Specifically, pupils constricted when viewing digits that evoked brighter synesthetic colors and dilated to digits that evoked darker synesthetic colors. Conceptual cross-over correspondences, which are consistent at the group level, can also be observed in the general population (Rich et al., 2005; Rouw et al., 2014; Simner et al., 2005). Along with the conscious perception of color, a main feature of synesthetic color experiences is that they happen automatically (e.g., Simner and Bain, 2013). As increases in mental effort are tightly coupled with pupillary dilations (e.g., Bumke, 1911; Kahneman, 1973; Strauch et al., 2022b; Koevoet et al., 2024; Mathôt, 2018), we could put this to a direct test. First, generating and maintaining mental images is effortful and produces according pupil dilations (Kay et al., 2022; Kosslyn et al., 1988; Laeng and Sulutvedt, 2014; Bumke, 1911; Henderson et al., 2018).