If you’re stopped for drunk driving in Massachusetts, the police will likely ask you to perform a field sobriety test. (See my post: Drunk Driving (OUI) Investigations: Comply or Deny?)
The field sobriety test will include a horizontal gaze nystagmus (HGN) test.
The Supreme Judicial Court thoroughly explains this test in Commonwealth v. Sands:
The HGN test consists of three parts which measure various aspects of these involuntary movements which cause nystagmus. In the first part, the “smooth pursuit” test, the officer asks the defendant to first look straight ahead, focusing on an object, such as a pen, which the officer then moves back and forth horizontally. As the driver follows the path of the pen, the officer looks to see whether the eyes move smoothly from side to side, or whether they exhibit nystagmus, characterized by an unsteady, bouncing movement. In the second part, the “maximum deviation” test, the officer moves the pen horizontally to the limit of the driver’s field of vision, and watches the eyes for bouncing at the extremes. In the third part, the “forty-five degree” test, the officer again moves the object from side to side, asking the driver to follow the movement with his eyes. The officer watches for the onset of the nystagmus prior to a forty-five degree angle between the driver’s nose and the position of the object. The underlying theory is that there is a strong correlation between the degree of a person’s intoxication and the angle at which the person’s eyes begin to exhibit evidence of nystagmus. Intoxicants, such as alcohol, interrupt various tracking and focusing functions of the retina which can be observed through a series of simple tests.
While a police officer may use an HGN test to assess your level of impairment, he cannot testify about the test’s results at trial. This is because the test is considered “scientific” and the average police officer lacks the credentials needed to speak about a complex medical condition such as nystagmus. There are, in fact, 34 different causes of nystagmus. And only a specialized physician would have the expertise needed to testify that alcohol was to blame for a person’s abnormal eye movements.
Therefore, a criminal defense attorney should always file a motion in limine asking the judge to exclude officer testimony on the HGN test during trial. Such motion are usually granted without question.