When arrested for a DWI, a motorist will be asked by the investigating police officer to perform a series of tests which are commonly known as SFST, or standardized field sobriety tests. The three tests that are generally required are the walk and turn, the one leg stand, and the horizontal gaze nystagmus, also referred to as HGN. This post will focus on the HGN.
Nystagmus is an involuntary jerking of the eyes, and can be caused by a number of factors, some having to do with the ingestion of substances or alcohol, but some due to medical reasons. For example, nystagmus can be due to a brain injury or tumor, an inner ear disorder, or impairment due to use of alcohol or drugs. The HGN is performed by the officer using a penlight or other stimulus, and the stimulus is placed just above eye level and 12-15 inches from the nose. The officer then requests that the person follow that stimulus with their eyes, while looking for evidence of nystagmus.
According to the National Highway Traffic Safety Administration Manual, if the nystagmus test is failed by the motorist, (four or more clues are seen) this is proof that the driver has a blood alcohol concentration of .10% or above in 77% of cases. The HGN is deemed to be the most reliable of the SFST’s, with the walk and turn being 68% reliable, and the one leg stand at 65% reliability.
As with other field sobriety tests, the design of the “divided attention” tests is twofold: to determine coordination and whether the motorist can follow directions, checking both at the same time. The theory is that if the person is impaired, they may be able to follow instructions, or perform the test, but not both. The HGN is not a true coordination test as are the walk and turn and one leg stand, which involve balance, concentration, and coordination, since HGN is in fact a physical reaction which cannot be controlled by the person.
The officer gives the following instructions in sum or substance: “I’m checking your eyes. Keep your head still and follow the stimulus with your eyes only. Follow the stimulus until I tell you to stop. Have you understood all of the instructions I’ve given you?”
The three parts to the nystagmus test are:
The person is asked to follow the stimulus first with each eye, and the officer is looking for the clue of what is known as “lack of smooth pursuit”, meaning that the eye begins to jerk as it is following the stimulus to the left and right;
The person is required to follow the stimulus to what is known as “maximum deviation”, or as far to the left and to the right as they can. After four seconds, if there is distinct nystagmus, this is the second clue of intoxication according to the test;
The person follows the stimulus and nystagmus begins at a 45 degree angle. This is the third clue purporting to establish intoxication. Supposedly, the more impaired the person is, the sooner the nystagmus will be noticeable.
The HGN is not evidence of the BAC (blood alcohol concentration) of the motorist, but rather is offered by the prosecution as proof of impairment, along with the other field sobriety tests and chemical test results if a breathalyzer, urine or blood was taken from the driver.
If you or a loved one are arrested for a DWI, another crime or traffic infraction, contact the Westchester County DWI Defense Lawyers online or toll free at 888-761-7633 for a free initial consultation with an experienced defense attorney to discuss your legal rights and options.