What is the HGN or "follow my finger" eye test?

Horizontal gaze nystagmus is the pen-light test. The officer moves a stimulus across your field of vision and watches for involuntary jerking at certain points. Six clues, three per eye.

Here's what matters legally, and it's specific to Florida. The Fourth District — which covers this area — has held HGN is scientific evidence, not simply an observation an officer can describe. Its relevance is outweighed by the danger of unfair prejudice and jury confusion unless the traditional predicates for scientific evidence are satisfied. Lay testimony on HGN results wasn't permitted.

That's a meaningful limitation, because officers frequently describe HGN as their most reliable indicator, and the foundation isn't always laid correctly.

A separate line of authority addresses whether a properly qualified drug recognition expert can supply testimony permitting HGN to be considered for probable cause.

Worth knowing: nystagmus has causes besides alcohol — certain medications, medical conditions, fatigue, and even the strobe of the patrol car's own lights.