When the interface programs the user
I recently went to a meeting of a group of electronic-music enthusiasts, one of who mentioned an essay he had recently read. I think it was by Brian Eno, and its main point was that when you're doing creative work with software, you may think you're programming the software; but it is just as true that the software is programming you.The point is that every major piece of software has a user interface, and every such interface encourages (and sometimes forces) its users to think in a particularly way. By making some tasks easier and others harder, the interface encourages its users to think in terms of the easier tasks and ignore the more difficult ones.
As a simple example, consider a piano-like keyboard on a synthesizer. One feature that is present on some, but not all, such keyboards is aftertouch--the idea that while you're holding a key down, you can press it harder to transmit a signal to the synthesizer. If you are using a keyboard with aftertouch, you can control how a note sounds after you have already started playing it. Accordingly, the presence or absence of aftertouch affects kind of music that people make.
Once one realizes that user interfaces can program their users, examples of this phenomenon appear everywhere. I'd like to invite readers to point out particular interesting examples; perhaps I will take some up in detail in future postings.If you are using a keyboard with aftertouch, you can control how a note sounds after you have already started playing it.