[Homer-users] Odd HRF shape: possible explanations?

[Scholkmann Felix]

Dear Luca,


>From my own fNIRS studies done with neonates and from published papers I know that there are several factors that are causing the HRF of neonates being different compared to the HRF of adults:


(1) Normaly, the amount of movement artifacts in fNIRS signals from neonates is larger compared to recordings from adults. This, in combination with the filtering, can cause distortions of the shape of the HRF. In oder to prevent this, all trials that are heavily contaminated with artifacts should be rejected. If the artifacts are not that severe the signal can be reconstructed. There are several methods available. For example, the algorithm published in 2010 [1] (I can send you the Matlab code if you need it), or other methods as implemented in NIRS-SPM and Homer.


(2) According to findings from Chen et al. [2] the neuro-vascular coupling comprises two components: a fast and a slow mechanical change in the endothelium. The fast component (due to endothelial hyperpolarization) has a high propagation speed (about 2 mm/s), the slow one (due to calcium wave-associated dilation) propagated much slower. These two compoents form the shape of the HRF. In adults, these two mechanism work quite well - which is not the case in neonates. In neonates, one mechanis is well developed, the other not - as shown by Kozberg et al. [3]:

"Our data suggest that there are at least two vascular mechanisms at work in normal neurovascular coupling: (i) a global, delayed constriction process, present from early development and (ii) a localized initial hyperemia that develops gradually and is not in place at birth."

So, in neonates (depending on the age), the neuro-vascular coupling is different and will produce different HRFs than in adults.


(3) fNIRS signals are contaminated with physiological signals coming from the extracerebral compartment (skalp blood flow) and systemic physiology affecting the global cerebral blood flow. See the review of 2016: [4]. CW-NIRS systems with only one source-detector system are susceptible to these artifacts.

If there is a global changes in blood pressure, the fNIRS signals will be affeced. This is especially true for neonates - as nicely shown by Kozberg et al. [3]:

"We also observed that stimulus-evoked systemic blood pressure increases can cause apparent adult-like positive BOLD cortical responses in neonates consistent with immature autoregulation. We conclude that the parallel development and complex interplay of autoregulation, neurovascular coupling, and neuronal function in the developing brain should be carefully considered when interpreting hemodynamic measures such as fMRI BOLD in infants and children."

See also the figures in this paper that visualize the problem.


(4) And finally, the shape of the HRF depends on the type of filtering applied. The main parameters are the lower and upper cutoff-frequencies of the bandpass-filter and the filter type (as well as the order of the filter). Changing these parameters will change the shapes. In addition, the shapes depend also on the choise of either using the mean or the median for calculating the block-averages. I recommend using the median.


In summary, there are many factors involved causing "abnormal" HRF shapes in fNIRS recordings of neonates. It's a mixture of physiology and technology that determines how the shapes will look. The best was is to report precisely all the signal processing steps performed and to conduct additional physiological measurements (e.g. hear rate, SaO2, respiration rate, etc.) during the fNIRS experiments in order to allow a correct physiological interpretation of the fNIRS signals.


Kind regards,

Felix


[1] http://iopscience.iop.org/article/10.1088/0967-3334/31/5/004/meta

[2] http://jaha.ahajournals.org/content/3/3/e000787

[3] https://www.ncbi.nlm.nih.gov/pubmed/23426630

[4] http://neurophotonics.spiedigitallibrary.org/article.aspx?articleid=2502725






________________________________
Von: homer-users-bounces at nmr.mgh.harvard.edu <homer-users-bounces at nmr.mgh.harvard.edu> im Auftrag von Cecile Issard <cecile.issard at etu.parisdescartes.fr>
Gesendet: Sonntag, 12. März 2017 17:10
An: homer-users at nmr.mgh.harvard.edu
Betreff: Re: [Homer-users] Odd HRF shape: possible explanations?


Hello Luca,

What are the 40 s you average? Only one block? Or a block + silence? It seems to me that 50 to 80 ms is very short in between blocks.

Best,

Cécile Issard
PhD student
Laboratoire Psychologie de la Perception
Université Paris Descartes- CNRS
Paris, France

Le 11 mars 2017 5:06 PM, Luca Filippin <luca.filippin at gmail.com> a écrit :
Hi,
   we've been using Homer2 to analyze some data collected on newborns and we get some puzzling HRF shapes as result, averaged over a 40 secs temporal window The function has in some cases multiple humps: it rises then descends a bit and then rises again, descends again and so on, 2 or more times. In some other cases it simply rises and doesn't fall at all.

Would you have any explanations for that?

We believe that it has something to do with the stimuli design.

Our stimuli are sounds of variable length between 18 and 22 secs, roughly. Each sound is made of block of N repeated short sound segments (for example: BI), interleaved with other blocks of M sound segments of the same type BI, but shorter duration:

N x BI (pause) M x shorter BI  (variable pause) N x BI (variable pause) M x shorter BI  (variable pause) ...

where "variable pause" is a silent segment of duration between 50 and 80 ms.

We wonder if  there's a way to do the analysis so to disentagle the contribution of each block of sound segment within a sound stimulus. We play different sounds during the experiments: each sounds onset is separated by roughly 50 seconds. The problem would be perhaps, the estimation of the baseline.

Many thanks for your help,
Luca




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