Forgive what will be a series of very beginner questions.
I have never processed physiological data from scratch before. We use a GE Healthcare Discovery MR750 3T MR System and obtain PPG and RESP data.
My first issue is I have not been able to find online WHAT exactly the y-axis is when the data is obtained. I understand the PPG, "Pulse monitoring uses a photopulse sensor to detect blood flow in the vascular bed of the patient’s finger" so seemingly is a measure of heart beats while, "The Respiratory bellows detects the motion of the abdominal wall" but if anyone knows exactly what the numbers correspond to that would be really great to know. I've emailed a few scan techs and a physicist at my scanning location and surprisingly have not received any useful reply.

However that's not the main issue. I would like to use these physiological signals as regressors when preprocessing my task data and understand I can get these regressors with RetroTS. However when I graphed the raw RESP and PPG data from 3 subjects from the same experimental run they look pretty different. I attached two screenshots showing each measure. I suppose I'm just wondering how to handle physiological data in general: Should either of these measure THEMSELVES be preprocessed or "cleaned" before running RetroTS? For example, subject 1's (blue) RESP data drops to 0 sometimes. Is this bad? If so how should I handle it? Is there more information or resources anyone can provide about this?

Thanks so much for your insight.
Lauren