HORIZONTAL GAZE NYSTAGMUS
“Nystagmus” means an involuntary jerking of the eyes. Horizontal Gaze Nystagmus (HGN) refers to an involuntary jerking occurring as the eyes gaze toward the side. In addition to being involuntary, the person experiencing the nystagmus is unaware that the jerking is happening.
According to the 2006 NHTSA Student Manual, involuntary jerking of the eyes becomes readily noticeable when a person is impaired. As a person’s blood alcohol concentration increases, the eyes will begin to jerk sooner as they move to the side.
Officers are instructed that Horizontal Gaze Nystagmus (HGN) is the most reliable field sobriety test. In administering the HGN test, the officer has the suspect follow the motion of a small stimulus with the eyes only. The stimulus may be the tip of a pen or penlight, an eraser on a pencil, or their finger tip, whichever contrasts with the background.
When the HGN test is administered always begin with subjects left eye. Each eye is examined for three specific clues.
° As the eye moves from side to side, does it move smoothly or does it jerk noticeably? (According to officers’ training, as people become impaired by alcohol, their eyes exhibit a lack of smooth pursuit as they move from side to side.)
° When the eye moves as far to the side as possible and is kept at that position for several seconds, does it jerk distinctly? (Distinct and sustained nystagmus at maximum deviation is another clue of impairment which officers attempt to observe.)
° As the eye moves toward the side, does it start to jerk prior to a 45-degree angle? (Onset of nystagmus prior to 45-degrees is another clue of impairment.)
Officers a trained that as a person’s blood alcohol concentration increases it is more likely these clues will appear. The maximum number of clues that may appear in one eye is three. The maximum total number for any suspect is six. The original research shows that if four or more clues are evident, it is likely that the suspect’s blood alcohol concentration is above 0.10. With four-or-more clues present, validation studies show that this test is 77% accurate.
VERTICAL GAZE NYSTAGMUS
Vertical Gaze Nystagmus is an involuntary jerking of the eyes (up and down) which occurs when the eyes gaze upward at maximum elevation. Although this type of nystagmus was not addressed in the original research, officers are trained that field experience has indicated that the presence of Vertical Gaze Nystagmus has proven to be reliable indicator of high doses of alcohol for that individual or certain other drugs.
ONE-LEG STAND
Officer’s are trained that the One-Leg Stand test also has been validated through NHTSA’s research program. It is divided attention test consisting of two stages:
º Instructions Stage; and,
º Balance and Counting Stage.
In the Instruction Stage, the subject must stand with feet together, keep arms at sides, and listen to instructions. This divides the subject’s attention between a balancing task (maintaining a stance) and an information processing task (listening to and remembering instructions.)
In the Balance and Counting Stage, the subject must raise one leg, either leg, with the foot approximately six inches off the ground, keeping raised foot parallel to the ground. While looking at the elevated foot, count out loud in the following manner: “one thousand and one”, “one thousand and two”, “one thousand and three” until told to stop. This divides the subject’s attention between Balancing (standing on one foot) and small muscle control (counting out loud).
The timing for a thirty-second period by the officer is an important part of the One-Leg Stand test. The original research has shown that many impaired subjects are able to stand on one leg for up to 25 seconds, but that few can do so far 30 seconds.
One-Leg Stand is also administered and interpreted in a standardized manner.
Officers carefully observe the suspect’s performance and look for four specific clues:
º sways while balancing;
º uses arms to balance;
º hops;
º puts foot down.
Inability to complete the One-Leg Stand test occurs when the suspect:
º puts the foot down three or more times, during the 30-second period;
º cannot do the test.
The original research shows that, when the suspect produces two or more clues or is unable to complete the test, it is likely that the BAC is above 0.10. This criterion has been shown to be accurate 65 percent of the time.
Walk-and-Turn
Officers are trained that the “Walk-and-Turn” is a test that has been validated through extensive research sponsored by the National Highway Traffic Safety Administration (NHTSA). It is a divided attention test consisting of two stages:
° Instructions Stage; and,
° Walking Stage,
In the Instructions Stage, the subject must stand with their feet in heel-to-toe position, keep their arms at their sides, and listen to the instructions. The Instructions Stage divides the subject’s attention between a balancing task (standing while maintaining the heel-to-toe position) and an information processing task (listening to and remembering instructions).
In the Walking Stage the subject takes nine heel-to-toe steps, turns in a prescribed manner, and take nine heel-to-toe steps back, while counting the steps out loud, while watching their feet. During the turn, the subject keeps their front foot on the line, turn in a prescribed manner, and use the other foot to take several small steps to complete the turn. The Walking Stage divides the subject’s attention among a balancing task (walking heel-to-toe and turning); a small muscle control task (counting out loud); and a short-term memory task (recalling the number of steps and the turning instructions).
The Walk-and-Turn test is administered and interpreted in a standardized manner, i.e., the same way every time. Officers administering the Walk-and-Turn test observe the suspect’s performance for eight clues:
º can’t balance during instructions;
º starts too soon;
º stops while walking;
º doesn’t touch heel-to-toe;
º steps off line;
º uses arms to balance;
º loses balance on turn or turns incorrectly; and,
º takes the wrong number of steps.
Inability to complete the Walk-and-Turn test occurs when the suspect.
º steps off the line three or more times;
º is in danger of falling;
º cannot do the test.
Original research shows that if a suspect exhibits two or more of the clues, or cannot complete the test, the suspect’s BAC is likely to be above 0.10. This criterion has been shown to be accurate 68 percent of the time.