AFE4400SPO2EVM: Why is the PPG Signal acquired by the AFE 44x0 GUI software on the LED IR seems like a mirror image of what it should be?

I use the AFE44x0SPO2EVM development kit together with MatLab and am getting this signal from PPG (mVolts x Seg). dicroctic notch is on the left of the peak

According to several bibliographical references the PPG signal should read: dicroctic notch is on the right of the peak.

Desenvolvimento de um Sensor deFotopletismografia para Monitorização Cardíaca para aplicação no Pulso (Rui Miguel Silva Martins, Coimbra, 2010);

What should I do to extract an acquired signal? What is the reason for this "mirroring" of the PPG signal? 

Here is a similar question, but it was not very clear to me: e2e.ti.com/.../436227