The question of whether marijuana, or its primary psychoactive compound delta-9-tetrahydrocannabinol (THC), causes Horizontal Gaze Nystagmus (HGN) is central to roadside impairment testing. HGN is a specific, involuntary eye movement used in Field Sobriety Tests (FSTs) to determine impairment. The reliability of this test for cannabis impairment, compared to alcohol impairment, depends entirely on the differing physiological effects of these substances on the central nervous system.
Defining Horizontal Gaze Nystagmus
Horizontal Gaze Nystagmus describes an involuntary, rapid, and rhythmic jerking of the eye that occurs when the eye is held at maximum lateral deviation. This motion is caused by a temporary disruption of the central nervous system’s ability to coordinate eye movements and maintain a stable gaze. The body’s vestibular system works with the oculomotor system to keep the eyes fixed on a target. When this finely tuned system is compromised by certain substances, the smooth pursuit movement breaks down, manifesting as involuntary jerking.
The HGN test involves an officer observing a subject’s eyes as they follow a stimulus moved slowly from the center to the side of the face. Officers look for three distinct clues in each eye: the inability to follow the stimulus smoothly, distinct jerking when the eye is held at its maximum lateral angle, and the onset of jerking before the eye reaches a 45-degree angle.
The Established Link: Alcohol and Nystagmus
The relationship between alcohol consumption and HGN is well-documented and forms the basis for its inclusion in standardized testing procedures. Alcohol acts as a central nervous system (CNS) depressant, which directly interferes with the brain mechanisms governing eye movement control. This interference reliably produces HGN by disrupting the function of the cerebellum and the vestibular system.
As alcohol concentration rises, the eyes’ ability to track smoothly is impaired, and the involuntary jerking becomes more pronounced. The degree of HGN observed correlates strongly with a person’s blood alcohol concentration (BAC). The onset of jerking at an angle of 45 degrees or less is a specific indicator suggesting the BAC may be at or above the legal limit. This predictable relationship makes HGN the most accurate of the three Standardized Field Sobriety Tests for indicating alcohol impairment.
Scientific Consensus on THC and HGN
Scientific studies agree that THC does not reliably induce true Horizontal Gaze Nystagmus in the same manner or degree as alcohol. This difference occurs because THC does not operate as a generalized central nervous system depressant like ethanol. Instead, THC interacts with the endocannabinoid system, resulting in a different profile of psychoactive effects and physical symptoms.
The specific physiological pathway leading to the involuntary jerking of HGN is typically not activated by THC alone. Law enforcement personnel trained as Drug Recognition Experts (DREs) are taught that the absence of HGN is an expected finding when a subject is solely impaired by cannabis. While some research notes issues with smooth pursuit, these subtle differences do not consistently produce the specific signs of HGN observed with alcohol impairment. Therefore, HGN testing is considered an unreliable marker for determining cannabis impairment in isolation.
Other Cannabis-Related Visual Indicators of Impairment
Since HGN is not a consistent indicator of cannabis use, other visual signs are utilized to assess potential impairment. The most commonly noted visual indicator is Lack of Convergence (LOC), which refers to the inability of the eyes to turn inward and cross properly when focusing on an object moving toward the bridge of the nose. LOC is included in the Drug Recognition Expert (DRE) evaluation protocol and is considered a much more predictive sign of cannabis impairment than HGN.
Inability to converge the eyes demonstrates a breakdown in the coordination of the six muscles surrounding each eye, a common effect of acute THC exposure. Other physiological changes frequently observed with cannabis impairment form a specific profile used by trained experts:
- Lack of Convergence.
- Distinct pupil dilation (mydriasis).
- Slowed or poor reaction to light.
- Increased pulse rate and elevated blood pressure.