Which researcher first realized that certain "psychic secretions" pointed to a simple but important form of learning?

That researcher was Ivan Pavlov.
These psychic secretions are related to his experiments with dogs - he noticed that dogs would start salivating at the mere sight of food, or person delivering them food, or even the sound of their footsteps (because they knew food was coming). That's why he started experimenting even further - each time he brought them food, he would ring a bell, so eventually the dogs would start salivating every time they heard a bell because they were expecting food.