An alcohol dose affects the central nervous system (CNS)—the predominant effect being a depression of central functions. This means that the higher the dose of alcohol, the more the CNS is depressed. The most highly integrated brain functions are involved first; when the brain cortex is released from its functions of integrating and control, processes related to judgment and behavior occur in a disorganized fashion and the proper operation of behavioral tasks becomes disrupted.

The effects of alcohol are biphasic, and the phases depend on the dose and the rate of administration. With higher alcohol concentrations, central depressant effects dominate. Low concentrations seem to stimulate various functions by inhibiting the control mechanisms. This is seen in animal studies as decreased motor activity with large doses of alcohol and increased activity with small doses. In humans, very small doses of alcohol do not necessarily impair performance, and the tension-relieving effects of alcohol can sometimes be seen in some tests of short duration. However, there is no reason to overestimate this occasionally observed pseudostimulant effect of alcohol; in actuality, alcohol impairs various skills that are needed to cope with everyday routines.

Several investigators have demonstrated that alcohol does induce a larger decrease in test performances requiring hand-eye coordination, whereas simple tests of cognitive ability show less of a decrease. When more complex cognitive functions are studied, however, low to moderate BACs (0.3— 1 g/l) impair sensory tasks and sensorimotor skills less than they do complex cognitive behavior, such as performing two tasks simultaneously (''divided attention''). It thus seems that alcohol impairs the rate of information processing by slowing the ability to switch attention from one to another sensory input to motor control, without significantly impairing sensory motor functions as such. In fact, moderate BACs (less than 1 g/l) are not associated with dramatic changes in such basic neurophysio-logical mechanisms as neuromuscular transmission or the conduction velocity of motor nerves. Alcohol effects are thus better seen in situations where the information load is increased and highly integrated functions are needed for the task.

It is well known that the muscles of the eye and eye movements often easily reflect the CNS depression caused by alcohol. One of the most sensitive signs is the appearance of lateral nystagmus; small twitches or vibration in the position of the eye are seen when the person looks to the side. The angle of the gaze at which the nystagmus appears correlates with the alcohol dose: On average, a BAC of 0.5 g/l induces nystagmus at a 45-degree angle of deviation, whereas a BAC of 1 g/l produces nystagmus even at the 35-degree angle. Also, saccadic eye movements (from one fixation point to another) become slower with BACs of 0.8 g/l to 1 g/l. All this indicates that people who are drunk have a narrower sector of intact vision than people who are sober. Visual information becomes disrupted if eyes must be turned to the side to detect stimuli, or if eyes must be moved quickly from one point to another.

Several types of tests measure skilled performance in tasks related to driving behavior. Tracking tasks involve hand-to-eye coordination, and the task is to keep an object on a prescribed path by controlling its position through turning a steering wheel. Impairment of performance is seen at BACs of as little as 0.7 milligrams per milliliter (mg/ml). Choice reaction task refers to a situation where aural or visual stimuli (or both) need response according to rules that necessitate mental processing before giving the answer. In traffic, driving requires a division of attention between a tracking task and surveillance of the environment. When a driver must process information from more than one source concomitantly—by adding sudden reaction tasks to the tracking task—very low BACs are sufficient to produce significant impairment of performance.

Clinical tests for drunkenness include many simple tasks that are easy to measure even in field conditions. These can be divided into three subtests. (1) Motor subtests consist of measuring a person's ability to walk along a straight line with eyes open and closed; maintain a steady turning gait; fit the tips of index fingers together with eyes closed, and collect small objects (e.g., matches) from the floor. (2) Vestibular subtests assess the person's body sway, with eyes open and closed, and nystagmus. (3) Mental subtests assess the driver's ability to subtract backward, orientation as to time, and overall behavior. The performance in each subtest is graded from 0 to 3, but these clinical tests are not very sensitive to small BACs (nystagmus exlcuded), and there is great individual variation. The use of these clinical tests for drunkenness in field conditions has greatly diminished since portable BREATHALYZERS became available. The tests are most useful in situations where one has to decide whether to take a blood test for detection of other drugs when no alcohol is found in the driver's breath. Unfortunately, tests developed to detect alcohol effects are less sensitive to the effects of BENZODIAZEPINES and other CNS depressant drugs.

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