Observed values of Δσ_⋆ against colour excess E(B − V) in the three selected regions for the four galaxies in the sample. We represent the mean values of Δσ_⋆ estimated in equally spaced age bins. We also represent the 1σ uncertainty interval for these mean values as colour-shaded regions. The typical error in the Δσ_⋆ estimate is represented as the grey-shaded area. In general we find vertical distributions of the mean values of Δσ_⋆. The only exception is the ring of NGC 1097 where we find increasing values of Δσ_⋆ when E(B − V) increases.