A constable pulls over a car in which a young person was dri…

A constable pulls over a car in which a young person was driving erratically.  The officer smells alcohol on the driver’s breath.  He asks the young man to leave the car and performs a Field Sobriety Test.  He has the young man walk along a line, and stand on one foot.  The alcohol-sensitive part of the brain which is being tested most is the ___________.