The pen-and-eyes test is one of the strangest things that happens during a DUI stop, and it is also one of the most trusted and least understood. I am Joel Brand, and I defend DUI cases across California. Officers move a pen back and forth, watch your eyes, and then declare that what they saw proves you were drunk. The horizontal gaze nystagmus test deserves real scrutiny, because so much rests on something you cannot see and cannot verify. Here is how it actually works and why I challenge it.
What the test claims to measure
The horizontal gaze nystagmus test, usually called HGN, looks for an involuntary jerking of the eyes as they move toward the side. This jerking, called nystagmus, can become more pronounced with alcohol, and officers are trained to count specific clues as your eyes follow the pen. The theory is that the more clues the officer observes, the more likely you are impaired. I break down the details in the HGN field sobriety test article.
You cannot see what the officer claims
Here is the fundamental problem. You have no idea what your own eyes are doing. Unlike the walk-and-turn, where you can at least feel whether you stepped off the line, the HGN is entirely the officer's interpretation of tiny movements only they claim to have seen. You cannot confirm or dispute it in the moment, and even on video the jerking is often impossible for a layperson to make out. The evidence is, in essence, the officer's word about something invisible to everyone else.
Nystagmus has many causes
The biggest scientific weakness of the HGN is that nystagmus is not unique to alcohol. It can be caused by fatigue, certain medications, caffeine, neurological conditions, inner-ear problems, and even occurs naturally in some people. There are dozens of recognized causes that have nothing to do with intoxication. When an officer attributes the jerking to alcohol, they are drawing a conclusion that ignores all the other explanations, and that leap is exactly what a good defense examines.
How easily it is administered wrong
The HGN has a specific protocol, the speed of the pen, the distance from your face, the number of passes, and how long the eye is held at maximum deviation. Officers frequently deviate from that protocol, moving too fast, skipping steps, or rushing the test in the field. When the test is not administered exactly as required, the results become unreliable, and the deviations are often visible on the bodycam footage even when the eye movements are not.
Conditions at the roadside undermine it
Roadside conditions make the HGN even shakier. Flashing patrol lights, passing traffic, headlights, and a moving environment can all affect eye movement and interfere with the test. An eye following a pen against a backdrop of strobing lights is not being tested under anything like ideal conditions. Those environmental factors can produce the very movements the officer counts against you.
The officer already suspected you
Like all field sobriety tests, the HGN is scored by the person who already decided you might be drunk. There is no neutral measurement, no machine reading, just the officer's subjective count of clues. I make this broader point in my post on why field sobriety tests are not pass or fail. The HGN is the most extreme example, because it is the test where the subjectivity is hidden behind a veneer of scientific procedure.
It was optional in the first place
For most adults who are not on DUI probation, field sobriety tests including the HGN are voluntary, a point I explain in refusing field sobriety tests. Many people submit to the eye test believing they have no choice, when in fact they could have declined. That does not help after the fact, but it underscores that the HGN is an investigative tool the officer uses to build a case, not a neutral diagnostic.
How courts treat HGN
California courts allow HGN evidence, but with limits. The officer can describe the clues they observed, but there are real constraints on using the test to claim a specific blood alcohol level. Understanding those limits is part of keeping the HGN in its proper place, as one piece of subjective observation rather than as scientific proof of a number. The distinction matters when the test is presented to a jury.
How I challenge it
Challenging the HGN involves examining how it was administered, what conditions it was performed under, whether the officer is properly trained and followed protocol, and what innocent explanations exist for any nystagmus. I describe this broader approach in the unfair field sobriety test defense. Cross-examination on the specifics of the test often reveals that the officer's confident conclusion rests on a shaky foundation.
Why it matters to your case
The HGN often carries more weight with a jury than it deserves, precisely because it sounds scientific. That is exactly why it should never go unchallenged. When the test is properly examined, its conclusions frequently weaken, and a case that leaned heavily on the eye test can look very different. The broader menu of these challenges is in my guide to the top DUI defenses.
The bottom line
The HGN eye test is the least transparent and most subjective of the standardized field sobriety tests, resting on movements you cannot see, with dozens of innocent causes and a protocol that is easy to get wrong. It deserves serious scrutiny, not automatic acceptance. If the eye test was central to your arrest, that is exactly the kind of evidence worth challenging, because its scientific-sounding label often conceals how little a layperson, or a jury, can actually verify about it. Get a free written case analysis below, or call me directly at (888) 271-6644. I answer my own phone, 24/7. You can also read more from the DUI blog.