The HGN Test: Horizontal Gaze Nystagmus
Helpful Information for Residents of Dearborn & Wayne County
The Standardized Field Sobriety tests include three tests that have been approved by the National Highway Traffic Safety Administration for the assessment of a drunk driver. These tests have undergone research and are believed to provide a certain level of accuracy that is increased when the tests are used together. When an officer pulls a driver over believing they may be intoxicated, much of their decision whether or not to make an arrest will be based off their opinion. This is where testing comes in and these can be used to help an officer make a more accurate judgment. There are three tests; the Walk-and-Turn, One-Leg-Stand and HGN test.
Horizontal Gaze Nystagmus
The HGN test will focus on the issue of horizontal gaze nystagmus. Nystagmus is the involuntary jerking of the eye that happens when a disturbance occurs to the oculomotor control of the eye or the inner ear. It can happen in two ways. The first is known as jerk nystagmus and happens when the eye move from one point slowly and corrects itself through a fast motion known as saccadic. The second is referred to as pendular nystagmus and occurs when the eye moves back and forth between two directions, similar to a pendulum. A horizontal jerking occurs when the eye moves to the sides, and this is known as horizontal gaze nystagmus.
Alcohol's Effect on Nystagmus
The characteristics of this jerking can be changed for those that have alcohol in their system. It can begin sooner on when the eye deviates to the side, or the movement may be exaggerated. Alcohol can have a strong influence on the central nervous system, influencing the behavior and control a person has over themselves. Since the central nervous system manages a number of important functions of the body, alcohol may alter a number of these. That is not to say though that a suspect that exhibits these traits is in fact intoxicated and there are a number of other reasons that can lead to similar results. Alcohol is responsible for two types of nystagmus; positional alcohol nystagmus and horizontal gaze nystagmus.
Positional gaze nystagmus is the result of an outside liquid being present in the body in unequal levels between the fluid in the canals of the vestibular and the blood of the person. This can throw off the orientation of a person, including the eyes which rely on the vestibular system for steadiness. When alcohol is in a person's system this can cause positional alcohol nystagmus and can result from having a higher amount of alcohol in the vestibular system or having higher alcohol fluid in the blood stream. Gaze nystagmus is the result of a disruption in the nervous system that causes a jerking of the eye where it lags then corrects itself while following an object. When alcohol is involved, this will be exaggerated and can affect the eye when it is looking straight and moves to the side or up. The smoothness and control is hindered and this can be witnessed by an officer.
Carrying out the HGN Test
While the other field sobriety tests will be looking more at coordination, balance and ability to follow instructions, this test is simpler in that it just assesses what happens when the suspect tries to move their eye. An officer will ask them to follow an object with their eyes, such as a flash light or pen. When the test is being administered, the suspect should be looking away from a street so that the lights of cars do not throw off the motion of their eyes. They will also need to take off glasses and an officer will ask if they have contact lenses in. The officer will hold the object they are using about 12 to 15 inches from the face of the subject at a level that is slightly above their eyes. The suspect will need to follow the object with only their eyes while their head remains still. For those that are unable to keep their head from moving, they can hold their head or chin to keep it stationary. The officer will look to see that both eyes move at the same time and have an equal pupil size. These can indicate problems such as a medical condition or blindness.
What an Officer Looks for
Officers will begin their assessment, looking for three indications in each eye. The first is the smoothness in how the eye travels. The left eye will be checked first to see if the suspect can follow the object without jerking. Next is distinct nystagmus at maximum deviation. The suspect will need to bring their eye as far to the side as they can and hold it there for a four second period. This may also be referred to as end point nystagmus and the officer will have the suspect keep their eye at maximum deviation to make sure that the movement of the object did not interfere with any jerking.
Lastly is the inspection of the angle of onset of nystagmus prior to forty five degrees. This is when the officer moves the object from the middle to the outside for a period of time that is about four seconds. They will be reviewing when any jerking begins to take place and if it is prior to the object moving to forty-five degrees from the center. All tests begin on the left eye and will then be done on the right side. HGN testing can have faults and some courts do not even take the findings of it into consideration. Those that demonstrate four or more of the indications are expected to be intoxicated and the NHTSA states this test has an 88% accuracy rate.