  We present a study of the stellar populations within the central regions of
four nearby barred galaxies, and use a novel technique to constrain the
duration of bar activity. We focus on the star formation 'desert', a region
within each of these galaxies where star formation appears to have been
suppressed by the bar. New H beta spectroscopic data are presented, and used to
produce spectroscopic line indices which are compared with theoretical
predictions from population synthesis models for simple stellar populations and
temporally truncated star formation histories. This analysis shows that the
dearth of star formation activity in these regions appears to have been
continuing for at least 1 Gyr, with timescales of several Gyr indicated for two
of the galaxies. This favours models in which strong bars can be long-lived
features of galaxies, but our results also indicate a significant diversity in
stellar population ages, and hence in the implied histories of bar activity in
these four galaxies.
