The Law Offices of Edward Harrington Heyburn, PC

Police believe that the HGN is the most reliable of all field sobriety tests because the observation of nystagmus, or involuntary jerking of the eyes, indicates that something is disrupting and interfering with the central nervous system. Whether that “something” is alcohol is a question to be answered by a variety of factors, including the observations of the police and ruling out other causes of nystagmus such as head trauma, disease, or other drugs.  It should be noted that he HGN is subject to many false positives that make it questionable outside of the court.

The principles behind HGN are rather simple. The movement of the eye is controlled by six muscles through the central nervous system control center. As alcohol disrupts the central nervous system, certain bodily functions are slowed down including this muscle control (this also accounts for the onset of slurred speech and unsteadiness). The result is that the eye can no longer move smoothly because the operation of the controlling muscles has been slowed. This causes an inability of the eye to control its movement and is seen as an involuntary “jerking” of the eye as it moves up and down (vertical) and from side to side (horizontal). This loss or inability to smoothly control the eyes is termed nystagmus.

It is important to note that, unlike the other field sobriety tests, nystagmus is completely involuntary – an impaired subject might be able to practice and master the walk-and-turn but cannot control how their eyes move.

Administration of the test:

If the person is wearing glasses, the officer requests they be removed.

To eliminate a medical disorder as the cause for nystagmus, both eyes are checked for equal tracking (can they follow an object together?) and equal pupil size. The officer positions the stimulus (pointer, pen, pencil, etc.) approximately 12-15 inches from the person’s nose and slightly above the eye level. The officer then moves the stimulus smoothly across the person’s entire field of vision. If eyes don’t track together it could indicate a possible medical disorder or injury. Also, if the pupils are not equal in size, this could indicate a head injury.

Beginning with left eye, the police officer looks for the following 3 validated clues or signs:

1. Lack of smooth pursuit – the officer moves the stimulus to the right, smoothly at a speed that requires approximately 2 seconds to bring the eye as far as it can go. The officer then moves the stimulus all the way back to the left and checks the right eye. The officer then repeats this portion of the test. As the eyes move from side to side, do they move smoothly or do they jerk noticeably? As people become impaired by alcohol, their eyes exhibit a lack of smooth pursuit as they move from side to side. When a person is not impaired their eyes will move smoothly like a marble rolling across a smooth plane of glass (as opposed to a marble moving across sandpaper).

2. Distinct and sustained nystagmus at maximum deviation – the officer moves the stimulus to the right until the left eye has gone as far to the side as possible (no white should be showing at the corner of the eye). The officer will hold this position for a minimum of 4–6 seconds and observe the eye. The officer then moves the stimulus all the way across the person’s face to check the right eye. This portion of the test should also be repeated. When the eye moves as far to the side as possible and is kept at that position for 4–6 seconds, the officer is looking to see whether it jerks distinctly and is sustained.

3. Onset of nystagmus prior to 45-degrees – the officer then moves the stimulus toward the right, taking about 4 seconds for the stimulus to reach the edge of the subject’s shoulder. The officer carefully watches the left eye for any sign of jerking. If the officer notices jerking of the eye, the officer should stop and verify that the jerking continues. Next the officer checks the right eye. The officer moves the stimulus to the left, taking 4 seconds to reach an approximate 45-degree angle, again repeating this portion of the test. As the eye moves toward the side, the officer is looking to see whether it starts to jerk before it has moved through a 45-degree angle.

Results:

Since there are three clues and two eyes, the maximum number of clues that may appear in any given person is six. The original research shows that if four or more clues are evident, the central nervous system has been impaired and, if that impairment is due to the consumption of alcohol, it is likely that the subject’s blood alcohol concentration is above 0.10%. With four-or-more clues present, this test is 77% accurate.

According to NHTSA research, HGN is the most reliable of the three standardized tests and alone can provide valid indications to support arrest decisions at a BAC of 0.08%.

Here is a video that demonstrates the test and what police are looking for.