p485fig13.06 (left column) An inverted-U occurs in conditioned reinforcer strength as a function of the ISI between the CS and the US. Why is learning attenuated at 0 ISI?  (right column) Some classical conditioning data that illustrate the inverted-U in conditioning as a function of the ISI.<BR>||  InterStimulus Interval (ISI) effect. Data from (Dmith etal 1969; Schneiderman, Gormezano 1964).