What physiological response is measured during the Horizontal Gaze Nystagmus test?

The Horizontal Gaze Nystagmus (HGN) test specifically measures the involuntary jerking of the eyes as they gaze to the side. This physiological response occurs when the eyes are unable to smoothly follow a moving object, which is an indication of potential impairment often associated with alcohol consumption.

When a person's level of intoxication increases, the brain's ability to control eye movement diminishes, leading to noticeable eye jerking. During the HGN test, law enforcement officers observe this jerking as the individual follows a stimulus, usually a pen or finger, moved horizontally across their line of sight. The presence of this involuntary eye movement is a key indicator used to assess whether a person may be under the influence of alcohol.

The other options relate to different phenomena that do not specifically measure the same response as the HGN test. Rapid eye movement during tracking refers generally to eye movements and does not specify the involuntary jerking aspect related to impairment. Eye redness is typically a result of alcohol and not a measure used in the HGN test. Pupil dilation in response to light is a normal physiological response that does not indicate impairment related to alcohol. All these distinctions highlight why the involuntary jerking of the eyes is the correct focus for