'Make the light as bright as the sound is loud.' This is a typical instruction in experiments dealing with the cross-modal matching of stimuli. According to the theory of global psychophysics (Luce, Steingrimsson, & Narens, 2010; Heller, 2021), in such a cross-modal task, the perceived stimulus intensities are judged against respondent-generated internal reference intensities, all represented on a common psychological scale. The internal references are distinguished with respect to their role in the experimental setup, that is, whether they pertain to the standard or to the comparison stimulus in the matching task.

By testing the theory of global psychophysics on cross-modal data, the present study aims at thoroughly investigating the role-sensitivity of the internal reference intensities. Therefore, we replicate a classical experiment by Stevens and Marks (1965), whose participants adjusted the brightness of a light to the perceived loudness of a noise sound and vice versa. Complementing the traditional group-level analysis, we evaluate and model the data on the individual level. We find that the cross-modal matching curves differ in slope, and show a regression effect as reported in the classical literature.

In order to experimentally manipulate the internal references' role-(in)dependence, we discuss a paired comparison task with an adaptive staircase procedure as psychophysical method. With it, the subject is instructed to indicate the more intense stimulus. So there is no 'standard' or 'comparison' stimulus for the subject and the internal references are expected to be role-independent. As a result, the 'regression effect' should vanish.