But wait just a second - even though this "test" is couched in scientific terms, the follow-the-pen show on the video is far from being either scientific or valid.
Nystagmus is defined by NHTSA as "an involuntary jerking of the eyes." NHTSA states that alcohol causes horizontal gaze nystagmus.
The American Academy of Opthamology defines the condition as "is an unintentional jittery movement of the eyes" that "usually involves both eyes and is usually exaggerated by looking in a particular direction."
So, for your police officer trained to diagnose eye conditions by another police officer, nystagmus is an indicator of intoxication. To a trained medical practitioner, however, nystagmus is a medical condition.
When an officer detects "nystagmus" someone is likely to be placed under arrest and hand-cuffed. When a trained medical practitioner detects nystagmus, on the other hand, he is going to conduct a thorough evaluation and will more than likely call in another specialist to examine the patient.
When an officer conducts the pen-and-eye test in the field, there are no controls for the effects of weather, lighting or traffic conditions. The officer must estimate the time for each pass of the pen and the distance the pen travels to the side. When a trained medical practitioner conducts an eye test to determine if nystagmus is present, the test is performed in a controlled environment with medical instruments by a doctor who went to medical school.
Finally, when an officer detects "nystagmus," he is looking for evidence to confirm his suspicion that the motorist is intoxicated. When a trained medical practitioner detects nystagmus, his job is to eliminate all possible causes until he can diagnose the underlying cause.
The scientific literature indicates a myriad of conditions responsible for nystagmus - from epilepsy to Parkinson's disease and from muscular dysfunction to head trauma. But, according to the police officer's diagnosis, the only cause for nystagmus of a driver being tested at the side of the road is alcohol consumption.