Bilinguals: do they switch off one language?

Recently it happened again: in the middle of a lecture about the history of psychology, I used a German expression. I didn't even notice it at first, only the puzzled look of my students told me that something was wrong.

As I'm even thinking now more and more in English, it surprises me that occasionally German words would unconsiously sneak in when I'm speaking English. A group of Dutch psychologists now found an explanation:

Bilinguals Are Unable To 'Turn Off' A Language Completely, Study Shows

ScienceDaily (2009-08-19) -- With a vast majority of the world speaking more than one language, it is no wonder that psychologists are interested in its effect on cognitive functioning. For instance, how does the human brain switch between languages? Are we able to seamlessly activate one language and disregard knowledge of other languages completely? ... > read full article