During the Horizontal Gaze Nystagmus (HGN) test, which eye is tested first?

In the Horizontal Gaze Nystagmus (HGN) test, the left eye is tested first to provide a consistent and standardized approach to the evaluation of a subject's eye movement. This procedure is crucial in helping officers determine potential impairment due to substances such as alcohol.

Beginning with the left eye allows the officer to maintain a systematic method in conducting the test, ensuring that all subjects are evaluated in the same manner, which is vital for the reliability of the results. The HGN test specifically looks for involuntary jerking of the eyes at various angles as the eyes track a stimulus, and starting with one eye then moving to the other helps to create a clear and orderly assessment process.

Testing both eyes simultaneously would not provide the focused detail needed to observe each eye's reaction to the stimulus. It is also important that the examiner maintains this consistency to avoid any bias or confusion during the evaluation process. The choice to begin with the left eye ties back into standard protocols in field sobriety testing, underscoring its significance in law enforcement practices.