Horizontal Gaze Nystagmus (HGN) otherwise known as the follow the finger test
The officer is not gauging whether you can follow his finger/pen/stylus with your eyes; he is looking for nystagmus (involuntary jerking of the eye) at certain stages in the test. You cannot feel the jerking, It does not affect your vision and you do not know whether the officer observed it or not. If the officer videotaped the stop and SFSE, someone trained in SFSE can review the video to see whether the officer performed the test properly. If there is no video, the officer may be questioned about his proficiency in HGN. Some officers "learn" HGN from watching other officers and have not actually completed the training and certification. These officers usually do not have the expertise to survive a good cross exam by an experienced DUI attorney. There are several things that may cause HGN other than alcohol consumption, but if your Blood Alcohol Concentration (BAC) is .08 or greater, there will be nystagmus.
There are only six clues scored on HGN: lack of smooth pursuit in the left eye, lack of smooth pursuit in the right eye, distinct and sustained nystagmus in the left eye, distinct and sustained nystagmus in the right eye, the onset of nystagmus prior to 45 degrees in the left eye and the onset of nystagmus prior to 45 degrees in the right eye. The officer will also observe for vertical nystagmus. The clues are progressive, meaning that you cannot have distinct and sustained nystagmus without also having lack of smooth pursuit; you cannot have the onset of nystagmus prior to 45 degrees without having lack of smooth pursuit and distinct and sustained nystagmus. You also cannot have nystagmus in one eye and not the other in a valid test. If the driver is scored four out of six clues or more, it is indicative of a BAC of .08 or greater. Vertical nystagmus indicates whether the BAC is high for the individual being tested, but is not a clue used in scoring.
HGN and drugs
Nystagmus will occur with the use of some, but not all drugs. Marijuana, hallucinogens, central nervous system stimulants and narcotic analgesics will not cause nystagmus. Impairment from central nervous system depressants (including alcohol), Inhalants and dissociative anesthetics will cause nystagmus. There are additional field evaluations for determining whether the driver is under the influence of drugs. Some officers have the mistaken belief that vertical nystagmus, by itself, indicates drug use. This is not true and the case should be reviewed by a qualified drug recognition expert.
The Walk and Turn (WAT) otherwise known as "heel to toe"
The walk and turn is the second field sobriety evaluation. Drivers often think they "passed" this test only to discover later that they "failed." The confusion comes from the driver thinking that the officer is only looking to see if the driver can walk a straight line or walk heel-to-toe. The officer is trained to observe for eight separate clues during the WAT.
Scoring the Walk and Turn
There are two stages, the instructional stage and the walking stage. During the instructional stage there are only TWO clues; cannot maintain balance (scored when the driver steps out of the instructional position whether the driver lost his balance or not) and starts too soon. Too often officers score other invalid "clues" during this stage, but unless the defense attorney knows that these are the only two clues recognized during the instructional stage, it may be used against the driver during motions or trial. The walking stage has six clues: stops while walking, does not touch heel to toe, steps off line, raises arms for balance (more than 6 inches from the body), turns improperly or loses balance during turn, and takes the wrong number of steps. If the officer scores two or more clues from the combined eight clues, the driver "failed" the test
Challenging the Walk and Turn
Often the clues the officer observed are due to improper or incomplete instructions. If the officer leaves out part of the instruction or modifies the instructions, it may invalidate the test. Every officer certified by NHTSA in DWI Detection and Standardized Field Sobriety Testing, was required to pass an evaluation, including administering the instructions for this test, but over time they often cut corners or leave steps out. Officers will sometimes ask a general qualifying question, such as, "is there any reason you cannot perform this test" before the driver even knows what the test will be. It is the officer's responsibility to medically qualify the driver before the test, not the driver's. Back or leg injuries may invalidate the test results.
One Leg Stand (OLS)
The one leg stand is the third test in the SFSE. During this test the officer has the driver stand on one leg and count while the officer times the driver for thirty seconds. There are only four clues in the OLS: puts foot down, raises arms for balance, sways, and hops. If the officer scores two or more clues during this test, the driver "failed" the test. The OLS may be invalid if the officer failed to medically qualify the driver before administering the test. The "sway" clue may be subjective and should be reviewed if it is one of the clues used to fail the driver. Other factors such as environmental (weather, traffic, and road conditions) or other distractions may be used to challenge a failed test. Because NHTSA studies have ranked the OLS to be more accurate a predictor of impairment than the WAT, if the driver passed the OLS, but failed the WAT, the results of the OLS may be used to challenge the results of the WAT.
Testing to NHTSA Standards
The reason for standardizing the field sobriety evaluations in the first place was to allow the use of study results to correlate the score to a specific BAC. When used by law enforcement, the SFSE is a powerful tool for making DWI/DUI arrests. But NHTSA recognizes that the SFSE are only reliable when done properly. Page VIII-19 of the latest Student Manuel reads, "IT IS NECESSARY TO EMPHASIZE THIS VALIDATION APPLIES ONLY WHEN: THE TESTS ARE ADMINISTERED IN THE PRESCRIBED, STANDARDIZED MANNER THE STANDARDIZED CLUES ARE USED TO ASSESS THE SUSPECT'S PERFORMANCE THE STANDARDIZED CRITERIA ARE EMPLOYED TO INTERPRET THAT PERFORMANCE" This leaves room for the experienced DWI/DUI defense attorney find the discrepancies in the administration of the SFSE and challenge the results.