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Neuroscience
Talking Is Like Walking
Both speech and locomotion engage the same brain regions.
Posted December 16, 2014
Just as we alternate left and right steps to produce paces when we walk, we alternate consonant and vowel sounds to produce syllables when we speak. It’s in this sense that talking is like walking.
Walking involves the exquisite coordination of dozens of muscles. When you walk, you do much more than just move your feet. Rather, your whole body gets into the act. As you move your right foot forward, your left arm swings back.
You take a step, and then your left foot moves forward while your right arm swings back. And with each step, your hips swivel left and right while your spine sways forward and back. This pattern repeats at a rate of up to a hundred steps, or fifty paces, per minute.
Talking also requires the finely tuned movements of many muscles to get the jaws, lips, and tongue into proper position for each speech sound. Moreover, a normal speaking rate is about two hundred syllables per minute, twice the pace of an Olympic sprinter.
When we consider the parallels between walking and talking, it’s not surprising to find that many of the brain structures that control locomotion also get enlisted for speech. Based on brain-imaging studies and clinical evidence from patients with brain damage, scientists have identified three brain systems that regulate both walking and talking.
The anterior cingulate cortex lighting up in an MRI. The supplementary motor area is behind it. (To the right in the picture).
The first system initiates and maintains smooth continuous movement, whether we’re striding or speaking. Scientists think the supplementary motor cortex may trigger voluntary movements, while the anterior cingulate cortex monitors for and corrects errors in production. These two areas lie inside the longitudinal fissure, which runs from the space between the eyes to the back of the head, splitting the brain into left and right hemispheres.
Both areas light up in the MRI when people are asked to produce sentences as opposed to simply repeating them. Likewise, patients with brain damage in these areas tend not to produce utterances on their own, even though they can repeat what they hear. Thus, your decision to get up and walk across the room and your decision to utter a sentence may both originate in the same brain regions.
Colorful images are useful for locating functional regions, but they also lure us into thinking of the brain as a Lego model.
The second system includes the traditional areas for motor and speech production. A swath of brain surface running from ear to ear across the top of the head contains a map of the body’s skeletal muscles. Of course, the left side of primary motor cortex controls the right side of the body, and vice versa. But this body map is also hanging upside down, with the feet at the top and the face at the bottom.
This is the area where the overall motor plans for walking and talking are assembled. Touch the side of your head in front of your left ear, and you’re pointing at Broca’s area, the traditionally recognized center for speech production. Small wonder it’s located there, right next to the motor cortex area that controls the face muscles.
Damage to the basal ganglia or cerebellum (lower right) disrupts the rhythm of walking and talking.
The third system consists of a number of motor loops that coordinate the muscle movements involved in either walking or talking. One motor loop includes the cerebellum, a walnut-sized structure at the back of the brain. Patients with damage to the cerebellum can still walk, but with a jerky, awkward gait. Likewise, they can still talk, but with poor articulation and at a much slower rate than normal.
Another motor loop runs through the basal ganglia, which regulate the motor system by inhibiting inappropriate behaviors. Parkinson’s is a disease of the basal ganglia, and these patients suffer from tremor at rest, rigidity, and slowness of movement. Their speech articulation is similarly impacted.
The traditional view holds that language processing is lateralized to the left hemisphere. This model is based on nineteenth-century clinical observations that brain damage to the left hemisphere often leads to language deficits while damage to the right hemisphere rarely does.
Some higher-level language functions are lateralized. Patients with left-brain damage often struggle with syntactic processing. Likewise, patients with right-brain damage falter with figurative language, taking every statement literally.
Twentieth-first century brain science is moving away from the “Lego” model—one piece for this function, another piece for that function. It’s better to view the brain as a massive set of highly integrated and overlapping circuits, each taking part in multiple functions. When we walk or talk, many areas of the brain get into the act.
References
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Bohland, J. W., Bullock, D., & Guenther, F. H. (2010). Neural representations and mechanisms for the performance of simple speech sequences. Journal of Cognitive Neuroscience, 22, 1504–1529.
Graziano, M.S.A. (2008). The intelligent movement machine. Oxford: Oxford University Press.
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MacKenzie, C. (2011). Dysarthria in stroke: A narrative review of its description and the outcome of intervention. International Journal of Speech-Language Pathology, 13, 125–136.
Richardson, J. D., Fillmore, P., Rorden, C., LaPointe, L. L., & Fridriksson, J. (2012). Re-establishing Broca’s initial findings. Brain and Language, 123, 125–130.
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David Ludden is the author of The Psychology of Language: An Integrated Approach (SAGE Publications.
