Horizontal gaze nystagmus refers to an involuntary jerking of the eye as a suspect gazes towards their periphery of their vision.  Nystagmus becomes readily noticeable when a person is impaired.  The greater the impairment, the sooner the eyes will begin to involuntarily bounce as the person starts looking toward their periphery. 

The horizontal gaze nystagmus test (or HGN), is the most reliable field sobriety test.

The test itself is administered as follows:
(1) the officer has the subject follow a small stimulus such as a pen, penlight, eraser on a pencil, or the tip of the officers finger, anything that contrasts with the background;
(2) the officer positions the stimulus 12-15 inches from the suspect's nose and slightly above eye level;
(3) the officer is to check that both pupils are the same size and both track together by making a pass;
(4) the HGN is to begin with the subjects left eye with a pass to the officer's right.  The officer is to move the stimulus at a speed that requires approximately 2 seconds to bring the suspect's eye as far to the periphery as possible;
(5) the officer is to move the stimulus all the way to the periphery of the suspect's right eye, again taking approximately 2 seconds out to the periphery and two seconds back, looking for clues both out and back:
(6) the officer repeats the procedure. (this concludes the smooth pursuit portion of the test);
(7) the officer checks for distinct nystagmus at maximum deviation beginning with the left eye by moving the stimulus to the edge of the periphery.  No white should be showing in the corner of the eye at maximum deviation.;
(8) the officer is to hold the eye at that position for no less than 4 seconds and look for nystagmus.
(9) the office moves the stimulus all the way across the suspect's face and check the right eye, using the same procedure;
(10) the officer repeats the procedure. (this concludes the nystagmus at maximum deviation portion of the HGN);
(11) the officer now checks for onset of nystagmus prior to 45 degrees.  The officer begins with the suspect's left eye.
(12) The officer moves the stimulus at a speed that would take approximately 4 seconds for the stimulus to reach the edge of the suspect's shoulder;
(13) when the officer observes nystagmus, he is to hold in the position and verify the nystagmus:
(14) the officer notes the angle of onset of nystagmus:
(15) the officer repeats the procedure:
(16)

There are three specific clues the officer is looking for: (1) as the eye moves from side to side, does it move smoothly, or does it jerk noticeably?; (2) when the eye moves as far to the side as possible and is kept in that position for several seconds, does it jerk distinctly?; (3) as the eye moves towards the periphery, does it start to jerk prior to a 45 degree angle? 
The more impaired the person is, the more likely it is that the officer will observe one or more of the above clues.
The maximum number of clues in each eye is three, with the total number of clues possible being six (3 for each eye).
Research has indicated that if four or more clues are evident, it is likely that the person's blood alcohol content is above a .10.  With 4 or more clues present, the HGN test is 77% accurate.

Defenses: nystagmus can be caused by factors other than alcohol or drug impairment.  Optokinetic nystagmus occurs when the eyes fixate on an object that suddenly moves out of sigh, or when the eyes watch sharply contrasting moving images.  Examples are strobe lights, rotating lights, or rapidly moving traffic in close proximity. 

Nystagmus can also naturally occur from pathological disorders such as brain damage or diseases of the inner ear.  In order to determine whether nystagmus is attributable to a pathological disorder, the officer should determine pupil size, resting nystagmus and tracking ability.  Specifically, if the pupils are not equal size, if the eyes bounce looking straight ahead, and if the eyes do not track together, a pathological condition may be present.  

Maximum deviation- when an officer checks for nystagmus at maximum deviation (i.e. looking to the extreme periphery of vision) there is some natural nystagmus.  However, the nystagmus is not pronounced, nor sustained.  When a person is impaired, the nystagmus is pronouced at maximum deviation and is sustained for more than 4 seconds.  If the officer does not hold the stimulus at maximum deviation for more than 4 seconds, the nystagmus observed could be naturally occurring nystagmus. 

With respects to the onset of nystagmus prior to 45 degrees, if the officer moves the stimulus too fast, he will miss the angle at which he observes nystagmus.

Nystagmus can be caused from substances other than alcohol.  Specifically, some seizure medications and some other prescribed drugs can cause nystagmus.

The 9 step walk and turn test is a divided attention test.  The person submitting to the test must pay attention to the instruction phase of the test and then apply the instructions to the walking phase of the test. 

Instruction phase: in the instruction phase, suspects are to stand with their feet in the heel to toe position, keep their arms at their sides, and listen to the instructions.  The instruction phase divides the subject's attention between a balancing task, and hearing and comprehending instructions. 
Specifically, the officer is to give the following instructions:
(1) "place your left foot on the line" (demonstrate);
(2) "place your right foot on the line ahead of the left foot, with the heel of the right foot against the toe of your left foot." (demonstrate);
(3) "place your arms down at your sides" (demonstrate);
(4) "maintain this position until I have completed the instructions.  Do not start to walk until told to do so."
(5) "do you understand the instructions so far?" (ensure understanding)
(6) "when I tell you to start, take nine heel to toe steps, turn, and take nine heel to toe steps back" (demonstrate 3 heel to toe steps);
(7) "when you turn, keep the front foot on the line, and turn by taking a series of small steps with the other foot, like this". (demonstrate)
(8) while you are walking, keep your arms at your sides, watch your feet at all times, and count your steps out loud"
(9) "once you start walking, don't stop until you have completed the test."
(10) "do you understand the instructions?" (make sure suspect understands)
(11) "begin, and count your first step from the heel to toe position as one."

