  Seismic electric signals have been found to obey the ubiquitous $1/f^a$
behavior [{\em Phys. Rev. E} {\bf 66}, 011902(2002)]. The newly introduced
concept of natural time enables the study of the dynamic evolution of a complex
system and identifies when the system enters the critical stage. On the basis
of this concept, a simple model is proposed here which exhibits the $1/f^a$
behavior with $a$ close to unity. Furthermore, we present recent data of
electric signals, which when analyzed in the natural time domain are found to
exhibit {\em critical} dynamics and hence can be classified as seismic electric
signals.
