The interstimulus interval is the temporal interval between the offset of one stimulus

Stimulus (physiology)

In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....

 to the onset of another. For instance, Max Wertheimer

Max Wertheimer

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 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 ISI is simply the time between the two flashes.

The term interstimulus intervals is also relevant for classical conditioning

Classical conditioning

Classical conditioning is a form of conditioning that was first demonstrated by Ivan Pavlov...

, usually denoting the time between the unconditioned stimulus (food) and the conditioned stimulus (bell or whistle). It is particularly relevant in Eyeblink conditioning

Eyeblink conditioning

Eyeblink conditioning is a form of classical conditioning that has been used extensively to study neural structures and mechanisms that underlie learning and memory. The procedure is relatively simple and usually consists of pairing an auditory or visual stimulus with an eyeblink-eliciting...

, in which it has been found that the interstimulus interval is a major factor in creating an optimal learning paradigm.