The interonset interval, or IOI, is the interval between onsets of stimuli, or, more specifically, the time between the (perceived) beginning of one note and that of the next note.
(Prepared by Paul M. Cohen, 2/6/10, 9:07 PM)
The IOI is generally considered to be the strongest contributor to accent and pulse perception. One study cited by Parncutt definitively demonstrates that the durational accent is more significant than dynamic accenting: while an increase in 2 dB registers as a dynamic accent, an increase of twice that amount (4 dB) is required to counteract the influence of a durational accent (Parncutt 1994, 426).
While IOI generally increases the longer the duration, saturation points exist on both ends of the durational spectrum. Increases in duration beyond approximately 1 s no longer result in an increase in durational accentation (Parncutt 1994, 429). Beyond the saturation point of approximately 50 – 100 ms, individual events cease to be recognized as such and the effective IOI for each note becomes 0 (Parncutt 1994, 430).