612.1Horizontal Gaze Nystagmus (“HGN”)

Law and Issues
Last Updated: 12/01/23

Key Concepts

  • Horizontal Gaze Nystagmus, or “HGN,” refers to a jerking or stuttering type of movement observed in a person’s eyes that occurs as a result of consuming alcohol or other nervous system depressants.
  • HGN can be observed and documented through a simple series of visual tests administered by a properly trained examiner.
  • Rule 702(a1) states that evidence of a properly administered HGN test by a qualified witness is admissible as evidence of impairment (but not a specific blood alcohol content); therefore, no further reliability determination under Daubert is required.

Basic Explanation of HGN 

Horizontal Gaze Nystagmus (“HGN”) refers to a stuttering or jerking movement that can be observed in a person’s eyes after he or she has consumed alcohol and certain other drugs. As explained in this Resource Guide from NHTSA, NDAA, and APRI, the scientific rationale behind HGN is that alcohol is a central nervous system depressant, and therefore consuming it interferes with fine motor control of the muscles in the eyes, just as it interferes with more commonly observed aspects of motor control such as balance, coordination, and reflexes. HGN can be observed and measured through the administration of a series of simple tests thatinvolve asking the subject to track the movement of a pen or other stimulus using only his or her eyes, and noting whether the jerking movement is visible as the subject tracks the stimulus, or when the subject attempts to hold a steady gaze on the stimulus near the edge of his or her field of view. 

Practice Pointer

Alternate explanations for nystagmus 
One of the most common defense strategies against HGN evidence is to argue that the nystagmus could have been caused by something other than alcohol, such as the flashing lights of the patrol car, a medical issue such as brain damage, or simple eye fatigue. Each of these alternate explanations is discussed and refuted in the Resource Guide cited above. Prosecutors (and testifying officers) should familiarize themselves with these arguments in advance, and be ready to respond. For example, the state may need to follow up on redirect examination to clarify that the subject was facing away from the patrol car’s flashing lights, or verify that the officer held the stimulus at maximum deviation for the full recommended time to observe the nystagmus, so it was not just a “natural tremor” as the person’s eye “caught up” to the stimulus. 

Under Rule 702(a1), expert testimony on HGN is admissible “solely on the issue of impairment and not on the issue of specific alcohol concentration….” G.S. 8C-1, Rule 702. This means that a properly qualified witness can testify that he or she observed HGN and give an opinion as to the subject’s “impairment” based on that, but the witness may not testify that based on the HGN test he or she concluded the defendant had a particular BAC number such as “at least .15 or higher.” See State v. Torrence, 247 N.C. App. 232 (2016) (prejudicial error where officer testified to a specific alcohol concentration); State v. Turbyfill, 243 N.C. App. 183 (2015) (officer’s testimony as to the defendant’s BAC appears to have violated Rule 702(a1) but the error did not have a probable impact on the verdict).  

Rule 702(a1) and Admissibility 

Rule 702 was amended in 2006 and again in 2017, and it now explicitly permits expert testimony on HGN under certain conditions. See G.S. 8C-1, Rule 702(a1)State v. Smart, 195 N.C. App. 752 (2009). The amended rule now provides that “notwithstanding any other provision of law, a witness may give expert testimony solely on the issue of impairment and not on the issue of specific alcohol concentration level relating to . . . [t]he results of a[n] [HGN] Test when the test is administered in accordance with the person’s training, by a person who has successfully completed training in HGN.” G.S. 8C-1, Rule 702(a1). 

By its express terms, Rule 702(a1) therefore removes the requirement that the state must prove that the horizontal gaze nystagmus testing method is sufficiently reliable in every case. However, the results of the test are only admissible if the state shows that the witness has successfully completed training in HGN, and that the test was administered in accordance with that training. See G.S. 8C-1, Rule 702(a1)(1). Additionally, if the witness holds a current certification from DHHS as a Drug Recognition Expert (“DRE”), the witness may testify as to whether the person was under the influence of an impairing substance, and the category of that substance. See G.S. 8C-1, Rule 702(a1)(2).

Historically, there was some uncertainty over how trial courts should reconcile the explicit legislative authorization of HGN admissibility in Rule 702(a1) with a judge’s normal obligation under Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993), and Rule 702(a) to act as a “gatekeeper” and determine whether proposed expert testimony is reliable before allowing it into evidence.

Recent appellate cases have resolved this issue, and conclusively held the statutory authorization in Rule 702(a1) is controlling as to this reliability question; therefore, the trial court does not conduct an independent reliability determination on HGN. As long as the state meets the statutory foundational requirements, the expert testimony on HGN is admissible. See State v. Younts, 254 N.C. App. 581 (2017) (so holding – trial court was not required to make its own reliability determination before admitting HGN testimony); State v. Godwin, 369 N.C. 604 (2017) (“Furthermore, with the 2006 amendment to Rule 702, our General Assembly clearly signaled that the results of the HGN test are sufficiently reliable to be admitted into the courts of this State.”); State v. Smart, 195 N.C. App. 752 (2009) (“We interpret this amendment to Rule 702(a1) as obviating the need for the State to prove that the HGN testing method is sufficiently reliable.”). 

Notwithstanding the clear language in these recent cases, the trial judge might still exclude expert testimony about HGN if, for example, he or she found that it was unreliable because the officer failed to administer the test correctly. See, e.g., Commonwealth v. Camblin, 31 N.E.3d 1102, 1111 (Mass. 2015) (recognizing that the legislature has the power to prescribe the rules of evidence and the methods of proof for trial but stating that this power “does not mean that the reliability of every type of?evidence?the?Legislature?may deem?admissible, particularly in a criminal case, is automatically insulated from challenge and review on reliability grounds”).

Finally, although the better practice is to do so formally, the court is not required to expressly determine that a witness is qualified in HGN; such a determination can be implied from the record. See State v. Godwin, 369 N.C. 604 (2017) (holding that the trial court implicitly found that the witness was qualified to testify, but noting that “the appellate division's ability to review the trial court's oral order would have benefited from the inclusion of additional facts supporting its determination that [the] Officer . . . was qualified to testify as an expert regarding his observations of defendant's performance during the HGN test”).  

Practice Pointer

Expert’s report? 
Rule 702(a1) states that “a witness may give expert testimony” about HGN – so, in cases where the state’s discovery obligations apply (e.g., DWIs originally charged in superior court, or other felony offenses involving HGN), the state must comply with G.S. 15A-903(a)(2) by giving notice to the defense that it reasonably expects to call the witness as an expert, and turning over “a report of the results of any examinations or tests conducted by the expert.” In most cases, this “report” will simply be the narrative in the officer’s arrest report.

Portions of this entry were excerpted from the North Carolina Superior Court Judges’ Benchbook, “Criminal Evidence: Expert Testimony,” Aug. 2017, Jessica Smith.