  Concentrated suspensions of Brownian hard-spheres in water are an epitome for
understanding the glassy dynamics of both soft materials and supercooled
molecular liquids. From an experimental point of view, such systems are
especially suited to perform particle tracking easily, and, therefore, are a
benchmark for novel optical techniques, applicable when primary particles
cannot be resolved. Differential Variance Analysis (DVA) is one such novel
technique that simplifies significantly the characterization of structural
relaxation processes of soft glassy materials, since it is directly applicable
to digital image sequences of the sample. DVA succeeds in monitoring not only
the average dynamics, but also its spatio-temporal fluctuations, known as
dynamic heterogeneities. In this work, we study the dynamics of dense
suspensions of Brownian beads in water, imaged through digital
video-microscopy, by using both DVA and single-particle tracking. We focus on
two commonly used signatures of dynamic heterogeneities: the dynamic
susceptibility, $\chi_4$, and the non-Gaussian parameter, $\alpha_2$. By direct
comparison of these two quantities, we are able to highlight similarities and
differences. We do confirm that $\chi_4$ and $\alpha_2$ provide qualitatively
similar information, but we find quantitative discrepancies in the scalings of
characteristic time and length scale on approaching the glass transition.
