Alcohol does not affect visual contrast gain mechanisms

It has been suggested that acetylcholine plays a role in contrast discrimin ation performance and the regulation of visual contrast gain (Smith, 1996). Since alcohol has been shown to reduce levels of acetylcholine and contras t sensitivity, the present study measured the effects of alcohol on contras t discrimination and explored whether the deficits could be explained as a consequence of reduction in contrast gain. Detection thresholds and contras t increment thresholds under placebo and alcohol (0.06% BAG) conditions wer e measured in six volunteers. Alcohol was found to impair both detection an d discrimination of only high spatial frequencies. However, when the base c ontrasts used in the increment threshold task were equal multiples of detec tion threshold, no alcohol-induced changes in increment thresholds were obt ained at any spatial frequency. We conclude that alcohol impairs contrast d iscrimination performance but that no change in contrast gain mechanisms ne ed be postulated to account for the data.