The data does not appear to me to represent a function. There is a lot of verticality to the data. It’s doesn’t appear to me to be logistic, exponential, or a higher degree polynomial. What is the best model? I don’t know. I’m not a statistician or a neuroscientist. Maybe they have reasons for doing a linear fit in this case that we aren’t aware of. Typically experts have reasons for what they do and their reasons would seem reasonable to me if I knew what they did.