Unraveling the Filled Duration Illusion and its Stability in Repeated Measurements

Interval timing plays an essential role in various types of behavior including perception and production of music. However, subjectively perceived intervals may substantially differ from their objective durations. One of the phenomena, the filled duration illusion (FDI), is well described in the literature; however, there are still many questions to address concerning mechanisms behind this phenomenon. To further unravel the FDI, we asked 61 healthy adults to reproduce the duration of various acoustic stimuli (from 2 to 3 seconds). We used empty intervals (marked by two short tones) and filled intervals: a continuous tone or rhythmical tone sequences in legato or staccato. We demonstrated that the reproduction of empty intervals was shorter than reproduction of all filled intervals, whereas the reproduction of rhythmic intervals was the longest. Therefore, we clearly demonstrated and distinguished both types of the FDI—the sustained sound illusion and the divided time illusion—and documented their test-retest stability in two subsequent measurements. Moreover, we confirmed the effect of tone pitch on the reproduction—higher pitch tones were judged as longer. By testing all the mentioned phenomena in repeated measurements, we demonstrated the stability of the illusions and prepared the ground for an investigation of more complex musical stimuli.