Which of the following statements concerning nootropics is not correct?

a) They improve the highest integrative brain functions (memory, learning, understanding, thinking and the capacity for concentration)

b) They stimulate the bulbar respiratory center

c) They stimulate existing neuronal synapses to optimum performance (adaptive capacity)

d) They stimulate existing neuronal synapses to damaging influences, such as disturbances of the energy and neurotransmitter metabolism or ischemia (protective capacity)