Academically known as the Horizontal Gaze Nystagmus (HGN) test, the “follow my finger” command is probably best associated with a roadside sobriety test along with walking a straight line and touching your finger to your nose. It’s also a test your primary care physician or optometrist uses, though sometimes they will substitute a penlight moved side-to-side and up and down in your field of vision.

Nystagmus technically means involuntary, sometimes jerky, rhythmic eye movements. Strictly speaking, the roadside impairment version of the HGN is looking for indications of alcohol or drug impairment as they manifest in a person’s ability to focus and control their eye movements. Whether or not a person is a habitual user of drugs, alcohol, or other stimulants, when consumed in larger amounts any of these can affect eye movement.

Simply put, alcohol and similar sedatives or stimulants can relax a person’s eye muscles causing the pupil to dilate, it can impair general physical coordination including in the eye slowing eye movement, it can reduce contrast sensitivity making it more difficult to see in low light, and it can disrupt the alignment of your eyes causing double vision.

Eye Muscle Function Test

Whether it’s on the side of the road or in a doctor’s office, when someone is asking you to “follow their finger,” they are generally looking to assess the functionality of your eye muscles. Since consumption of drugs and alcohol in large amounts can impair the function of your eye muscles, this can be one indication that a person is under the influence.

A physician or optometrist, however, is observing for the same condition, but using that information to reach different conclusions.

In short, a doctor is looking to see if your eyes have their full range of motion and can move through that full range of motion smoothly. What are called saccadic eye movements, the jerky, shifting, uncontrolled repositioning of the eye, is an indication that a person does not have smooth control of their eye movements.

Doctors are also looking for conjugated eye movements, when both eyes are aligned, moving and working together. Double vision would be an indication that a person’s eyes are not aligned and working together.

Eye Muscle Function Test Indications

Assuming a doctor, or in some cases a police officer, observes any concerns with the movement of a person’s eyes, there are different conclusions that might be drawn from the observations.

Complications with eye movement can indicate nerve or muscle issues. There are specific nerves and muscles that control eye movement and function, and early detection of concerns can be addressed by a physician.

Vision and eye movement irregularities can also be indicative of concussion or brain impairment, or neurological disorders including stroke, epilepsy, or Alzheimer’s disease.

On the side of the road, a police officer is looking to observe at least four out of a possible six indications in a person’s eye movement and coordination to determine whether they are impaired by alcohol or drugs. These effects are generally short-term and not indicative of more significant medical concerns.

Our eyes are moving a mile a minute to keep our world in focus, process information, and focus how we coordinate ourselves in space and move through the world. They do this constantly and almost imperceptibly, but when they fall even just a bit off kilter and out of alignment, a close examination of the movement of our eyes simply following a pen across our field of vision can impart a lot of vital information if the right person is observing it.