A generation of children learned to read with Dick and Jane. I wasn’t one of them, but I still know the traditional format. See Dick. See Dick run. Run, Dick, run. As the young readers progressed, they must have encountered more complex sentences on their journey to reading. Imagine they encounter See Jane throw the ball to Dick. Does it matter whether Jane threw the ball with her left or right hand? Probably not. But it may matter whether the child reading throws a ball with her left or right hand…because that may determine just how our brains understand “throw”.
This is not just about the fact that language is lateralized – processed mostly by just one side of the brain, usually the left side. That idea has been so ingrained in me that I am sometimes surprised when others haven’t heard of it. I picked up that fact long before I was a cognitive psychologist myself, perhaps because I am left handed and therefore more likely to have language in my right brain. The idea is so familiar to me that I had begun to dismiss it as simple and uninteresting; most likely, to my mind, one half of the brain was dominant from early in development, and so was in a better position to take on challenging language tasks and more demanding fine-motor control activities. Ho-hum.
The possibility that handedness could change the way individual words are processed, though, takes us beyond that simple explanation.
The key brain region under consideration is not one usually associated with language: the premotor cortex, aptly named because it seems to be involved in planning movements, before the motor cortex actually sets the movements in motion. When you see that coat someone has tossed casually on the floor, it’s your premotor cortex that starts planning the actions needed to bend down, grasp it, and hang it up (or throw it at its owner, depending on your inclinations). Naturally, the premotor cortex that gets active will be on the side of the body you’re going to use: plan to grasp something with your left hand, and we’ll see activity in the right premotor cortex only; plan to bend both knees, and we’ll see activity both the left and right premotor cortex.
Language alone can be enough to get the premotor cortex fired up, though. When you were reading bend, grasp, and throw in that last paragraph, your premotor cortex was firing up some quick mental representations of each action. It would be simple enough to dismiss this as a useful enough reaction to a verb – it might be an instruction, after all, and you’d have a head start on following it – but it may actually be necessary to you understanding the word in the first place.
According to the theory of embodied cognition, we need to imagine ourselves acting out a verb to understand it. Right-handed people reading the word throw don’t just activate their left premotor cortex in case the word prompts them to throw something; they do it as part of actually figuring out what that string of letters actually means. Which would explain why right-handers trying to decide if throw is a word are faster if the hand areas of the left premotor cortex are given a boost with a little transcranial magnetic stimulation. They don’t get a boost if the right premotor cortex is stimulated, because most right-handed people don’t throw things with their left hand, and they don’t get a boost for non-hand-related words like kneel or wander (although those would probably get a boost if leg-related regions had been activated instead).
Take this idea back to Dick and Jane, then, and it may be that little children learning to read are tying verbs like throw to mental images of themselves acting out those verbs; they way they are learning to read is, ever so slightly, different depending on whether they use their right or left hand for each word they learn.
The implications are far greater than just the notion that some people understand throw with their right brains and others understand it with their left brains; handedness is just a handy (pun intended) way to demonstrate that this premotor activity isn’t a result of reading the word, but necessary for understanding it. So just how much of our understanding of other words relies on areas not usually associated with language? Are sensation areas needed to understand hot and cold, and emotion areas needed to understand happy and afraid? And if we need body representations to understand those words, what does that mean for abstract notions like truth?
We shouldn’t go too far in our thinking just yet, because this study does have one major flaw: it only looks at right-handed people. There are alternative explanations to be considered, until we can show that stimulating the right premotor cortex helps the left-handed. (Personally, I’d liked to see even finer analyses: I’m relatively ambidextrous, so do I understand write with my right hemisphere, but throw and chop with my left?). But at the very least, I can say that the question of handed and language has just become a lot more interesting.
Willems RM, Labruna L, D’Esposito M, Ivry R, & Casasanto D (2011). A functional role for the motor system in language understanding: evidence from theta-burst transcranial magnetic stimulation. Psychological Science, 22 (7), 849-854 PMID: 21705521