Spatial influences may be introduced to an experimental task by manipulations performed on the stimulus or the response or by virtue of the type of stimuli under study. Identification of spatial influences is especially pertinent in investigations of laterality, as isolation of processing to one hemisphere may inadvertently introduce spatial confounds. Because, however, space is not a relevant task feature, it may not always be obvious that it should be taken into consideration. Failure to anticipate these spatial influences can affect the conclusions drawn from results. The current work examines potential spatial influences in an experimental paradigm previously used to investigate perceptual asymmetries for duration estimation in which both stimulus presentation and response selection were lateralized. Potential spatial influences (including the spatial-temporal association of response codes – STEARC, spatial attention, and the Simon effect) are identified and systematically tested over 5 experiments. Results suggest that previously observed perceptual asymmetries in this experimental paradigm may be the result of a spatial confound, specifically, that of the Simon effect. Using vertical response options with the lateral stimulus presentation, however, mitigated the spatial influence. Altogether, the current work demonstrates the importance of carefully considering potential spatial confounds prior to commencement of laterality studies.