What is required of the suspect's eyes during the HGN test instructions?

During the Horizontal Gaze Nystagmus (HGN) test, it is essential for the suspect to follow the examiner's stimulus, usually a pen or flashlight, with their eyes only. This means that the suspect should not move their head but rather keep their head still while tracking the stimulus. This instruction is crucial because it allows the examiner to observe the way the eyes move and any indicators of nystagmus, which can be affected by alcohol or other drugs.

Focusing solely with the eyes ensures that any involuntary eye movements are clearly visible, allowing for a proper assessment of impairment. Other instructions or actions like focusing on the examiner's face or looking straight ahead do not accommodate the specific requirements of the HGN test, as they do not allow for the necessary observation of eye movement patterns. Closing the eyes would completely eliminate the ability to conduct the test effectively, making it impossible for the examiner to assess nystagmus.