The proportion-of-the-total duration rule (PTD) [Kidd and Watson, J. Acoust. Soc. Am. 92, 3109--3118 (1992)] states that the detectability of a change in a component of a tonal sequence can be predicted by the duration of the changed component relative to the sequence as a whole. A similar idea has been used in dimensional distinctiveness theory (DDT) [Neath, Mem. Cognit. 21, 689-698 (1993)] to account for primacy, recency, and other serial position effects in memory. An item will be remembered if it is more distinct along some dimension relative to possible competitors. These experiments explore the relation between DDT and PTD by examining the effect of inter-stimulus interval (ISI) on the detection of a change in one tone of a tonal sequence for each serial position in the sequence. Results show that, with increasing ISI, performance on the first items increases whereas performance on the final items decreases, as predicted by DDT. In addition, the interaction of PTD and ISI was explored for each serial position. This research combines theories proposed in the psychophysical and memory areas and suggests that a comprehensive principle based on relative distinctiveness can account for both perceptual and memory effects.