A 12-year-old boy presents with progressive clumsiness and difficulty walking. He walks 'like a drunken-man' and has experienced frequent falls. His muscle tone and strength in all 4 limbs are slightly decreased. When he is asked to stand with his eyes closed and with both feet close together, he sways from side to side, unable to stand still. Which of the following brain regions is most likely affected and caused the symptoms described above?