During traffic stops for suspected drunk driving, police often go through a battery of standardized field sobriety tests most people have either witnessed being performed on the side of the road, in the movies or have personally had to participate in. One of which is the horizontal gaze nystagmus, or simply the HGN test.

For those who don't know, this test typically requires an individual to follow the movement of an object - typically a finger or the tip of a pen - while the officer watches the person's eyes. Based on the officer's evaluation, the person may be let go or arrested for driving while intoxicated.

But what are police looking for when they perform this test and how does this indicate intoxication? The answer lies in several articles, including one written for the International Drug Evaluation and Classification Program.

A look at the nystagmus test

Alcohol has a profound effect on our bodies as most people are well aware, but sometimes in ways we're not aware. Like other central nervous system (CNS)-depressants, alcohol affects the neural centers of the cerebellum and brainstem, which control eye movements among other things.

When a person becomes intoxicated, their eyes unknowingly begin to drift then suddenly jerk back when trying to maintain focus on a target. This is what officers are looking for when they perform the HGN test. Police are looking for the sudden jerky movements the eye typically makes while under the influence of alcohol. Coupled with other factors, an officer may use the findings from this test as grounds for arrest.

The HGN test isn't perfect

As the National Highway Traffic Safety Administration pointed out in a 2007 publication entitled "The Robustness of the Horizontal Gaze Nystagmus Test," this test can be refuted in the courtroom for a number of reasons, particularly if an officer's decides to deviate from NHTSA guidelines regarding the test. False positives - and even false negatives - are possible depending on factors such as fatigue or neurological damage, explains the DECP.

Simply put, relying on the findings of this test alone could lead to wrongful arrests or even wrongful convictions for DWI, which is why it's important for police to follow proper procedure before making any damaging claims. By failing to follow proper procedure or having inadequate training, an officer could risk violating an individual's rights and creating serious legal problems for them down the road.