Horizontal gaze nystagmus or HGN is a very scientific, complex examination. Under proper circumstances, competent medical professionals can use HGN as a diagnostic tool for measuring gross neurological dysfunction often associated with head trauma.
Law enforcement officers rely heavily on HGN to form an opinion that a driver is impaired by alcohol.
Did you know there are 86 types of nystagmus, the overwhelming majority of which do not involve alcohol?
One of those types is optokinectic nystagmus. This is an involuntary movement of the eyes that occurs in response to a moving stimuli. The eyes track a feature of the stimulus and then rapidly move in the opposite direction.
I took this video of my son to show how optokinectic nystagmus can be induced by slow moving objects. The response to the stimuli is obvious. I can assure you no alcohol was involved (he had oatmeal and orange juice for breakfast!)
If I can induce optokinetic nystagmus with a school bus, then law enforcement officers can certainly induce it by not deactivating their overhead emergency lights. Other factors come into play, too, in traffic stops where the emergency lights are turned off or the driver is positioned to face away from the patrol cruiser. These include passing traffic, vehicle headlamps, emergency light reflection against road signage and nearby flashing signs.
The important thing to keep in mind is that unlike doctors, law enforcement officers do not have the requisite training and skill to properly evaluate HGN and differentiate it from types that are not alcohol related.