The integrity of the human perception is much, much more difficult, yet correspondingly more important, because what really matters is what the human finally perceives, not the mere data that is delivered. How can we ever determine whether someone has disturbed a message?
Concrete parts are amenable to comparison before sending and after reception. Messages are significantly more fluid. As discussed elsewhere, a web page may be viewed in a virtually infinite number of ways, with different fonts, color schemes, or graphics handling. Even the sensory modality it is rendered into can change, with someone perhaps listening to a computer reading the web page, or reading it through a specialized Braille interface. At first, it would seem impossible to say whether a supposed modification is truly a modification, or merely another legitimate way of perceiving the message.
Yet even though there may be an effectively infinite number of ways of viewing my web page, and an effectively infinite number of ways of viewing the New York Time's homepage, it is immediately obvious that there is some real difference. The answer lies in the fact that the message is not independent of the concrete parts that went into making it. No matter what settings you set your browser to, my web page will never have the same content of the New York Time's page, because they have articles, advertisements, and other things that my web page simply does not have, and vice versa. It is this point that has been missed in previous debates on the Internet about the real-world examples of message integrity attacks, because it is easy to be blinded by the apparently infinite number of ways of experiencing a particular message. (This is another manifestation of the common misconception that any infinite set must contain all possibilities within it. This is obviously false; the set of "all integers" contains infinite numbers, yet no matter how long you look, you will never find 1.5. Similarly, even though there are an incredibly wide variety of experiencing a given web page, perhaps even infinite in some sense, the content the web page is based on always remains constant.)