What is the Horizontal Gaze Nystagmus test?
It sounds like something off an eye-doctor’s chart, but the HGN is fairly simple. Otherwise known as the Penlight Test, it’s one of three Field Sobriety Tests police use in addition to the breathalyzer. Nystagmus means “involuntary jerking” and is a common result of intoxication. If you’ve been pulled over in Maryland and police officer suspects you’ve been drinking, he/she will probably perform this test first.
Holding a pen or pocketlight about a foot away from the subject’s face, the police ask the subject to follow the end of the pen with their eyes only. They will also hold the object steady at the corner and center of your vision as a further test of focus. Eyes jerking away and back to the object, otherwise called deviation, is typical of intoxication. Measuring the amount of deviation is meant to clue police into how intoxicated a person likely is.
Three cues are used to detect intoxication:
1. Lack of smooth pursuit: That is, the eyes jerk away and back to the object while following.
2. Sustained and distinct nystagmus when object is held at the corner of vision.
3. Nystagmus begins when the object is held in the center of a subject’s vision.
In other words, you can’t keep your eyes on the prize.
The HGN is controversial. Nearly thirty other common factors have been shown to cause nystagmus, high blood pressure and caffeine being only two. In Maryland, it’s been ruled that the HGN can only test for the presence of alcohol and not the degree of impairment.
Here’s a Youtube video of the test being performed. This should give you a better idea of what it looks like.
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