Interstimulus interval

The interstimulus interval (often abbreviated as ISI) is the temporal interval between the offset of one stimulus to the onset of another. For instance, Max Wertheimer did experiments with two stationary, flashing lights that at some interstimulus intervals appeared to the subject as moving instead of stationary. In these experiments, the interstimulus interval is simply the time between the two flashes.

The term interstimulus intervals is also relevant for classical conditioning, usually denoting the time between the unconditioned stimulus (food) and the conditioned stimulus (bell or whistle). It is particularly relevant in Eyeblink conditioning, in which it has been found that the interstimulus interval is a major factor in creating an optimal learning paradigm.