A 63-year-old man is brought to the emergency department after being found unresponsive at home. He was last seen well 2 hours ago. On arrival, he is somnolent and does not open his eyes to voice. He withdraws from painful stimuli but does not follow commands. Pupils are equal and reactive to light. Corneal reflexes are intact. Oculocephalic (doll’s eyes) reflex is present. Respirations are regular at 14/min and oxygen saturation is 98% on room air. Noncontrast CT head shows no acute hemorrhage. CT angiography suggests occlusion of a perforating branch supplying the paramedian midbrain. Damage to which of the following structures best explains this patient’s decreased arousal?

(Assume no intoxication or metabolic derangement.)