Scientists are divided on the connection between linguistic and the motor systems. Some neuroscientists believe the two are highly tied together. In their view, for instance, in order to understand the word “hammering” our brain sets in motion the same cerebral structures used to perform the action of hammering. This supposition is in line with theories of embodied cognition, according to which the nature of the human mind is modeled upon the body, its shape, and the way it interacts with the world.
Other studies have called into question dependence of the linguistic system on the motor one, actually uncovering dissociation between the two domains. Paola Mengotti, of SISSA, and his team have put to the test a theoretical model to account for such inconsistencies.
Linguistic and Motor Function
“A connection between linguistic and motor functions has been observed, but only under certain circumstances,” says Mengotti.The study involved 57 patients with left brain damage, a “very large sample group for this type of studies” points out Mengotti. Patients with left brain damage are often affected by language disorders, (aphasias) and, simultaneously, motor disorders (apraxias).
The researchers saw that involvement of the motor system depends upon the “type” of gesture: meaningful gestures activate the structures connected to semantic processing, while meaningless gestures, like waving your hands in the air dismissively for example, are mainly based on motor decoding. “This way we have clarified the inconsistencies shown by previous studies, which did not distinguish between the categories of gestures.” explained Mengotti.
“The truly interesting aspect of our study is the new analysis we have employed”, concludes Mengotti.
The voxel based lesion symptom mapping, is a cerebral visualization technique. A voxel is a three dimensional volume element, analogous to a pixel which is a picture element in 2 dimensions. By investigating the properties of each voxel in the brain during the test, it is possible see which ones are functioning or malfunctioning- a lesion is where some process is not occurring correctly- when a particular symptom happens. It locates very accurately where in the brain a defect is that can be “mapped” or linked to a symptom.
“With the help of this technique we were able to establish that damage to the angular gyrus, a region of the brain in the parietal cortex, is connected to a drop in performance in the imitation of meaningless gestures, without affecting performance on linguistic tests, while damage to supramarginal gyrus is associated with a drop in performance both regarding meaningful gestures and on some linguistic tests.”
Mengotti, P. et al. Selective imitation impairments differentially interact with language processing, Brain (2013) 136 (8): 2602-2618. doi: 10.1093/brain/awt194