While we do have some basic knowledge of how genjutsu is cancelled, it does seem to be something that Kishimoto hasn't really defined well. For example, Shikamaru was able to break himself out of the same technique by using his own shadow manipulation to break his own finger. It seems to me that Kishimoto always seems to find a convient way to get people out of genjutsu when necessary but he has only really loosely defined how it can be done. In this case, the point that Sasuke and Itachi were perfectly capable of performing their own genjutsu on each other but they could not use Susanoo seems convenient rather than logical based on what we have already seen.
http://read.mangashare.com/Naruto/ch...8/page018.html
It could be that using genjutsu was simply the easiest option at the time but that is not really made clear one way or another. If Shikamaru was able to use his shadow bind to counter genjutsu at his level there should have been multiple outs for Itachi and Sasuke who are not only some of the highest level genjutsu users but also have a large variety of abilities to draw upon. Also, from what I can tell, neither the series nor the databooks have defined the difference between visual genjutsu and sound based genjutsu, only that they activate under different circumstances as one is generated by sound while the other relies upon visual ques. To the best of my knowledge though, no difference has been defined when it comes to breaking free from them. If memory serves it was stated that the sharingan has the power to see through genjutsu but it doesn't make any mention of different types causing different problems.
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