  Power spectrum densities for the number of tick quotes per minute (market
activity) on three currency markets (USD/JPY, EUR/USD, and JPY/EUR) for periods
from January 1999 to December 2000 are analyzed. We find some peaks on the
power spectrum densities at a few minutes. We develop the double-threshold
agent model and confirm that stochastic resonance occurs for the market
activity of this model. We propose a hypothesis that the periodicities found on
the power spectrum densities can be observed due to stochastic resonance.
