Exactly. The visual system produces a very stable activation pattern, relative to other senses and multidimensional objects. The noise inherent in those other systems is probably the best protection of our inner privacy. There simply isn't a code to be read because there's little systematicity from one moment, or person, to the next. The visual system is as good as it gets and even then only when you're looking at a prescribed object.

In other words, ignore the PR about downloading thoughts and dreams. We can decode some stuff from fMRI activity patterns but we're soon swamped by noise, even when using machine learning techniques. The data just isn't there. It's like trying to predict the weather two weeks from now. The system is inherently noisy.