A single spaced TS before or after massed training induces LTM. A, B, Four spaced TSs (ISI of 15 min) induce 24 and 48 h LTM (A), whereas massed TSs (ISI of 1 s) do not (B). C, D, Three massed TSs contribute to LTM induction when a single spaced TS is administered either 45 min after (C) or 45 min before (D) the massed TSs. Training patterns are illustrated above their respective datasets. In this and subsequent figures, data are expressed as mean ± SEM.