In the midbrain, the regulate not only arousal generally but also the ability to pay attention to specific stimuli.

The right answer is reticular formation.

The reticular formation is a nervous structure of the brain stem at the interface of the autonomous, motor and sensory systems. It intervenes in the regulation of major vital functions (such as sleep-wake cycles), control of reflex or stereotyped motor activities, such as walking or postural tone, and in cognitive functions such as attention. Because of its central role in the regulation of alertness, lesions of the reticular formation often result in coma.

well when looking at this if you think about it   negativley and positivley   react with each other and when the first layer gets full they add layers it starts with 2 on the first layer and it constantly goes on until the official number is there