  Star formation depends strongly both on the local environment of galaxies,
and on the internal dynamics of the interstellar medium. To disentangle the two
effects, we obtained, in the framework of the AMIGA project, Ha and Gunn r
photometric data for more than 200 spiral galaxies lying in very low-density
regions of the local Universe. We characterise the Ha emission, tracing current
star formation, of the 45 largest and less inclined galaxies observed for which
we estimate the torques between the gas and the bulk of the optical matter. We
could subsequently study the Ha morphological aspect of these isolated spiral
galaxies. Using Fourier analysis, we focus on the modes of the spiral arms and
also on the strength of the bars, computing the torques between the gas and
newly formed stars (Ha) and the bulk of the optical matter (Gunn r). We
interpret the various bar/spiral morphologies observed in terms of the secular
evolution experienced by galaxies in isolation. We also classify the different
spatial distributions of star forming regions in barred galaxies. The observed
frequency of particular patterns brings constraints on the lifetime of the
various evolution phases. We propose an evolutive sequence accounting for the
transitions between the different phases we could observe. Isolated galaxies
appear not to be preferentially barred or unbarred. Through numerical
simulations, trying to fit the Ha distributions yields constraints on the star
formation law, which is likely to differ from a genuine Schmidt law. In
particular, it is probable that the relative velocity of the gas in the bar
also needs to be taken into account.
