Ethanol impairs saccadic and smooth pursuit eye movements without producing self-reports of sedation

It has been suggested that eye movements provide a sensitive tool to assess the sedative-like properties of drugs. However, the relationship between sedative-like subjective effects and impairment in eye movements is not clear. For example, it is not clear whether drugs with stimulant-like effects can also impair eye movements. This study evaluated whether ethanol, a drug with both sedative-like and stimulant-like properties, impairs eye movements, and whether the impairment observed after ethanol is related to its sedative properties. Twenty healthy men and women, aged 21 to 35, consumed beverages containing placebo or ethanol (0.4 or 0.8 g/kg) on three separate laboratory sessions, in randomized order. Eye movement and psychomotor and subjective responses were assessed before and at regular intervals for 3 hr after ingestion of the beverage. Subjects were divided post hoc into two groups, based on their sedative-like or stimulant-like subjective responses to ethanol. Nine subjects reported increases in sedative-like effects after ethanol and 11 reported decreases in sedative-like effects, and increases in stimulant-like effects, after alcohol. Despite their distinctly different subjective responses to ethanol, the groups did not differ in the magnitude, time-course or quality of responses on the eye movement measures. In both groups, ethanol decreased peak saccadic eye velocity and smooth pursuit gain. These results demonstrate the dissociation between impairment in eye movements and subjective feelings of sedation after ethanol, and show that eye movements can be impaired even when subjects are reporting stimulant-like effects. The findings suggest that impaired eye movements are not a nonselective index of sedation, but may be related to specific drug actions on brain regions involved in generating these eye movements.