Spatial distributions of LTP/LTD were investigated in the CA1 area of hippocampal slices by optical imaging method. A pair of electrical pulse stimuli was used to stimulate the schaffer collateral (Stim. A) and the stratum oriens (Stim. B) with various sets of the relative timing (τ). LTP was induced with -10ms<τ<10ms, τ=0ms led to the widest and strongest LTP. On the other hand, τ=±20ms induced LTD. In particular, this LTD at τ=20ms was something that had not been observed by others using cultured hippocampal neurons. The LTD at τ=20ms was blocked by bicuculline (GABA (gamma-aminobutyric acid) receptor antagonist). The importance of precise spike timing in hippocampal CA1 network is discussed in the light of computational neuroscience involving the covariance learning rule.