Answer:
He is showing a decreased responsiveness to a stimulus.
Step-by-step explanation:
According to conditioning theory, habituation is the decreased responsiveness to a stimulus after repeated presentations of that stimulus. The term is used to describe behavior that has not been conditioned and is product of unconscious processes, like reflexes.
In this case, the stimulus has not been associated with any reinforcement, the baby's response to the sound of the keys is a reflex which, after several presentations of the same stimulus, decreases.