EDIT: To sum up all the crap below:
Yes, our responses can be accurately predicted, so we technically do not have "free choice".
However, we can't actually predict them, so it does'nt matter anyway.
Sentience is just a measure of the brain's ability to create new concepts based on old ones. For example, a dog may be farily intelligent, but they cannot come up with the concept that a car may pull a trailer despite the fact that they know full well that a car is something that goes forward, and a trailer is something that needs to be pulled.
The brain's processing ability is based on a complex network of neurons that run via electrochemical processes. What we are is based on said processes; the network that is formed is what differentiates Cab Calloway from Mao. The fact that we cannot accurately predict said processes down to the sub-atomic level (see heisenberg uncertianty principle) means that, even with the same set of stimuli and past experiences, our responses may vary, if only very slightly.
(In other words, if you ask someone to pick a number between 1 and 5, they might pick 2 or 3; it's not entirely possible to calculate it exactly.)
Of course, if you had a computer of infinite size and exact data of the positions of the protons, neutrons, and electrons within the atoms that make up a brain, it might be possible to predict what we do. However, such a computer would have to be larger than the universe itself, so that's not really a possibility.