I just noticed that in the online documentation, the definition of FANN_THESHHOLD_SYMMETRIC is :
FANN_THRESHOLD_SYMMETRIC Threshold activation function.
x < 0 -> y = 0, x >= 0 -> y = 1
Reading the code, we have this (fann.c):
neuron_it->value = (fann_type) ((neuron_sum < 0) ? -multiplier : multiplier);
So the definition would rather be :
x < 0 -> y = -1, x >= 0 -> y = 1