What is assessed during the Horizontal Gaze Nystagmus (HGN) test?

During the Horizontal Gaze Nystagmus (HGN) test, the primary focus is on the involuntary jerking of the eyes. This particular phenomenon occurs when the eyes are moved laterally, and it can be indicative of impairment. When a person is under the influence of alcohol or certain drugs, the smooth tracking of the eyes may be disrupted, leading to noticeable jerking that can be observed as they follow a stimulus, such as a pen or finger, moving horizontally across their field of vision.

The presence and degree of this nystagmus are critical indicators used by law enforcement to assess potential impairment. Medical literature supports that the rate and angle at which this jerking occurs can provide objective data on a person's blood alcohol concentration level, typically becoming more pronounced as impairment increases. Therefore, option B accurately represents what is specifically assessed during the HGN test.