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Motivated by a series of reported experiments and their controversial results, the present work investigated if volunteers could causally affect an optical double-slit system by mental efforts alone. The participants' task in the experimental sessions alternated between intending an increase in the (real-time feedback-informed) amount of light diffracted through a specific single slit versus relaxing their intentional effort. In total, 240 sessions contributed by 171 volunteers were recorded. The first 160 sessions were collected in an exploratory mode, and those data revealed statistically significant differences between the intention and relax conditions. The analysis method and variables of interest derived from the exploratory sessions were then pre-registered for the subsequent 80 formal sessions. The formal experiments, based on a directional hypothesis, were not statistically significant. A post hoc meta-analysis based on a bi-directional hypothesis, and applied to the same data, resulted in a 2.75 sigma outcome ($p=6.02\times10^{-3}; \, es=0.31 \pm 0.22$ 95\% CI). Directional and bi-directional analyses applied to an equal number of control sessions, all conducted without observers present, resulted in uniformly non-significant outcomes. Analysis of environmental factors did not reveal any artifactual sources that might have produced the significant bi-directional effect. While the pre-registered analysis did not support the existence of the investigated phenomenon, the post hoc findings warrant further investigation to formally test the bi-directional hypothesis.