  A growing part of the behavioral finance literature has addressed some of the
stylized facts of financial time series as macroscopic patterns emerging from
herding interactions among groups of agents with heterogeneous trading
strategies and a limited rationality. We extend a stochastic herding formalism
introduced for the modeling of decision making among financial agents, in order
to take also into account an external influence. In particular, we study the
amplification of an external signal imposed upon the agents by a mechanism of
resonance. This signal can be interpreted as an advertising or a public
perception in favor or against one of the two possible trading behaviors, thus
periodically breaking the symmetry of the system and acting as a continuously
varying exogenous shock. The conditions for the ensemble of agents to more
accurately follow the periodicity of the signal are studied, finding a maximum
in the response of the system for a given range of values of both the noise and
the frequency of the input signal.
