One way researchers study newborn development involves measuring how long infants continue to focus upon a nonchanging stimulus. This technique is referred to as

Habituation

Explanation:

Habituation: In psychology, the term "habituation" is described as one of the "psychological learning processes" in which an organism's response to a specific stimulus is being decreased after the organism is repeatedly exposed to the given stimulus.

The concept of habituation underlies the concept that an organism including animals and human beings gains the tendency to surpass or ignore a specific stimulus if they have been exposed repeatedly to the given stimulus.

In the question above, the given technique is referred to as habituation.