The first of the tests is the horizontal gaze nystagmus test, or HGN. The idea behind this test is that the presence of alcohol or certain drugs in a person’s body impacts fine muscle movements, and the eyes are the best place to observe that. The officer starts by having the driver stand still with their eyes watching a small “stimulus”, like the tip of a pen, a pencil eraser, or a penlight. Since it’s usually dark outside when officers are conducting this test, a penlight is probably the most commonly used.
The officer then tells the driver to follow the stimulus with their eyes only, and to not move their head. They’ll say “follow it as far as you can without moving your head,” or something similar. The officer is taught to look for bumpy, “jerking” movement in the eyes. This is called horizontal gaze nystagmus, and it can be caused by many things, including high blood pressure, nicotine, antihistamines, and the flu. It can also be caused by the presence of alcohol in the body.
As with the other SFSTs, there are a list of “clues” that the officers are looking for. They are to examine each eye for three clues, always starting with the left eye. The maximum number of clues for each eye is three, so the maximum number of clues on the HGN test is six. The three clues are:
- Lack of smooth pursuit – the driver’s eye jumps from point to point instead of smoothly following the pen all the way to the side
- Distinct and sustained nystagmus at maximum deviation – when the driver’s eye has gone as far as it can to the side, the eye twitches
- Onset of nystagmus prior to 45 degrees – the driver’s eye begins to jerk before reaching a 45-degree angle
There are plenty of reasons to doubt the accuracy of these “clues,” but that’s what the NHTSA teaches the officers to look for. Typically, an officer’s report will read something like this:
Notice how the officer went in order from 1 through 3 with each clue and reported a total of six out of six clues. Also notice how the officer mentions vertical nystagmus. Vertical nystagmus doesn’t have the same history of testing that horizontal nystagmus does, but that doesn’t stop the NHTSA from mentioning it in their training programs or officers from talking about it in their reports.
Next, let’s look at the Walk and Turn, or WAT.