A generalized magnitude system for space, time, and quantity? A cautionary note

We investigated the claim that larger stimuli are perceived to last longer (Xuan, Zhang, He, & Chen, 2007). This claim, along with other similar claims of interactions between magnitude representations, is frequently used to support the generalized magnitude system hypothesis-the suggestion that the brain represents information about different magnitudes (e.g., time, space, and quantity) via a common mechanism. It is not clear, however, whether the size of a stimulus genuinely affects the perceived duration of the stimulus or simply biases decisions about duration. This was addressed using duration "equality judgments," which have been proposed to measure perceived duration unconfounded by decisional bias-in contrast to "comparative judgments," which are generally considered bias-prone. Using equality judgments, we failed to find support for the claim that larger stimuli are perceived to last longer, despite replicating the original effect reported by Xuan et al. (2007) using comparative judgments. Instead, unexpectedly, larger stimuli were judged-though not necessarily perceived-as shorter in duration. This result casts doubt on the conclusions of a significant body of behavioral interference studies using comparative judgments, which support a generalized magnitude system. We also identify a hitherto unrecognized potential source of decisional bias associated with equality judgments.