An interesting aspect of Indian art music is the prominent place of improvisation in performance. We explore the influence of the structural constraints of the genre on raga motifs in the course of improvisation. Audio recordings of North Indian vocal concerts are analyzed to extract measurements of the defining parameters of the recurrent melodic phrases that characterize the raga in performance. While improvisation relates to choosing the sequence of phrases at the larger time scales, we show that the categorical nature of the musical melodies is preserved through the interaction of the phrases with the changing melodic and rhythmic contexts.