If you have been arrested for DWI, you may hear terms like Standardized Field Sobriety Tests, Horizontal Gaze Nystagmus Tests, or Vertical Gaze Nystagmus Tests. HGN and VGN are similar but they are not equally reliable. In most cases, the VGN should be suppressed and found to be inadmissible.
This article explains why VGN evidence is legally different from other sobriety tests and how Texas courts treat it.
What Is Vertical Gaze Nystagmus?
Vertical Gaze Nystagmus (VGN) refers to an involuntary jerking of the eyes (up and down) which occurs when the eyes gaze upward at maximum elevation. The jerking should be distinct and sustained. Some officers are trained to testify that the presence of VGN suggests a very high blood alcohol concentration. However, Texas courts have not recognized the scientific theory or technique for VGN as scientifically reliable. Its admissibility is questionable and should be excluded before trial.
Why VGN Is Different from other Sobriety Tests
You may have heard of Horizontal Gaze Nystagmus (HGN), the involuntary jerking of the eyes occurring as the eyes gaze to the side. Texas courts have ruled that the scientific theory and technique for HGN is scientifically reliable and may be admitted in court when the technique is properly applied on the occasion in question. According to the National Highway Traffic Safety Administration, the involuntary jerking of the eyes becomes readily noticeable when a person is impaired by alcohol and certain drug categories. As a person’s blood alcohol concentration (BAC) increases, the eyes will begin to jerk sooner as they gaze to the side.
However, that ruling applies only to HGN—not to VGN.
There is no blanket approval for VGN evidence. Courts have repeatedly stated that VGN has not been proven reliable through scientific research.
Texas Courts Have Rejected VGN without Scientific Proof
Texas appellate courts have held that VGN evidence may not be admitted as evidence unless the State first proves that it is scientifically reliable. One court explained that admitting VGN without that proof is an abuse of discretion and stated: “Without scientific proof behind this theory, we cannot find the evidence admissible.”
Why This Matters to Your DWI Case
Despite clear case law, officers still routinely mention VGN in reports and testimony. When that happens without proper scientific support, it can unfairly influence a jury.
It is therefore important in most cases to:
- Challenge VGN evidence before trial
- Prevent officers from presenting unproven scientific opinions
- Limit or exclude unreliable testimony
- Make sure the court follows Texas evidence rules
When VGN evidence is successfully challenged, the evidence should be redacted from all video of the incident and excluded from being admitted through testimony.
The Bottom Line
Not all sobriety tests carry the same weight in Texas courts. Vertical Gaze Nystagmus is not recognized as reliable science, and it should not be treated as proof of intoxication.
This article is for general information only and does not constitute legal advice. Every case is different.
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