Smooth Pursuit

By James Parker
/
March 9, 2022

What Is Smooth Pursuit?

Smooth pursuit is defined as a motion made by the eye that is free from jerky or erratic motion as it tracks an object in motion. Smooth pursuit is often a criteria that is tested by optometrists as well as law enforcement in specific situations.

Smooth pursuit tests are designed to test the central vestibular system. The vestibular sense informs the brain about the body’s motion, head position, and spatial orientation. This system is most frequently implemented in order to maintain the body’s sense of balance. 

Smooth pursuit tests assess the vestibular system by having subjects place their head in a device that has a colored dot visible. The dot will move from side to side in a smooth, predictable motion. It is the subject’s job to follow the dot exactly without moving their eyes too much and getting “ahead” or “behind” it. While the test may need to be administered several times to ensure that the data recorded is accurate, the smooth pursuit test can be invaluable to optometrists and neurologists who use it as an indicator of vestibular or neurological conditions.  

Key Takeaways

  • Smooth pursuit is the uninterrupted motion of the eye as it tracks an object in motion.
  • Neurologists and optometrists use smooth pursuit tests to analyze for any neurological or vestibular problems in subjects.
  • Law enforcement officers may administer a horizontal gaze nystagmus test in order to test for a suspected drunk driver’s lack of smooth pursuit despite conditions that may be unhelpful to the test.
  • If you have been charged with a DUI after allegedly failing a horizontal gaze nystagmus test, an experienced DUI attorney may be able to improve the outcome of your case by leveraging experience and expert testimony.

Smooth Pursuit and Driving Under the Influence (DUI) Charges

In addition to doctors and neurologists using smooth pursuit tests in order to determine whether a subject is suffering from neurological damage, law enforcement officers also attempt to test for smooth pursuit in the field. When a law enforcement official pulls over an individual suspected of driving under the influence, they may cite an individual’s lack of smooth pursuit as the justification for arrest. 

Law enforcement officers who suspect an individual of DUI can administer roadside sobriety tests, also known as field sobriety tests. These tests are theoretically designed to demonstrate to the officer whether or not an individual is likely to be intoxicated. Performance on these tests can create reasonable suspicion that will allow the officer to charge the individual with a DUI, administer a blood alcohol content (BAC) test, or search the vehicle for intoxicating substances. 

The most common method that an officer will use to test for smooth pursuit is known as the horizontal gaze nystagmus test. This test involves an officer taking a pen or other small object and moving it back and forth in the field of view of the subject. The subject will be instructed to follow the object back and forth to the best of their ability while the officer attempts to see whether or not the subject can follow the object with smooth pursuit. 

There are several flaws with this form of administration. First, in laboratory settings there are controls for circumstances including lighting. During a field sobriety test it is often dark and in addition to the officer’s flashlight, there are also the flashing lights of the squad car. These distractions can draw the eye’s attention, creating the illusion of jerky motion when in fact the eye is just struggling to focus on the object proposed.

Second, the officer is not an objective, trained professional. When an optometrist or neurologist administers a smooth pursuit test, they are doing so with the knowledge about the limitations of this testing format as well as computer analysis of the eye movement providing unbiased data regarding the eye motion. This data is then analyzed against a baseline chart to see if the recorded data contrasted with the established baseline.

By contrast, an officer who administers a horizontal gaze nystagmus test is not in a laboratory setting. There are confounding factors ranging from distractions, to anxiety, to exhaustion and there is no baseline for the driver to be compared against. Even if the officer has performed multiple horizontal gaze nystagmus tests before, those were also flawed, less than ideal circumstances that could not produce unassailable, objective results. 

Finally, officers have their thumbs on the scale. There is only one officer on the scene to check the driver’s behavior. Additionally, the single officer there already has a personal incentive to find fault with the driver’s test results since the officer pulled them over under the suspicion that they were driving drunk. This produces a conflict of interest that pushes the officer away from objectivity.

Bottom Line

Smooth pursuit is a high bar to clear and when you are being administered a horizontal gaze nystagmus test, it is likely that the officer has already determined whether or not they are going to try to attempt an arrest. If you have been charged with a DUI on the strength of a horizontal gaze nystagmus test, you need an experienced DUI attorney.

A DUI attorney can help you achieve the best possible outcome in your case. By leveraging expert testimony, cross examining witnesses, and challenging the evidence submitted, a DUI attorney can expose the flaws in having someone other than a medical professional examine for smooth pursuit and help improve the outcome of your chances of dismissal.

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