Ivan Pavlov writes a science article which includes an early description of the phenomenon of classical conditioning.
According to Pavlov:
"From our experiments it is very evident that the intensity of the stimulation is of essential importance. In contradistinction to this we must state with regard to acoustic impressions that very powerful stimuli, such as the violent ringing of a bell, were not, in comparison with weaker stimuli, quick to produce conditioned increase of function in the salivary glands.
It must be supposed that powerful acoustic stimuli produce in the body some other important reaction which hinders the development of the salivary reaction. ...
Stimulation by musical sounds or by noise in general is remarkably convenient for determining the discriminating or analytical faculty of the nervous system of the dog."