“…Of the many variables that influence the acquisition of Pavlovian conditioned responses (CRs) one of the most thoroughly investigated is that of trial spacing. It is well established that lengthening the interval between pairings of a conditioned stimulus (CS) and an unconditioned stimulus (US) facilitates the acquisition of CRs in a variety of conditioning procedures (e.g., Domjan, 1980; Gibbon, Baldock, Locurto, Gold, & Terrace, 1977; Gormezano & Moore, 1969; Kaplan, 1984; Prokasy, Grant, & Meyers, 1958; Salafia, Mis, Terry, Bartosiak, & Daston, 1973; Spence & Norris, 1950; Terrace, Gibbon, Farrell, & Baldock, 1975; Yeo, 1976; see also Durlach, 1989). The impression left by this body of evidence is that a general principle of learning has been established—namely, that learning proceeds more readily when training trials are spaced than when they are massed.…”