If you have been arrested for DWI/DUI, one of the field sobriety tests the police officer will usually have you do is to check your eyes for a nystagmus (an involuntary jerking of the eye). In this video, the nystagmus is clearly visible. The person in the video also had a BAC of .14 at the time. It is important to note things other than alcohol can cause a nystagmus, including the officer improperly administering the test. If you are charged with drunk driving in New Hampshire, call an experienced New Hampshire DWI Lawyer to see if the officer administered the test properly, or other defense you may have.

9. 5. 2 Horizontal Gaze Nystagmus (HGN)


The first test is the horizontal gaze nystagmus test (Pen/ Eye test). A nystagmus is an involuntary jerking of the eye. The officer, when moving a stimulus (pen, finger) across the person's face, is looking to see if the eye "jerks" instead of moving smoothly.

Based upon some research, by NHTSA, if someone has a nystagmus, there is a high probability that the person has a BAC of 0.10.(The 2006 student manual does not include the HGN Robustness study as it concluded was after the manual was released. This study shows just how much of a junk science HGN is, with incredibly high false positives).

There are a few different types of Nystagmus. The officer is looking for Gaze Nystagmus[1] Other types that may be relevant to a DWI defense include:

Post-rotational[2] (Nystagmus which is caused after a person stops spinning. Although, this usually only lasts a few seconds),

Caloric Nystagmus[3] - Caused when fluid in one ear is different in temperature than the other ear.(Which could possibly occur while a window is down, and cold rain entering one ear while the warm car heater is aimed at the other ear).

Optokinetic[4] Nystagmus - Occurs when eyes fixate on an object that suddenly moves out of sight, or when the eyes watch sharply contrasting moving images (Possibly if client is facing the officer's flashing blue lights/takedown lights.)

Resting Nystagmus[5] - Occurs when the eyes are facing straight ahead.

NaturalNystagmus - Some people have a natural nystagmus. Some officers will ask the person if they have it. I am not sure why the person would ever know if they had it.


However, one of the main problems with this test is THINGS OTHER THAN ALCOHOL CAUSE A NYSTAGMUS. For example: “It is undisputed that there are many factors that can cause nystagmus: problems in an individual's inner ear labyrinth; physiological problems such as influenza, streptococcus infection, vertigo, epilepsy, or measles; conditions such as eye muscle fatigue, sunstroke, or glaucoma; changes in atmospheric pressure; and consumption of substances such as caffeine, nicotine, or aspirin. See 1 R. Erwin, Defense of Drunk Driving Cases § 10.09[5], at 10-43 (3d ed. 1999) (Defense of Drunk Driving); M. Rouleau, Unreliability of the Horizontal Gaze Nystagmus Test, 4 Am.Jur.Proof of Facts 3d 439 § 9, at 455 (1989) (4 Am.Jur. POF 3d); National Highway Traffic Safety Administration (NHTSA), United States Department of Transportation (DOT), No. DOT HS-0806512, Improved Sobriety Testing (1984) (1984 NHTSA Instruction Manual), reprinted in 1 Defense of Drunk Driving § 10.99[2], app. at 10-93.[6]

One Court has specifically listed 38 different possible causes of nystagmus other than alcohol:

“(1) problems with the inner ear labyrinth; (2) irrigating the ears with warm or cold water under peculiar weather conditions; (3) influenza; (4) streptococcus infection; (5) vertigo; (6) measles; (7) syphilis; (8) arteriosclerosis; (9) muscular dystrophy; (10) multiple sclerosis; (11) Korchaff's syndrome; (12) brain hemorrhage; (13) epilepsy; (14) hypertension; (15) motion sickness; (16) sunstroke; (17) eye strain; (18) eye muscle fatigue; (19) glaucoma; (20) changes in atmospheric pressure; (21) consumption of excessive amounts of caffeine; (22) excessive exposure to nicotine; (23) aspirin; (24) circadian rhythms; (25) acute trauma to the head; (26) chronic trauma to the head; (27) some prescription drugs, tranquilizers, pain medications, anticonvulsants; (28) barbiturates; (29) disorders of the vestibular apparatus and brain stem; (30) cerebellum dysfunction; (31) heredity; (32) diet; (33) toxins; (34) exposure to solvents, PCBS, dry cleaning fumes, carbon monoxide; (34) extreme chilling; (35) eye muscle imbalance; (36) lesions; (37) continuous movement of the visual field past the eyes, i.e., looking from a moving train; (38) antihistamine use.[7] Administering the HGN

To administer the test correctly, the officer must do the following:

Preliminary qualifications -The entire test is administered by holding a stimulus (pen, finger) approximately 12-15 inches away from the subjects nose slightly above eye level[8]. Check for equal tracking, equal pupil size, resting nystagmus[9]. The officer does this by moving the stimulus across the person’s entire full line of vision. Although the manual does not give an exact amount of time, generally 1-2 seconds per eye is a fair speed. If the officer goes too fast, he cannot see if the eyes track, as eyes can only follow an object up to a certain speed. Three Steps of HGN

Step 1: Lack of smooth pursuit[10]

For all 3 steps, the officer will be checking each eye two times.

