  Dust is known to drift and grow in protoplanetary discs, which results in
dust segregation over the disc extent. Maps of the spectral index $\alpha$ are
a common tool for studying the dust content in protoplanetary discs. The
analysis of observationally derived maps reveals significant gradients of the
spectral index, confirming that dust evolves in the disc, but a more detailed
information about the dust redistribution is required to make inferences about
the early stages of dust growth. We calculated the spectral index maps based on
the results of numerical hydrodynamical simulations using the FEOSAD code,
which allows studying a long-term dynamics of a self-gravitating viscous disc
populated with coagulating, drifting, and fragmenting dust. Here we demonstrate
that values of the spectral index estimated for different wavelength intervals
within the far-infrared and radio bands reveal the presence of dust grains of
various sizes. Specifically, we show that the disc regions with the maximal
spectral index in a specific wavelength interval are the regions with the
prevalence of dust grains of a specific size. Thus, a set of spectral index
maps derived using different wavelength intervals can be used to recover the
dust size-distribution over the disc extent.
