Word learning is a part of human faculty and is dependent upon two processes that are a result of activity coming from the posterior superior temporal (pST) and inferior parietal (IP) brain regions toward the prefrontal cortex (dorsal stream) and the temporal pole (ventral stream). The ventral stream is responsible for forming visual and linguistic information into representations of sentence structure and its meaning. The dorsal stream is responsible for sound-to-movement forming, articulation, complex arrangement in the verbal area, and how speech is stored and retrieved from memory. In an EEG study the connection between the IP lobe and Broca’s area was evaluated while musicians and non-musicians learned made up words presented as connected auditory streams. Musicians did better than non-musicians in behavioural sense, which was reflected by a higher sensitivity index (d’). This may be because of the increased left-hemispheric theta coherence in the dorsal stream, while non-musicians’ right hemisphere is more active. There were no differences between the groups during the test or a controlled condition, which points to a task-specific intertwining between musical expertise, functional connectivity, and word learning.

In the last two decades, musicians have served as a reliable and powerful model for studying functional and structural plasticity in brain regions supporting auditory perception, motor control, and recently also higher cognitive functions. However, such brain changes should not be considered as ability only musicians are capable of, but rather as part of interacting neural networks that are well synced. This view is supported by previous diffusion tensor imaging (DTI) studies showing white matter differences in a number of fibre tracts, including the arcuate fasciculus (AF), and different subdivisions of brain areas.

Evidence shows that plastic changes in the auditory-related cortex (ARC) and changed neural network traits cause behavioural advantages of musicians in processing speech cues manipulated in terms of voice-onset time, pitch, duration, timbre, rhythm, and prosody. These advantages also seem to be present in attention, short-term memory, working memory, and inhibition. Although, it is not fully discovered how these advantages develop, but it is suspected that certain shared functions between the domains of speech and music may play a role.

Music training has also shown to help with speech processing and distribution. Dittinger and colleagues conducted a study where they investigated neural signatures underlying word learning. Their results have shown advantage in musically trained people rather than the untrained ones, but only when the subjects had to use semantic memory. This could be explained by a shift in the N400 component.

Learning new words in a different language requires a combination of interplay between auditory perception, memory functions, and articulation. The dorsal and ventral streams are structured in a way to aid with these functions. Clinical observations have shown that damage to these streams results in loss of quality of the functions required for effective word learning.

With the help of multimodal imaging approach, Lopez-Barroso and colleagues reconstructed the posterior, the anterior, and the long segment of the AF, and discovered a positive correlation between functional and structural connectivity among Wernicke’s area and Broca’s territory (i.e., long segment of the AF) and the ability to remember made-up words presented in the form of auditory streams. These results show the correlation between learning of made-up words and an increased recruitment in the left dorsal stream. Another study done by Catani and co-workers focused on the relationship between the degree of asymmetry of the long segment of the AF and verbal memory performance. It was found that participants characterized by a more symmetric distribution of this fibre bundle were better at remembering the previously learned lexical items compared to those with a strong left-hemispheric asymmetry. Previous fMRI studies focused on word learning found increased brain activity in neocortical areas situated along the two processing streams and accommodating lexical-semantic processes, including the IP lobe, Broca’s area, and the middle-posterior part of the middle temporal gyrus (MTG).

In the present EEG study, word learning mechanisms in 15 healthy pianists and non-musicians of German origin were evaluated with the help of a paradigm that has previously been shown to recruit the left dorsal stream. Dynamic electrophysiological coupling mechanisms between specific brain regions of the dorsal and ventral streams during word learning were the areas that were focused on. Scalp-EEG data were collected while musicians and non-musicians learned made up words presented in the form of connected speech streams. EEG was used to collect scalp EEG data, validating the inverse-space solution, selecting the frequency band and brain regions of interest that represented spectral-density distribution in the dorsal stream, and assessing theta (θ) coherences. We evaluated functional connectivity between the IP lobe and ventral part of the prefrontal cortex, and predicted that the behavioral advantage of musicians in word learning would be thanks to an increased left-hemispheric asymmetry.

The results found a task-specific relationship between the stronger left-asymmetric recruitment of the dorsal stream in the θ frequency range in musicians and word learning. This suggests that word learning is easier for individuals in whom that specific brain area is more active. However, this does not mean such result was achieved by experience-determined brain changes, but that some brains are predisposed to such type of maturation. It should be noted that this study is only a single step taken into understanding functional role of θ. The EEG-based functional connectivity only enables an approximation of the neural sources contributing to word learning with a low spatial resolution. More studies are necessary to conduct in order to understand this topic.

More information: SCIENTIFIC REPORTS | (2018) 8:4565 | DOI:10.1038/s41598-018-22942-1