Numerous studies have found that top-down processes can affect perceptions. This study examines some of the issues involved in designing field experiments aimed at discovering whether top-down mental processes affect perceptions, and, if so, how the influence takes place. Lee, Frederick, and Ariely (2006) (LFA) attempt to go further by testing whether expectations affect perception directly, by altering how sensory receptors and/or the brain's processing centers interpret a outside stimulus-or indirectly, for example, by changing the amount of attention paid to the outside stimulus. In order to test the robustness of the findings in LFA, this paper reports the results of a field experiment similar to the one analyzed in LFA. The field experiment, designed to address some potential confounding factors in this type of research, confirms that expectations can alter perceptions. However, it also shows that heterogeneity across individuals can play a role in determining the nature of this effect, a finding that complicates the interpretation of results such as those in LFA. To frame the analysis, this paper discusses the difficulties in designing this type of experiment, makes some improvements to existing designs, and suggests some ways of eliminating the confounding influences that remain.