The Horizontal Gaze Nystagmus (HGN) Field Sobriety Test is regarded as the most "scientific" or "technical" of the field sobriety tests. It is based on nystagmus, which is the inability to hold one's eyes steady on a stationary or moving target, causing the eyes to produce a jerking movement. Alcohol consumption causes this involuntary jerking of the eye (nystagmus). It has been proven that when under the influence of alcohol, a person's smooth and accurate control of his or her eye movements will deteriorate. The HGN field sobriety test is supposed to help police officers identify drivers with a blood alcohol content in the .08-.12 range and to detect drivers who do not necessarily exhibit exaggerated characteristics of impairment and who may not display any gross coordination and balance problems. However, HGN does not prove alcohol intoxication without corroborating evidence.

The HGN field sobriety test is administered by requiring a subject to follow an object, such as a pen, with his or her eyes. The administering officer will place the object above eye-level and about twelve to fifteen inches from the subject's face. Next, the officer instructs the subject of the test to follow the object with his or her eyes only, and not move his or her head. The officer then conducts the test looking for three “cues” in each eye that HGN is present, which would indicate alcohol impairment. The HGN cues are lack of smooth pursuit, distinct nystagmus at maximum deviation, and onset of nystagmus prior to 45 degrees. If at least four cues are present, it is likely that the person performing the test has a blood alcohol content ("BAC") of at least .10, which his above the .08 legal limit in Massachusetts. Police officers are not qualified to make precise numerical correlations between HGN and blood alcohol level.

As a lay witness, a police officer may testify to the administration and results of field sobriety tests that measure a person's balance, coordination, and acuity of mind in understanding and performing simple instructions, as a juror understands from common experience and knowledge that intoxication leads to diminished balance, coordination, and mental acuity.

However, because the HGN test is based on an underlying scientific proposition, expert testimony is required to have HGN results admitted at a Massachusetts DUI trial. Testimony from an expert, such as an ophthalmologist or neurologist, is required, because the HGN test’s scientific basis is beyond the common knowledge or understanding of the lay juror. It is rare for the prosecution to produce the required expert in order to admit HGN results and, therefore, most Massachusetts DUI Lawyers automatically file a motion to exclude HGN evidence.

For HGN results to be reliable in a Mass. DUI case, the prosecutor must establish that the officer was qualified to administer the test and interpret the field sobriety test results. It must also be demonstrated that the test was administered and evaluated correctly. For example, if an officer skips one of the three procedures inherent in HGN testing, the test will be deemed unreliable in a DUI trial. 

HGN may not be a valid indicator of alcohol intoxication in drunk driving cases where there is an accident and the driver may have suffered a head injury. Such drivers often experience blurred vision and dizziness or other symptoms which render the test an unreliable indicator of whether a person was driving under the influence of alcohol.