A 65-year-old woman complains of recurrent episodes of sudden-onset dizziness and nausea. She notices an abrupt onset of a spinning sensation when rolling over or sitting up in bed. The symptoms last for 30 seconds and then completely resolve. She has no hearing change or other neurologic symptoms, and her physical examination is completely normal. A Dix-Hallpike maneuver reproduces her symptoms. Which of the following findings on vestibular testing favors the diagnosis of benign paroxysmal positional vertigo (BPPV) over central positional vertigo?