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The Sobriety Eye Test: How It Works, Limitations, and Your Rights

When you hear the term “sobriety eye test,” you might wonder what it entails. This test, also known as the Horizontal Gaze Nystagmus (HGN) test, is a part of field sobriety testing conducted by police officers to assess whether a driver has consumed too much alcohol. 

During the test, an officer observes a driver’s eyes and their ability to focus on an object while moving it horizontally.

The HGN test is designed to detect involuntary jerking movements in the eyes, which can be exaggerated by alcohol consumption. 

When administering this test, the officer will ask you to follow a pen or another object with your eyes, carefully inspecting for any signs of impairment, such as jerking, an inability to smoothly follow the object, or a distinct angle of onset. 

It’s important to note that while the HGN test can be an accurate indicator of impairment, there are other factors that could cause similar eye movements.

As one component of field sobriety tests, the HGN test is commonly used in conjunction with other methods like the walk-and-turn test and the one-leg stand test. 

Together, these tests provide law enforcement officers with a comprehensive assessment of a driver’s physical and cognitive abilities, helping determine whether they’re under the influence of alcohol or other substances.

Understanding the Sobriety Eye Test

The sobriety eye test, also known as the horizontal gaze nystagmus (HGN) test, is a common field sobriety test used by law enforcement officers to help determine if a driver is under the influence of alcohol or drugs.

Improper use or interpretation of this test may lead to false positives, so it’s important to understand how it works and its limitations.

During the test, an officer will instruct you to follow an object, such as a pen or finger, with your eyes. 

They will move the object horizontally at a distance of 12 to 15 inches away from your face. The officer will be observing if your eyes exhibit an involuntary jerking motion, known as nystagmus when reaching an angle greater than 45°.

There are a few key factors to keep in mind when it comes to the HGN test:

  1. Accuracy: The sobriety eye test is considered one of the most accurate field sobriety tests, with an approximate 77 percent accuracy rating in detecting blood alcohol contents that exceed the legal limit.
  2. Distractions: It is crucial for the test to be conducted in an environment free from visual distractions such as traffic and patrol lights, as they can affect the results.
  3. Health Conditions: Some medical conditions and medications can cause nystagmus in a person, leading to a false positive result. Inform the officer of any conditions or medications you may have taken prior to the test.
  4. Proper Administration: Ensure that the officer administering the test has been trained properly and is following the correct procedure, as this can greatly impact the test’s reliability.

By understanding the sobriety eye test and its limitations, you can confidently undergo the test if you are ever in a situation where it is required. Remember that being transparent with the officer and knowing your rights are essential when it comes to field sobriety tests.

Related: Why You Don’t Need to Be an Alcoholic to Have a Drinking Problem

close up of an eye ball
What is a sobriety eye test?

Components of the Sobriety Eye Test

A sobriety eye test, also known as a horizontal gaze nystagmus test or an HGN test, is used by law enforcement officers to determine if a driver is impaired due to alcohol or other substances. 

In this section, we will discuss the main components of the sobriety eye test to help you understand what to expect during this examination.

Nystagmus Test

The Nystagmus Test is the primary component of the sobriety eye test. It involves checking for involuntary jerking movements of the eyes, known as horizontal gaze nystagmus (HGN). Alcohol and some other substances can exaggerate this jerking movement, which is why it is considered evidence of impairment.

During the test, you will be asked to follow an object, such as a pen or a finger, with your eyes. The officer will move the object from side to side, observing your eye movements and looking for signs of nystagmus.

Pupil Size

Another important aspect of the sobriety eye test is the examination of your pupil size. Substance use can cause your pupils to become either dilated (larger than normal) or constricted (smaller than normal). 

The officer will examine your eyes to determine if your pupils are an appropriate size for current lighting conditions.

Some common factors affecting pupil size are:

  • Ambient light
  • Emotional state
  • Medications
  • Fatigue

It’s important for officers to consider these factors when interpreting pupil size during a sobriety test.

Reaction to Light

Lastly, the sobriety eye test checks your reaction to light. This involves the officer shining a light into your eyes briefly and observing your pupils’ response. Your pupils should constrict, or become smaller, when exposed to bright light.

A slow or absent response to light can be another indicator of impairment. 

Keep in mind that certain medications and medical conditions can also affect your eyes’ reaction to light, so it’s essential to notify the officer if any such factors may apply to you.

Process of the Sobriety Eye Test

A police officer conducting a field eye sobriety test
How the sobriety eye test is conducted

Administering the Test

First, the officer will inform you that they are going to examine your eyes, instructing you to follow a stimulus, like a pen, with your eyes only. They will move the object from side to side, and monitor your eye movements closely, looking for any involuntary jerking or erratic motion.

Scoring the Test

The HGN test’s reliability primarily lies in its correct administration and interpretation. As your eyes follow the stimulus, your ability to smoothly track the object’s movement provides the officer with critical information. In general, there are three main categories of clues that the officer will use to score the test:

  1. The smoothness of eye pursuit: Difficulty in following the object smoothly can be a sign of impairment.
  2. Distinct nystagmus at maximum deviation: If your eyes show pronounced jerking when looking as far to the side as possible, this could indicate impairment.
  3. Onset of nystagmus before 45 degrees: The earlier the jerking begins as your eyes follow the object, the more likely it is to be a sign of impairment.

Based on these observations, an officer can make a determination about your level of impairment. However, it is essential to consider that other factors, such as medical conditions or medications, might affect the HGN test’s accuracy. If subjected to this test, it’s crucial to inform the officer of any such factors that could alter the results.

Accuracy and Controversies

Reliability of the Test

While the HGN test can be a useful tool, its reliability has been a subject of debate.

When conducting the HGN test, officers examine your eyes and their ability to focus while following a moving object, like a pen or a finger. 

The presence of nystagmus (an involuntary eye-jerking) can indicate alcohol or drug impairment. However, there are multiple kinds of nystagmus, and not all are influenced by alcohol.

Some factors can affect the reliability of the HGN test:

  1. External distractions: Traffic or patrol lights may interfere with your ability to focus.
  2. Medical conditions: Certain medical issues might cause nystagmus unrelated to alcohol or drugs.
  3. Proper test administration: The accuracy of the test greatly depends on the officer’s training and experience.

The use of the HGN test in DUI cases has led to legal challenges and controversies, particularly concerning its admissibility as evidence in court.

In some jurisdictions, courts may consider the HGN test as scientific evidence, requiring the prosecution to prove its reliability before admitting it. 

In other areas, it might only be seen as observational evidence that supports the officer’s determination of your impairment.

Knowing the legal standards in your area is crucial to understanding the potential consequences of an HGN test.

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