The determinant factor is the actual seat of fatigue. Correlation doesnt mean causation.
IIRC, the way this is done in laboratories is to create M-waves in 3 steps, before, after, and during a muscular contraction. A decline in M-wave amplitude indicates impairment in the processes which convert axon potential into sarcolemmal action potentials. This in effect is a beautiful measurement for neuromuscular propagation.
There are lab tests and experiments for central factors, excitation contraction coupling, and so on.
“Motivation” should be understood in this context as the will to generate maximal maximum speed, or maximum speed-strength.
Motivation is a key factor, but not enough. In addition to be motivated to generate max speed or max speed-strength, you must actually generate them. Cleaning 70% of your max will not cause significant CNS fatigue, even if you are totally motivated to lift it.
A pre-competition clean with a maximal weight which was not used before will cause fatigue. If you lack motivation in such a lift you will miss it. And also, such a lift would probably cause emotional distress even in seasoned lifters.
I think reading Charlie’s books will give you a good grasp on the practical side of CNS fatigue. It’s basically a all you need to know in practice. And also, that many trainers will be fast to yell CNS fatigue at anything which appear not easy to pinpoint, but this doesn’t mean they re right.
Also what James said about the CNS and control of body functions is very important.