Thanks for your tests. My render tests (on PC) present the same results: better with the option OFF (more contrast). I've just opened the Real World Bryce 4 book and Susan says (as I understood it) that the gamma correction is a Bryce 4 standard correction (the same for all computers and screens). For her, it's better to leave the Gamma correction option OFF and adjust the image in postwork (with Photoshop or other). Mac and PC seem to have different gestion of ICC profiles, it's perhaps better with mac?.