Positive and negative expectations crucially shape how we perceive our surroundings, including our emotional and cognitive abilities. Prior work shows that cognitive abilities in different domains can be modulated by placebo or nocebo effects, in clinical cohorts, but also in the absence of disease. However, these studies show inconclusive results and do not directly compare different expectation modulations with each other. This preregistered online study tested the effects of three types of expectations on cognitive performance in a visual attention task, using a mixed design. Participants (n = 197) completed this task before and after a written suggestion, which was information that white noise played during the task would either improve (deceptive placebo), improve through the placebo effect (open-label placebo), worsen (nocebo), or not affect (no suggestion) their focus and thus cognitive performance. In a complementary frequentist and Bayesian analysis framework, we observed evidence for learning effects such as faster reaction times or lower error rates from before to during the treatment. However, we found direct evidence of absence regarding any group-related differences. Interestingly, the nocebo group had lower impact ratings than both placebo conditions and the influence of the noise was rated as stronger compared to its expected influence at baseline. This work underscores the importance of distinguishing between subjective and objective effects when evaluating the influence of treatment suggestions on cognitive performance.