​Horizontal Gaze Nystagmus (HGN)

Walckner Law Office

​Attorney James Walckner​​

Movement of the eye is directed by six muscles controlled by the central nervous system (CNS).  Nystagmus is an involuntary movement of the eye that becomes visible when the CNS is affected.   The CNS can be affected by a variety of factors; one factor is the consumption of alcohol.  Alcohol slows the muscular control of the eyes, causing eye movement to appear as “jerking” and lacking smooth back-and-forth movement.

The admissibility of HGN test results in Massachusetts are often suppressed by rightfully arguing the technique is unrecognized by the scientific community and unsupported by expert witness testimony.

The HGN Test

Proper administration of the test requires police rule out other causes of CNS impairment, for instance, head trauma.  This important step is often overlooked or not properly documented.

If the driver is wearing glasses, they should be removed.  Both eyes are checked for equal tracking, meaning the eyes follow a moving object together, by having the driver follow a stimulus (finger or pen) with her eyes.  Both eyes are also checked for equal pupil dilation.  If the eyes do not track equally, or the pupils are of different size, a medical disorder, head injury, eye injury, or blindness should be inferred, and the test should not be conducted.

Phase 1​ - Smooth Pursuit: The officer next moves the stimulus from side to side, testing for smooth pursuit.  This refers to the eyes following the stimulus smoothly without jerking.

Phase 2​ - Distinct Nystagmus at Maximum Deviation: In the next step, the officer moves the stimulus out to the right side until the left eye has moved to the right placing the iris in the corner with no white showing.  Holding the stimulus in place at this point, the officer notes whether the eye remains still or jerks.  The right eye is tested by moving the stimulus to the left.


Phase 3​ -  Onset of Nystagmus Prior to 45 Degrees: The last step tests for the onset of nystagmus prior to 45 degrees.  In this phase, the stimulus is moved towards a point perpendicular to the driver’s right shoulder while the left eye is observed for jerking.  The right eye is tested by moving the stimulus to the left.

The presence of nystagmus in a single eye, during each phase, amounts to a “clue,” with a possible amount of six clues.  A driver demonstrating four or more clues out of the possible six clues is believed to have an impaired central nervous system.