The Horizontal Gaze Nystagmus (HGN)
The very first standardized test, and arguably the most accurate test, is the horizontal gaze nystagmus (HGN). Nystagmus is an involuntary jerking or bouncing of the eyeball that occurs when there is
a disturbance of the vestibular (inner ear) system or the oculomotor control of the eye. Horizontal
gaze nystagmus (HGN) refers to a lateral or horizontal jerking when the eye gazes to the side. In
the impaired driving context, alcohol consumption or consumption of certain other central
nervous system depressants, inhalants or phencyclidine, hinders the ability of the brain to correctly
control eye muscles, therefore causing the jerk or bounce associated with HGN. As the degree of
impairment becomes greater, the jerking or bouncing, i.e. the nystagmus, becomes more pronounced. This
is assessed in the horizontal gaze nystagmus test.
There are several types of nystagmus. Alcohol causes two types: alcohol gaze
nystagmus, which includes HGN, and positional alcohol nystagmus. Although alcohol causes both, alcohol
gaze nystagmus and positional alcohol nystagmus are very different and easily distinguishable.
Testing for positional alcohol nystagmus is not a part of the standardized field sobriety test battery.
While conducting this test, law enforcement officers are looking for six "clues" of impairment, three in each eye:
- Lack of Smooth Pursuit: The officer moves the object slowly but steadily from
the center of the subject's face towards the left ear. The left eye should smoothly follow the
object, but if the eye exhibits nystagmus, the officer notes the clue. The officer then checks the right
- Distinct Nystagmus at Maximum Deviation: Starting again from the center of
the suspect's face, the officer moves the object toward the left ear, bringing the eye as far over
as possible, and holds the object there for four seconds. The officer notes the clue if there is
a distinct and sustained nystagmus at this point. The officer holds the object at maximum
deviation for at least four seconds to ensure that quick movement of the object did not possibly cause
the nystagmus. The officer then checks the right eye. This is also referred to as "end-point"
- Angle of Onset of Nystagmus Prior to Forty-Five Degrees: The
officer moves the object at a speed that would take about four seconds for the object to reach the edge
of the suspect's left shoulder. The officer notes this clue if the point or angle at which the eye
begins to display nystagmus is before the object reaches forty-five degrees from the center of
the suspect's face. The officer then moves the object towards the suspect's right shoulder. For
safety reasons, law enforcement officers usually use no apparatus to estimate the forty-five
degree angle. Generally, forty-five degrees from center is at the point where the object is in front of
the tip of the subject's shoulder.
The bad news is that this test is very scientific and hard to understand. The good news is that this test is inadmissible in Tennessee courts, unless the officer is qualified as an expert in optometry. The other good news is that an officer is rarely qualified as an expert in optometry, which means that the most accurate indicator of impairment is normally not even admissible in court.