The effect of ingested alcohol on fusion latency at various viewing distances

Although diplopia is a frequently reported symptom of severe alcohol intoxication, there have been few past attempts to describe in any systematic fashion the specific impact of alcohol on fusion. In the present experiment, fusion latency was assessed for 8 male emmetropes. At the beginning of the alcohol condition, they consumed a drink containing 1.4 ml/kg of 95% ethanol, plus diluent. For the placebo condition, the drink consisted only of diluent. In each condition, fusion latency (the time required to fuse a binocularly visible target) was recorded for targets requiring a magnitude of vergence that varied from +10 through -2 meter angles, in 1-meter-angle steps. It was found that the far point of fusion approached the observer as a direct function of intoxication level. Furthermore, fusion latency increased for optically far and near targets as a systematic function of intoxication, with no effect of alcohol on intermediate targets. The results were consistent with previous findings of alcohol-induced narrowing of vergence range, suggesting that alcohol causes the farthest and nearest distances at which fusion latency is affected to approach an intermediate value. Measurable effects on fusion latency were observed with blood alcohol levels as low as 0.05%-0.06%, considerably below the legal limit for operating a motor vehicle in most states.