So begins the administration of the horizontal gaze nystagmus (HGN) test to the suspected drunk driver. The officer then begins the magical process of waving a pen back and forth in front of the driver's eyes to determine if he or she consumed any alcohol. Nevermind that, by that point, the driver has usually admitted to drinking "a couple" of beers or mixed drinks.
But instead of looking for signs of impairment, let's look for some signs that our driver is not impaired. Did the driver keep his or her head still? Did the driver follow the movement of the pen with his or her eyes? Was the driver standing still?
In order for the officer to administer the "test" correctly, he must be able to look into the driver's eyes. In order for him to look into the driver's eyes, the driver must be standing still and not swaying. The stance the driver takes during this test is the most normal stance he or she will take during the administration of the standardized field sobriety tests (or, as I prefer to call them, the police coordination exercises).
In order to "take" the pen and eye test, the driver must exhibit some fine motor skills. He or she must stand still and, without moving his or her head, follow an object being passed in front of the eyes. The driver must also understand the directions he or she was given by the officer - a test of mental faculties.
Think about it, if the driver can't stand still, can't keep his head still, can't follow the pen with his eyes and can't follow directions, the officer can't complete the test.
Therefore, the very fact that the officer completed the administration of the HGN test demonstrates that the driver still had the use of his or her mental and physical faculties.