  A phase space formulation of the filtering process upon an incident quantum
state is developed. This formulation can explain the results of both quantum
interference and delayed-choice experiments without making use of the
controversial wave-particle duality. Quantum particles are seen as localized
and indivisible concentrations of energy and/or mass, their probability
amplitude in phase space being described by the Wigner distribution function.
The wave or particle nature appears in experiments in which the interference
term of the Wigner distribution function is present or absent, respectively,
the filtering devices that modify the quantum wavefunction throughout the
set-up, from its generation to its final detection, being responsible for the
modification of the Wigner distribution function.
