The midbrain is extremely reduced in humans, but an important relay center, the reticular formation, passes through it. The neurons of the reticular formation extend all the way from the central core of the medulla, through the pons, the midbrain, and on into lower regions of the forebrain. It recieves input from virtually every sense and every part of the body and from many areas of the brain as well. The reticular formation plays a role in sleep and arousal, emotion, muscle tone, and certain movements and reflexes. It filters sensory inputs before they reach the conscious regions of the brain, although the selectivity of the filtering seems to be set by higher brain centers. Through a combination of genetically determined wiring and learning, the reticular formation "decides" which stimuli require attention. Important stimuli are forwarded to the conscious centers for processing, and unimportant stimuli are suppressed. The fact that a mother wakens upon hearing the faint cry of her infant but sleeps through loud traffic noise outside her window testifies to the effectiveness of the reticular formation in screening inputs to the brain and to the role of learning in determining the importance of sensory stimulation.