Effects of spectra and sound pressure levels on the occurrence of the gap transfer illusion

We conducted a series of phenomenological experiments to determine the requirements for the occurrence of the gap transfer illusion in stimulus patterns consisting of harmonic glides. The gap transfer illusion is an auditory phenomenon in which a temporal gap in a long glide is perceived as if it were in a crossing short glide that is physically continuous. We employed stimulus patterns of a long and a short three-harmonic-component glide crossing at their temporal middles, and varied the spectral slope of each glide independently. The gap transfer illusion took place only when the crossing glides had the same spectral slope. To examine whether we could generalise our finding we employed stimulus patterns of the same type and made more types of spectra. The gap transfer illusion took place only when the crossing glides had the same spectrum and the same sound pressure level (SPL) at the crossing point. We also used stimulus patterns consisting of single-component glides, and found that the gap transfer illusion occurred when the long and the short glide had the same intensity, and also when the short glide was somewhat less intense than the long glide. The equality or similarity of spectra and SPLs between crossing glides was crucial for the occurrence of the gap transfer illusion.