To check for lack of smooth pursuit, the officer moves the stimulus at a speed of two seconds from center position all the way to the side of the face. During this time, the officer is looking to see if the eye is able to pursue smoothly. Again, a nystagmus is an involuntary jerking of the eye. Many officers will testify that what they are looking for is similar to windshield wiper blades (as this example is in the training manual). If those wiper blades go across your windshield while it is wet, they will smoothly move. However, if the blades are old or the windshield is dry, the blades will jerk as they go back and forth[11].


Step 2: Distinct and sustained nystagmus at maximum deviation[12]

To do this, the officer moves the stimulus all the way to the side, so that no white is left showing in the corner eye. The officer then must hold the stimulus there for at least 4 seconds (but not longer than 30 seconds as this can cause fatigue nystagmus). The reason the officer must hold it that long, is because people will show a nystagmus at maximum deviation, even when unimpaired, for up to a few seconds[13].

This step is usually the one I can show the officer did incorrectly. Many officers forget about the sustained part (particularly if they use outdated training), and just move the pen all the way out.


Step 3: Onset prior to 45 degrees[14]

To do this step, the officer is trying to determine where a nystagmus is first present. The officer must move the stimulus at a rate that would take 4 seconds (so twice as long as LSP) to wherever the officer is guessing 45 degrees is. Once the officer sees the nystagmus, he is to hold the stimulus there to see if it remains.

To measure 45 degrees, the officer will usually say he went out 12-15 inches, which is the same distance he held the stimulus out. This angle will be 45 degrees, assuming his guess in inches is correct. Note however, this test is onset prior to 45 degrees. Many officers will make the mistake of saying they saw the nystagmus at 45 degrees. Further, some officers will say they measured 45 degrees by going out to the shoulder. However, everyone's shoulder width is different, so this does not sound like a very accurate method, to me at least. The manual states if no white is showing, the officer went too far, or the person has unusual eyes[15].

According to NHTSA, if the person has a nystagmus prior to 45 degrees, it is evident the BAC is above -0.08, as shown by research. (Whichresearch? I have no idea.)

Practice Tip: If it is evident that someone’s BAC is 0.08 or higher when there is onset prior to 45 degrees, you should argue it is evident the person’s BAC is less than 0.08 when there is no nystagmus prior to 45 degrees.

My belief, based upon other NHTSA research, and other actual scientific research, is that the NHTSA manual is false. According to Thorpes formula, BAC can be estimated as 50-angle of onset. So, at 45 degrees would be a 0.05, 44 degrees a 0.06, etc.

Further, look at the Robustness study. Get an expert, or hope you have an officer who likes to seem knowledgeable who took the time to read the study. The study is very favorable to the defense. It shows people have a nystagmus at as low as 0.02 BAC. The false positive rates are incredibly high. If you have an expert, I think you should argue Dahood should be revisited in light of this study showing just how inaccurate HGN is.

Practice Tip: In order to remember the timing, assuming the officer does the LSP first, it is 1-2 seconds for equal tracking, 2 for LSP, 4 seconds holding at maximum deviation, and 4 seconds to get to 45 degrees. So, the timing is 1,2,4,4. Scoring of HGN

If the officer observes 4 out of 6 clues, NHTSA research (according to the manual) shows 77% accuracy in determining a BAC above 0.10[16]

Each eye is checked twice, for the 3 separate clues. A clue can only be present for each step once per eye. So, while doing the LSP if the officer observes it twice in the left eye, and twice in the right eye, he can only count it once per eye. He would have noted 2 clues (1 for the left eye, and 1 for the right eye).


Practice Note - An odd number of clues will usually mean something is wrong. If a nystagmus is present in one eye, it should be present in the other. If it is not, one of two things likely happened. The officer either messed up and saw it when he shouldn't have, or didn't see it in the other eye when he should have. Or, the person has a medical condition (large disparity between the performance of the left and right eye may indicate a medical condition)[17].


9.5.3 Vertical Gaze Nystagmus (VGN)

This part of the test is optional, but often the officer will check for it.

To check for VGN, he basically does the distinct and sustained nystagmus, but goes up, instead of out to the side.

The officer is to take the stimulus, raise it so the eyes are elevated as far as possible, and hold it for approximately 4 seconds to see if jerking continues[18].

VGN is a sign of high BAC for that person (meaning someone with a high tolerance will take more alcohol to achieve), or a sign of impairment by drugs[19].

[1]Id. at VIII-4

[2]Id. at VII-3



[5]Id. at VIII-4

[6]State v. Dahood, 148 NH 723,729 (2002)

[7]Schultz v. State, 106 Md. App. 145 (1995)

[8] Student Manual 2006 at VIII-6

[9]Id. at  VIII-9

[10]Id. at VIII-7

[11]Id. at VIII-5

[12]Id at VIII-7

[13]Id. at  VIII-5

[14]Id. at  VIII-7

[15]Id. at  VIII-7

[16]Id at VIII-3

[17]Id at VIII-8

[18]Id at VIII-8