Why is HGN testing specific to alcohol impairment as opposed to other substances?

The reasoning for why HGN testing specifically indicates alcohol impairment rests on the unique way that alcohol affects the central nervous system. HGN, or Horizontal Gaze Nystagmus, refers to an involuntary jerking of the eyes that occurs when an individual looks to the side. This phenomenon is accentuated when a person has consumed alcohol because alcohol specifically depresses the functioning of the cerebellum and vestibular system, which are responsible for motor control and eye movement.

While various other substances can also influence eye movement and can lead to impairment, the distinctive effects of alcohol on these brain areas make HGN a reliable indicator of alcohol impairment specifically. The physiological response observed in HGN testing is highly associated with alcohol consumption rather than other substances, which may not produce the same pronounced eye movement effects.

Therefore, the validity and accuracy of HGN testing in determining alcohol impairment arise from the specific interaction between alcohol and the central nervous system, making it a targeted assessment for this substance.