In this edition of Ask a Neuroscientist, I crowdsource the answer to a question about the differences between how the brain processes spoken versus written language.
The question comes from Minski, who wrote:
"Does writing down what I think and saying what I think activate different parts of the brain and neuropathways? I feel I have an easier time writing than I do speaking, so I wonder.
Thank you for your time and knowledge!"
Below, I have reproduced (with Minski's permission) my response.
Last week I was at the Society for Neuroscience conference (think: 31,000 neuroscientists in the San Diego Convention Center), and I decided the best (read: funnest) way for me to tackle your question was to crowdsource.
So, late on night, at a bar filled with neuroscientists, I posed your question to 3 other Stanford Neuroscience PhD students. Here’s what we came up with, in our informal brainstorming session.
One graduate student remembered a series experiments involving split brain patients (whose corpus callosum, the part of the brain that connects the left and right hemispheres, is severed). So in these experiments, the researchers presented a picture to a patient in such a way that the image filled only ½ of the patients visual field. The idea here is that the visual image would only be captured by one eye, and therefore predominantly only be encoded by one side of the brain (this is a feature of how the human visual system is wired). So the image is presented to only one side of the brain, and then the patients are asked to pick a second object that was associated with the original picture, from a larger sample of objects. So people would pick the object associated with the original picture. And depending on which side of the brain was processing the original image, when the researchers asked the people why they picked the second object, they couldn’t tell the researchers why. But if the researchers then asked the people to write down their reason, the people were able to do that just fine.
Another example comes from the laboratory of Dr. Michael Gazzaniga. His patient, V.J. had her corpus callosum severed as a treatment for intractable epilepsy. After her surgery, V.J. was no longer able to write, but was able to speak and understand spoken language without any problem. So the idea here is that speech and writing are lateralized functions in the brain. Indeed, experiment conducted with the help of V.J. and other split brain patients have lead to the understanding that spoken languages are stored/encoded on the left side of the brain, whereas writing is controlled by the right side of the brain. For a more in-depth discussion of V.J. and the lateralization of speaking/writing, I highly recommend a 1996 article published in the New York Times, “Workings of Split Brain Challenge Notions of How Language Evolved”, written by Sandra Blakeslee.
With our discussion now focused on lateralization of behaviors, another graduate student mentioned a book by Stanislas Dehaene, called Reading in the Brain. This book talks about a lot of ideas, but the basic premise is that there are a lot of visual pathways that words/concepts can go through, that are completely independent from the pathways that those same word/concepts go through when you are hearing them. But at some point, there is a convergence of those various pathways - at some level, there is part of your brain that deals with semantics, where the representation of the written word ‘manatee’, meets the representation of the spoken sound ‘manatee’, and presumably the representation of the image below. So there is a region of your brain that is going to be encoding language, but there seem to be different neural pathways for accessing that general region (visual, verbal, aural). Our discussion reiterated the observation that some people display selective aphasia; for these folks, if you put a picture of a cat in front of them, and ask ‘What is that?’, you’ll get a response of ‘It’s, an animal. Not dog.’ But they won’t be able to say ‘cat’. And if you ask these people to write down that the picture is, they’ll be perfectly able to write the word ‘cat’.
So with these extraordinary examples, our conclusion was that it is not at all unreasonable to think that a person could be better at written language than verbal language, and at expressing their comprehension of language better through writing as opposed to speaking. And indeed this point has been highlighted in non-neuroscience based studies of the most effective ways to teach children: whether teachers should talk to the students or should draw on the board.
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