed by the tissue. In homeostasis, both the inflow of O2Hb and the formation of HHB should be constant, as the amount of oxygen being consumed by the tissue is equal to the amount of oxygen being carried towards the tissue. During activation of the tissue (e.g. excitation of brain areas or straining of muscles), oxygen is consumed within the tissue and hemodynamically the tissue responds by increasing the flow of blood toward that tissue. Combining both O2Hb and HHb yields the total hemoglobin concentration changes, which are linked to changes in total blood volume  in the tissue underneath the sensor. Which one of those entities you pick depends therefore on your research question, however plotting both O2Hb and HHb is preferred as this yields the most information. 

The text-book model is that the an increase in oxygen demand from the tissue instigates an increase in O2Hb due to neuro-vascular coupling as depicted by Scholkmann et al. in figure 5 of their 2014 review article (http://www.sciencedirect.com/science/article/pii/S1053811913004941). However, NIRS is sensitive for all changes in tissue properties, so if for example the brain is measured and something like blood pressure changes in the extracerebral tissue this is also picked up by the NIRS-signal. A beautiful example is the article by Luuk van de Rijt et al., in which a shallow reference channel was used to "filter out" the extracerebral effects: http://journal.frontiersin.org/article/10.3389/fnhum.2016.00048/full.

If there are problems with significance, you could try more trials, adaptation of your measurement protocol (make sure no hair is in between the optode and the skin), adaptation of the task or more sensitive equipment (here is where I mention I work for Artinis Medical Systems).

Best regards,

Mathijs Bronkhorst