According to some neuroscience research, not much.
In the AWP 2013 session “This Is Your Brain on Fiction,” Susan Hubbard discussed the 2012 New York Times article, “Your Brain on Fiction,” which references several recent studies of how our brains respond not only to literature in general but to metaphors specifically. Metaphors—comparisons between dissimilar things or ideas—add depth and complexity to our writing. When a metaphor is overused, however, it becomes a cliché—hackneyed, meaningless, dead as a doornail.
In “From novel to familiar: Tuning the brain for metaphors,” published in NeuroImage in February, 2012, researchers suggest that as metaphors become commonplace, our brains lose interest and show decreased activity in not just the meaning-making right hemisphere but in the language-centered left hemisphere, as well. We read a cliché as though it were literal, without any added mental activity on our part. Fresh, new metaphors, however, “require suppression of the literal sense of the sentence” because we must understand abstraction or similarities drawn between otherwise unrelated domains (E.R. Cardillo et al., NeuroImage 59 (2012) 3219).
It’s the difference between the phrase “the sea was angry,” which we read literally because we have heard it so often, without anthropomorphizing the sea in our mind, and the words uttered by George in a famous Seinfeld episode, “The sea was angry that day, my friends, like an old man trying to send back soup in a deli.”
Readers, writers, and teachers have known for a long time that clichés flatten language and meaning, and science is now showing us the how and why. This research doesn’t change what makes writing powerful, but it might poke us into taking the extra time to write with more care .
What does dead as a doornail mean, anyway? Used as long ago as the 14th century in Langland’s Piers Plowman, the phrase probably refers to large nails used in doors, as Gary Martin explains:
“Doornails are the large-headed studs that were used in earlier times for strength and more recently as decoration. The practice was to hammer the nail through and then bend the protruding end over to secure it. This process, similar to riveting, was called clenching. This may be the source of the ‘deadness’, as such a nail would be unusable afterwards.”
Being dead as a doornail implies that something is no longer usable for another purpose. And, with that, the commonplace metaphor becomes fresh again, at least for a little while.
Brain scan photo by Tim Sheerman-Chase: http://www.flickr.com/photos/tim_uk/. Nail photo by Stewart Black: http://www.flickr.com/photos/s2ublack/. Both photos made available by Creative Commons Attribution 2.0 Generic license.