Two-phase model analysis of the effects of interstimulus interval and masking task in human aversive classical conditioning.

Three levels (.5, 1.0, and 2.0 sec) of interstimulus interval (ISI) and presence vs absence of a masking task were manipulated in a human eyelid reflex conditioning situation. Through the use of a two-phase model of conditioning performance, it was concluded that (1) increasing the ISI and introducing a masking task increased the duration of Phase 1, (2) as ISI increased, the amount of conditioning decreased, (3) the trial-by-trial rate of change in response probability increased as a function of ISI, and (4) the masking task reduced operator limits. It was also noted that the typical ISI function is composed of rate effects at short ISis and conditioning limit effects at long ISIs and that the larger 0 at larger ISis does not follow from a model of the stimulus trace hypothesis.