Alcohol or ethanol is an addictive CNS depressant that has varying effects on the subject. In small doses, it appears to promote positive mood while the blood alcohol level is rising. But when the blood alcohol concentration (BAC) is falling, it causes mental impairment and depression.
Blood Alcohol Concentration Effect
|Less than 50 mg/dL||Drowsiness or a sensation of being 'at ease with the world'|
|50-150 mg/dL||Loss of muscular coordination, incoherent speech, visual impairment, tunnel vision|
|150-200 mg/dL||Severe Loss of mental faculties and possible onset of stupor|
|300-400 mg/dL||Unconsciousness, stupor and even death in excess of this level|
The legal limit of Blood Alcohol concentration for drivers in the US is 100 mg/dL. In some states it is even lower: 80 mg/dL
Gaze test or Horizontal Gaze Nystagmus Test - (HGN test) is one of the 3 Standardized Field Sobriety Tests (SFST) conducted by the police to check if the person is driving under the influence of alcohol. Nystagmus refers to involuntary rapid movements of the eyes that are caused by abnormal function in the areas of the brain that control eye movements. Nystagmus is caused by various conditions, one of which is alcohol consumption. Gaze test is a useful indicator of alcohol intoxication.
During the Gaze test, the police officer will hold a stimulus such as a flashlight or pen or even his finger in front of the driver's face and moves it sideways. The officer then observes the eyes of the driver as he follows the moving object horizontally with his eyes. The suspect's eyes may bounce or involuntarily jerk (nystagmus) if he is impaired due to alcohol consumption. The higher a person's blood alcohol content level, the more pronounced the nystagmus becomes.
As the subject's eyes follow the object, police will look for the following three indicators:
1. Whether the suspect was able to follow the object smoothly. Normal and sober person follows the stimulus whereas person under the influence of alcohol may have difficulty following the object.
2. The extent of stability of the eye at maximum deviation.
3. The angle at which the jerking begins. Jerking movements in the eye before the gaze reaches a 45-degree angle is indicative of a higher blood alcohol content level.
Gaze test is the most accurate test of all the sobriety tests and does not require any special equipment to conduct it. However gaze test requires a well trained police officer to read the eye movements.
Bibliography / Reference
Collection of Pages - Last revised Date: December 18, 2018