“Follow the Pen”: Why the Police Eye Test is Voodoo Science

By: John Guidry

Imagine driving late at night. You are in a bad part of town, alone. Suddenly, blue and red lights flash behind you. You pull over, nervous. (Who likes this hassle?). The officer shines a spotlight in your face and asks if you’ve been drinking.

• Your Answer: “Two beers.” (The worst, but most common response).
• The Request: “Please step out for some Field Sobriety Tests.”

The Setup: It’s dark. Traffic is whizzing by. You are nervous. And now, the officer wants you to perform roadside gymnastics that you probably couldn’t do sober.

• The Reality: FSTs are designed to make you fail. They give the same test to a 21-year-old athlete as they do to a 65-year-old with a bad knee.

Did you fail the roadside exercises because you were nervous, not drunk?

Anxiety is not a crime. Call John today at (407) 423-1117.

The Big Three: Standardized Tests

Decades ago, the government determined that three specific tests were the “most reliable” for detecting impairment. These are the ones you will likely face:

1. Walk and Turn: (Walking a straight line, heel-to-toe).
2. One Leg Stand: (Balancing on one foot while counting).
3. Horizontal Gaze Nystagmus (HGN): (The Eye Test).

The Flaw: These tests seek to divide your attention. They assume that if you can’t listen to instructions while balancing on one foot, you must be drunk. But studies show that fatigue, nervousness, and age can cause the exact same failures.

Deep Dive: The HGN (Eye) Test

The Horizontal Gaze Nystagmus (HGN) test is the one where the officer waves a pen in front of your face.

• The Theory: Alcohol causes an involuntary jerking of the eyeball (nystagmus) as it moves to the side.
• The Procedure: The officer looks for three clues:
  1. Lack of Smooth Pursuit: Does the eye jerk as it follows the pen?
  2. Distinct Nystagmus at Maximum Deviation: Does the eye jerk when held all the way to the side?
  3. Onset Prior to 45 Degrees: Does the jerking start before the eye reaches a 45-degree angle?

Why It’s “Voodoo Science” The officer claims this test is scientific proof of alcohol. It isn’t.

• False Positives: Nystagmus can be caused by Caffeine, Nicotine, Aspirin, and even Fatigue.
• The Tired Eye: Most people have nystagmus at “Maximum Deviation” simply due to eye strain. If the officer holds the pen there long enough, your eye will twitch, drunk or sober.
• The Environment: Doing this test on the side of a highway with strobing police lights in your face is hardly a “controlled scientific setting.”

Is the Eye Test Admissible?

Can the jury hear that your eyes twitched? That depends. Because HGN is considered “scientific” evidence, the State must lay a proper legal foundation before admitting it.

• The Requirement: The prosecutor must prove the officer is properly trained, educated, and administered the test exactly according to the manual.
• The Limit: Even if admitted, the officer cannot say, “His eye twitch meant he had a .12 BAC.” They can only say it indicated “impairment.”

If the officer moved the pen too fast, held it too close, or did not qualify as an expert, we file a Motion to Suppress to throw the eye test out of court.

John’s 2026 Update: The “AI Optometrist”

Note: In the past, we argued about what the officer saw. In 2026, we argue about what the computer saw.

1. AI-Enhanced Body Cams In 2026, many departments use Body Cams equipped with Real-Time Analysis Software.

• The Tech: As the officer moves the pen, the camera tracks your pupils. The AI gives a “Pass/Fail” score instantly on the officer’s display.
• The Threat: Prosecutors love this because it looks objective. “It’s not just the officer’s opinion; the computer confirmed the nystagmus.”
• The Defense: We subpoena the Source Code and the Calibration Logs. Often, these AI models are trained on perfect lighting conditions and fail miserably in the dark with flashing blue lights.

2. “Smart” Glasses Some DUI task forces now wear Smart Glasses specifically for HGN.

• The Data: These glasses record the exact angle of onset (e.g., “Jerking at 38 degrees”).
• The Flaw: Just like the manual test, the machine cannot distinguish between alcohol-induced nystagmus and caffeine-induced nystagmus. The machine measures the twitch, but it doesn’t know the cause. We bring in medical experts to prove that your “fail” was actually just exhaustion.

3. Video is Your Best Friend In 2026, virtually every stop is recorded in high definition.

• The Strategy: We watch the video in slow motion. If the officer claims you were “swaying” but the video shows you standing perfectly still, or if the officer moved the pen way too fast (a common trick), the video proves the police report is a lie.

Don’t Trust the Field Tests

The roadside is not a laboratory, and you are not a lab rat. If you were arrested based on these “gymnastics,” we can fight the results.

Call me at (407) 423-1117. Let’s review the video.

About John Guidry II

John Guidry II is a seasoned criminal defense attorney and founder of the Law Firm of John P. Guidry II, P.A., located in downtown Orlando next to the Orange County Courthouse, where he has practiced for over 30 years. With more than three decades of experience defending clients throughout Central Florida since 1993, Guidry has successfully defended thousands of cases in Orange, Seminole, Osceola, Brevard, Lake, and Volusia counties. He has built a reputation for his strategic approach to criminal defense, focusing on pretrial motions and case dismissals rather than jury trials.

Guidry earned both his Juris Doctorate and Master of Business Administration from St. Louis University in 1993. He is a member of the Florida Bar and the Florida Association of Criminal Defense Lawyers. His practice encompasses the full spectrum of Florida state criminal charges, with a particular emphasis on achieving favorable outcomes through thorough pretrial preparation and motion practice.

Beyond the courtroom, Guidry is a prolific legal educator who has authored over 400 articles on criminal defense topics. He shares his legal expertise through his popular YouTube channel, Instagram, and TikTok accounts, where he has built a substantial following of people eager to learn about the law. His educational content breaks down complex legal concepts into accessible information for the general public.

When not practicing law, Guidry enjoys tennis and pickleball, and loves to travel. Drawing from his background as a former recording studio owner and music video producer in the Orlando area, he brings a creative perspective to his legal practice and continues to apply his passion for video production to his educational content.