A 62-year-old man undergoes resection of a posterior fossa tumor. Postoperatively, he is awake and following commands but develops an abnormal breathing pattern characterized by frequent, shallow breaths with a reduced inspiratory time and decreased tidal volume. Pulse oximetry shows intermittent desaturations that improve when he takes deeper breaths on command. Cranial nerve exam shows no facial weakness; corneal reflex is intact; facial sensation is intact. Which of the following structures was most likely affected during surgery?

(Assume the lesion is within the rostral pons.)