  Spike-timing-dependent plasticity (STDP) has recently been shown in some
physiological studies. STDP depends on the precise temporal relationship of
pre- and post-synaptic spikes. Many authors have indicated that a precise
balance between long-term potentiation (LTP) and long-term depression (LTD) of
STDP is significant for a stable learning. However, a situation in which the
balance is maintained precisely is inconceivable in the brain. Using a method
of the statistical neurodynamics, we show robust retrieval properties of
spatio-temporal patterns in an associative memory model against the imbalance
between LTP and LTD. When the fluctuation of LTD is assumed to obey a Gaussian
distribution with mean 0 and variance $\delta^2$, the storage capacity takes a
finite value even at large $\delta$. This means that the balance between LTP
and LTD of STDP need not to be maintained precisely, but must be maintained on
average. Furthermore, we found that a basin of attraction becomes smaller as
$\delta$ increases while an initial critical overlap remains unchanged.
