Answer: The reflex circuit in our eyes that is responsible for keeping our vision trained on an object in the center, called the vestibulo-ocular reflex, is altered by alcohol.
When we move our head side to side (as if we were shaking our head “no”), the center of our vision normally stays centered and fixed on the object it was looking at before the head movement. This is because our nervous system naturally compensates for sideways shaking movements, which allows us to maintain focus on an object. This is due to a rapid reflex called the vestibulo-ocular reflex. It is known to be one of the fastest reflexes in the body. It is even present when our eyes are closed or in total darkness!
The vestibulo-ocular reflex relies on three neurons to convey the compensatory information to the ocular muscles. These muscles automatically move when there is a sideways motion of the head as detected by the vestibular system of the inner ear.
These neurons are sensitive to alcohol. Alcohol will delay the responding of the excitatory ocular muscles, causing the eye turn response to be significantly slower than in the sober state.
A common field test of sobriety as used by law enforcement is a test to track motion by the eyes. A police officer, after stopping a motorist suspected to be intoxicated, may ask them to follow a finger that moves back and forth. A slowness here may be indicative that alcohol is in the body.