A 58-year-old man with a 30-year history of heavy alcohol use presents with progressive gait disturbance. On examination, he walks with a wide-based, unsteady gait and cannot stand upright without support, but his finger-to-nose test and rapid alternating movements are relatively preserved. Imaging reveals selective degeneration of the anterior superior aspect of the structure marked **B** in the diagram. Which of the following best explains the predominant clinical finding in this patient?