Abstract
Previous literature has shown response expectancies to be a major determinant of the placebo effect in numerous contexts. There has been no research examining the role of response expectancies in vitamin supplements (such as B9 & B12), even though multiple meta-analysis have shown supplements to be ineffective in healthy populations. Here we report differing expectations producing differing cognitive results. Subjects (n=54) were divided into three equal groups and presented with stroop, digit span and 15-word memory (delayed & immediate) tasks. Before starting the tasks subjects were presented with a cup of water to drink, yet the symbolic meaning differed between groups. The control group simply ingested water and completed the tasks. The blind group was presented with two cups of yellow sparkling water, they were instructed that one of the cups contained high dose B12, while in reality both cups contained water. The last group was actively deceived. They were presented with one cup of yellow sparkling water and told they were in the high dose Vitamin B12 condition. In both the blind and deceived groups a waiting period, food dye, sparkling water, and the explanation of an expected effect acted to heighten response expectancies. As anticipated, a significant graded effect was found on the stroop task, with the control condition as the slowest and the deceived condition the fastest. None of the memory tasks showed significant differences between the groups, but this might have been due to a small sample size. These results indicate that different verbal instructions about certain and uncertain expectations of Vitamin B’s effects produce different placebo effects, which in turn trigger a change of behaviour leading to a significant reduction of time on a stroop task.
