A new method of evaluating neurosurgical patients provides fresh insight into how the brain produces speech
Researchers in the U.S. have developed a new and improved technique for mapping and protecting brain function in conscious patients about to undergo neurosurgery – by cooling small, localised brain regions to temporarily interrupt their workings and map the areas required for word formation and the timing of speech. They describe their method, and explain how the used it to investigate the brain mechanisms underlying the production of spoken language, in a new study just published in the journal Neuron.
Stories about people who remain awake during brain surgery appear regularly in the mass media. In 2008, for example, multiple news outlets reported that legendary Bluegrass musician Eddie Adcock not only remained conscious, but also played his banjo, during a three-and-a-half operation to treat a hand tremor. And last December, there were similar reports about the Spanish jazz musician Carlos Aguilera, who played his saxophone throughout a 12-hour operation to remove a brain tumour.
Such procedures are often described as “astonishing,” “revolutionary,” and, sometimes, “miraculous.” In fact, surgeons have been operating on the brains of conscious patients for nearly one hundred years: The method was developed in the 1920s by the pioneering Canadian neurosurgeon Wilder Penfield, as a way of locating the abnormal brain tissue causing epileptic seizures.
Penfield used electrodes to stimulate the surface of his patients’ brains, and deliberately kept them awake during surgery so that they could report the effects of the stimulation back to him. By stimulating the areas around the abnormal tissue, he could identify the tissue causing the seizures while also determining which of the surrounding areas are crucial for important functions like speech and movement. This way, he could remove the abnormal tissue without causing any collateral damage.
Penfield’s method of cortical stimulation is still used widely today but, somewhat ironically, can itself trigger epileptic seizures. The new method carries no such risk. Developed by Michael Long of New York University’s Langone Medical Center and his colleagues, it builds on an earlier method that used cooling probes to study the brain circuitry responsible for song production in zebra finches.
Long and his colleagues used a similar cooling device on 16 patients being evaluated for neurosurgical operations to treat their drug-resistant epilepsy. With the patients under local anaesthetic, the researchers used the device to cool 42 discrete brain regions, all of which have been previously implicated in speech production, by about 10°C each. Meanwhile, the patients were asked to recite the days of the week, or a simple string of numbers, so that their speech function could be assessed while each region was cooled.
Cooling interrupts cellular activity, and in some cases, the researchers found that it interfered with the patients’ ability to speak, slowing and blurring their speech. This effect was only temporary, however – brain function returned to normal immediately after the cooling device was removed from the brain, and all 16 patients subsequently recovered from their operations without any undesired side effects or other complications.
The results confirm earlier findings that the brain areas involved in speech production are mostly confined to the left hemisphere, and they also provide new insights into how the brain produces speech. Specifically, cooling a specific part of the left motor cortex altered the quality of the patients’ speech, whereas cooling of Broca’s Area in the left temporal lobe altered the timing of their speech.
These results shows that the motor cortex directs the muscle movements in the lips and tongue that are required for articulating speech, whereas Broca’s Area is needed for executing these movements in their proper sequence.
“This study confirms that cooling is a safe and effective means of protecting important brain centers during neurosurgery,” says Long. “[It] also represent[s] a major advance in the understanding of the roles played by the areas of the brain that enable us to form words.”
Long, M. A., et al. (2016). Functional Segregation of Cortical Regions Underlying Speech Timing and Articulation. Neuron 89: 1–7. DOI: 10.1016/j.neuron.2016.01.032 [Abstract]
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