What is tau_r and tau_d in 'exp2' kernel model?

[Jessica0218]

I'm trying to use OASIS for calcium deconvolution. I'm a beginner in this field and I don'know which kernel model is recommended.

In this part：
% pars: parameters for the specified convolution kernel. it has
% different shapes for differrent types of the convolution model:
% 'ar1': scalar
% 'ar2': 2 x 1 vector, [r_1, r_2]
% 'exp2': 2 x 1 vector, [tau_r, tau_d]
% 'kernel': maxISI x 1 vector, the kernel.

What's the difference between this three models?
I've tried the 'ar1' model but the result seems not good. Some calcium event are in the falling edge of the spikes, which is not expected.
Next I plan to use 'exp2'. It seems to be related to tau (the timescale of the sensor). GCaMP6f is used for imaging so the tau is 0.7s, but what value should tau_r, tau_d be?
Thank you very much.