Refers to an extraneous variable attributable to changes in the test units themselves that occur with the passage of time.

Maturation.

Explanation:

According to my research, I can say that based on the information provided within the question the term being defined in this statement is called Maturation. This term is used in many categories to refer to the changes over time until full maturation is achieved, which is the peak point in value.

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answer; i believe the correct answer is prefrontal cortex;