Word meaning acquired by auditory-motor associations: the role of the left perisylvian cortex.
Speech is largely based on the body motor and sensory experience. The question, which is crucial for understanding the brain mechanisms of human language, is how our brain transforms sensory-motor experience into word meaning. Multiple evidence hints that natural language acquisition involves biological mechanisms of associative learning. The ability to quickly acquire word-picture associations was shown to depend on reorganization in neocortical networks including the left temporal area, especially the left temporal pole, as well as temporoparietal, premotor, and prefrontal regions.
We developed an auditory-motor experimental procedure that allowed investigating neural underpinning of word meaning acquisition by way of associative "trial-and-error" learning that mimics important aspects of natural word learning. Participants were presented with eight pseudowords; four of them were assigned to specific body part movements during the course of learning – through commencing actions by one of participant’s left or right extremities and receiving a feedback. The other four pseudowords did not require actions and were used as controls. Magnetoencephalogram was recorded during passive listening of the pseudowords before and after learning. The cortical sources of the magnetic evoked responses were reconstructed using distributed source modeling.
We found a significant effect in the middle part of the STS/STG that mostly includes the auditory parabelt areas responsible for spectrotemporal analysis and initial steps of word recognition. Processing of new words also activated the posterior opercular part of the inferior frontal gyrus that is involved in subvocal rehearsal and articulatory coding of the perceived speech sounds, this fact emphasizing the role of articulatory sensory-motor experience in acquisition of word meaning. Our analysis did not reveal significant effects in the temporal pole or in the temporoparietal regions.
Juxtaposition of our findings with the current body of literature may imply that rooting the word meaning into one's sensory-motor experience is an initial stage, which is prerequisite but not sufficient for its embedding into the full associative structure of semantic memory.
Taken together, our findings show that learning of novel word meaning through word-action association selectively increased neural specificity for these words in the auditory areas responsible for spectrotemporal analysis, as well as in articulatory areas, both located in the left hemisphere. The extent of neural changes was linked to the degree of language learning, specifically implicating the physiological contribution of the left perisylvian cortex in the learning success.