What is an indication of impairment when observing nystagmus during the HGN test?

The indication of impairment during the Horizontal Gaze Nystagmus (HGN) test is observed when eye jerks occur before expected angles. During this test, a law enforcement officer checks the subject's eyes as they follow a moving object, typically a pen or the officer's finger. A key aspect of the HGN test is that, under normal conditions, the eyes should smoothly track the object up to a certain angle.

However, if nystagmus is present, the eyes will exhibit involuntary jerking movements before reaching the expected angle of 45 degrees. This early onset of jerking is a strong indicator of potential impairment, suggesting the individual's blood alcohol concentration (BAC) may be above the legal limit or that other substances affecting motor function are involved. The presence of this involuntary movement helps officers assess the degree of impairment effectively during DUI traffic stops.

In contrast, other scenarios do not indicate impairment. For instance, if the eyes remain still, track smoothly, or converge perfectly, these are signs of normal function and do not suggest any impairment related to alcohol or drugs. Therefore, the specific observation of eye jerks prior to the anticipated angle is a critical sign of impairment during the HGN test.