As we know alcohol weakens driving skills and increases the risk of accidents which leads to injury and/or death. In USA after drinking RTA case are 31% per year, in CHINA 34% and in INDIA 40% per year. Why do accidents happen after drinking?
Alcohol is well known for its ability to change psychomotor coordination of the body. However, obtaining valid objective measures of performance under intoxication is extremely difficult; this is because alcohol intoxication is not a noble state for the subject who is tested. There is no certain dose or amount which a person can consume. Although alcohol drinking is legal for a certain level and for a certain age group, it is illegal to drive under the influence of alcohol.
It is figured out that a candidate for such objective measures might have disturbed fine motor skills and automatic function of visual system. Both these functions are easy target to the effect of alcohol and other drugs as well. They are largely involuntary; therefore, they cannot be accepted for objective test.
When we drive after consuming alcohol our motion parallax, which is the ability to recover depth from retinal motion, is hampered. Motion parallax is an important part of the visual system which is needed for driving. Recent work indicates that the sense of depth from motion parallax relies on the slow eye movement system. It is well known that alcohol intoxication reduces the gain of this slow eye movement system, the basis for the “horizontal gaze nystagmus”field sobriety test.
Current studies show that alcohol intoxication also weakens the sense of depth from motion parallax due to its influence on the slow eye movement system. The observer’s thresholds in both active and passive motion parallax tasks are largely increased by severe alcohol intoxication. Perhaps, such a failure of motion parallax plays a role when intoxicated drivers must make quick judgments with what could be inaccurate or missing perceptual information about the location of obstacles around them.
Visual processing is of importance to driving. Drivers must be able to see objects and then extract information about the object, about its position and orientation. To do so, drivers must constantly move their eyes so that the fovea, the area with the best visual acuity approximately 6\9 and visual field 120º is stimulated to enable safe day time driving.
These eye movements are produced by two different systems. The fast eye movement system generates saccadic or ballistic eye movements to foveate an item of interest. The slow eye movement system produces tracking eye movements to maintain fixation on an object during movement of the object or movement of the observer.
Alcohol effect on both the fast and slow eye movement systems has been well studied¹. Studies have shown that after drinking Alcohol, intoxication slows the initiation and the speed of saccadic eye movements and reduces the gain of slow eye movements².Gain is a ratio of eye velocity / target velocity and should be close to 1.0 to maintain fixation on the moving target. With alcohol intoxication, the slow eye movements are too slow, gain is less than 1.0 and the visual system must recruit the fast eye movement to generate a “catch-up saccade. This produces the jerky eye movement, also called horizontal gaze nystagmus that is an important component of field sobriety tests³
The specific effects of alcohol on visual perceptual performance are also well known. For instance, contrast sensitivity is affected by alcohol⁴ but visual acuity and color vision remain largely unaffected. However, there remains a controversy regarding alcohol’s effect on depth perception from binocular stereopsis⁵.
Binocular stereopsis has been a subject of interest for many studies, the sense of depth from motion parallax is much more important in situations such as driving. Motion parallax is created when an observer moves through the environment. While moving, the visual system maintains fixation on a particular object by making a slow eye movement in the direction opposite the translation. From the observer’s view, objects closer than fixation, although remaining stationary in the environment, change in relative position in the direction opposite the translation.
On the contrary, objects farther away than fixation change relative position in the direction of observer translation. This motion parallax, or changes in relative position of objects at different distances, also creates movement on the observer’s retina. The object of fixation remains stationary on the retina, nearer objects move one way, and distant objects move in the opposite direction. For the sense of depth from motion parallax, the visual system relies on these retinal motions.
However, since these retinal motions are arguable the visual system also relies on slow eye movement system to provide “extra-retinal” information to distinguish which direction of retinal motion corresponds to near or far depth⁶. It also appears that identified depth is scaled with the gain or velocity of these eye movements.⁶ While it is well known that alcohol intoxication reduces the gain of slow eye movements, it is unknown whether alcohol intoxication weakens the perception of depth from motion parallax due to its influence on the eye movement system.
References: 1:Stapleton, Guthrie, & Linnoila, 1986, for a review with considerations of traffic safety,2:Moser, Heide & Kompf, 1998; Holdstock & de Wit, 1999,3:Forkiotis, 1986, Tiffany, 1986; Good & Augsburger, 1986; Belton 1987,4:Nicholson et al., 1995; Pearson & Timney,1998,5:Wallgren & Barry,1970; Watten & Lie, 1996, Hill and Tofollon, 1990,6:Nawrot, 1997,2000
Dr. Shiv Naresh Gupta