Optical predictions show that retinal image quality in the presence of specific combinations of astigmatism and coma improves over astigtmatism alone (de Gracia et al. Vision Research 50 (19), 2008-2014). We found that the changes in visual acuity upon combinations of coma and astigmatism are actually dependent on prior adaptation to astigmatism.
Visual performance matches optical predictions in non-astigmatic subjects, and to a lesser extent corrected astigmats. However habitually non-corrected astigmats do not experience an improvement when adding coma and astigmatism, and appear to be less sensitive to the induction of astigmatism. The results can be found in a paper just published: Gracia et al. Journal of Vision, 11(2): 5, 1-11 (2011)