Walking phase: the subject takes nine heel to toe steps, turns in a prescribed manner, and takes nine heel to toe steps back, while counting out loud, while watching their feet.  During the turn, the suspect keeps their front foot on the line, and makes a pivot turn with several small steps with the other foot.  The walking test divides a person's attention between a balancing task, small muscle control (counting out loud) and a short term memory task (recalling the number of steps and the turning instructions. 

There are eight possible clues: (1) can't balance during instruction; (2) starting to walk too soon; (3) stopping while walking; (4) fails to touch heel to toe; (5) steps off line; (6) uses arms for balance; (7) loses balance on turn or turns incorrectly; (8) takes the wrong number of steps.

Research indicates that if a suspect exhibits two or more clues, or cannot complete the test, the suspect's BAC is likely to be above .10.  When 2 or more clues are present, the test is accurate 68% of the time.   

Defenses: whether a suspect was given proper instructions is the initial analysis in the defense to a DUI.  If instructions are not properly given, then the failure to follow the instructions cannot be considered to be a clue by the officer. 
Oftentimes, an officer will treat the use of arms for balance or swaying during the instruction phase of the 9 step walk and turn as a clue, but in fact, it is not, so long as the suspect does not break heel to toe during the instruction phase.
In addition, walking slowly is not a clue.  Only when the suspect pauses for several seconds should a clue be noted.
Heel to toe does not actually mean heel to toe with no gap.  The NHTSA manual recognizes a tolerance of 1/2 inch between the heel and toe.
Steps off the line cannot be partial steps off the line, be it real or imaginary.  In order for a clue to be noted, the step must be entirely off the line.
Raising arms for balance is another frequently noted clue.  However, the suspect must raise his/her arms more than 6 inches to constitute a clue.
The test conditions frequently create a defense.  The test requires a designated straight line and conducted on a reasonably hard, dry, hard, level, nonslippery surface. 
Suspects over 65 years of age, persons with back, leg or inner ear problems should not perform the test. 
In addition, persons wearing heels in excess of 2 inches should be given the opportunity to remove their shoes when performing the test.

The one leg stand is also a divided attention test.  The suspect's attention is divided between the instruction phase and the balance and counting phase. 
Instruction phase: the suspect must stand with feet together, keep their arms at their sides, and listen to instructions.  The suspect's attention is divided between a balancing task and listening to and comprehending instructions.
Instruction phase: the officer is to give the following instructions:
(1) "stand with your feet together and your arms down at the sides, like this." (demonstrate).
(2) "do not start to perform the test until I tell you to do so."
(3) "do you understand the instructions so far?"
(4) "when I tell you to start, raise one leg, either leg, with the foot approximately 6 inches above the ground, keeping your raised foot parallel to the ground (demonstrate)
(5) "you must keep both legs straight, arms at your side".
(6) "while holding that position, count out loud in the following manner....one thousand and one, one thousand and two, one thousand and three, until told to stop." (demonstrate).
(7) "keep your arms at your sides at all times and keep watching the raised foot."
(8) "do you understand?" (ensures suspect understands test).
(9) "go ahead and perform the test." (officer should time the 30 seconds.  The test should be discontinued after 30 seconds.)
Balancing phase: the subject must raise one leg, either leg, with the foot approximately 6 inches off the ground, keeping the raise foot parallel to the ground.  While looking at the elevated foot, the suspect must count out loud in the following manner: "one thousand and one, one thousand and two" and so one until told to stop. 
The timing of the test is 30 seconds.  The significance of 30 seconds is that the research shows that many impaired subjects are able to stand on one leg for up to 25 seconds, but that few can do so for 30 seconds. 
There are 4 clues the officer is looking for: (1) swaying while balancing; (2) using arms to balance; (3) hopping; (4) putting foot down while counting.

Research indicates that when the suspect produces two or more clues or is unable to complete the test, it is likely that the BAC is above .10.  When two or more clues are observed, the test is accurate 65% of the time. 

Defenses: the test requires a reasonably dry, hard, level, and non slippery surface.  If a person counts slowly, the officer should terminate the test after the passage of 30 seconds, regardless of what number the suspect is counting.  Persons over the age of 65, persons with back, leg or inner ear problems should not perform the test.  Individuals wearing heels more than 2 inches should be given the opportunity to remove their shoes.