Neuroscientists map the activity of sensorimotor cortex for speech articulation.

A new study published in Nature reveals some of the dynamics of neural activity when people articulate syllables commonly used in English1. The researchers have taken advantage of a therapy that patients were about to follow for epilepsy. In some cases of epilepsy, doctors need to locate the region of the brain that induces seizures. To do so they place a series of electrodes right on the surface of the brain, under the cranium. The technique is called Electrocorticography. Since the patients were to wear that device for some time, they were asked if they would be willing to make some articulation exercises while the electrical activity on the surface of their brain was recorded through this device. Three of them have accepted, and the result is an unprecedented insight on the neural activities in a part of the brain, the ventral sensorimotor cortex, while subjects perform pronunciation exercises including sounds like “la”, “za”, “po”, “vu”, etc…

Illustration of neural activity recorded while patients articulated sounds such as “ka”, “gu”, etc… In A and B, you can see the neural activities regrouped in a 3D graph. Sets of sounds resulting in similar neural activity share the same color. In C and D, neural activities are grouped together in a hierarchical fashion, illustrating the degree of similarity in the brain activities observed during the production of each sound. E and F are statistical confirmation that the link between the neural activities and the different groups of sounds is significant. Everything to the left of the figure is taken at the consonant moment (“k….”) while everything to the right is taken at the vowel moment (“….a”). See the Figure legend for more details.

Figure reproduced with special permission from Nature, the license that applies to this blog does not include this figure.

One of the strengths of this paper is the way in which the neural activities are illustrated. The recordings come from many electrodes located above the brain, thus part of the results are illustrated using Principal Component Analysis. The technique is often referred to as a dimensionality-reduction technique – in simple words it means that it takes many complex signals and reduces those to a more simple illustration that contains the most informative signals or sets of signals. This is why despite the fact that more than 3 electrodes are recording activities, we can view the PCA of all those electrodes in a 3-dimensional graph, as illustrated in the figure to the left. Notice in (a) for instance how “su”, “sa” and “zi” are located close to each other. This means that the production of those sounds causes similar neural activities, while the production of other sounds such as “ka”, “gu” or “ja” induce very different activities.

The study provides a very detailed analysis of how neural activities evolve through time as each syllable is pronounced. The recording technique used differs from previously-used techniques such as functional magnetic resonance imaging due to its very high temporal resolution. With Electrocorticography, which is used in this study, it is possible to track changes in activities in the millisecond range. Thanks to this high temporal resolution, the researchers can characterize the dynamics of neural activity as the syllables are produced. Among the many notable advances in this huge article, they successfully recorded the convergence of neural activities as the subjects transferred to many different consonants to a single vowel, for instance they found at which point the recorded brain activities converged to similar patterns as subjects pronounced “gu”, “ku”, “lu” or “zu”. Further research using this technique and others might reveal how the brain puts together those syllables to make up coherent discourse.

References

1. Bouchard KE, Mesgarani N, Johnson K, Chang EF. (2013) Functional organization of human sensorimotor cortex for speech articulation. Nature 495, 327–332.

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