ИСТИНА

What makes a word a word? Word is commonly believed to be a structural unit of any human language, and word learning underlies language acquisition. However, it is rather challenging for neuroscience to describe how lexicality is established in the human brain, and how phonological word representations relate to lexicality. Any word may be considered within two domains: phonological and semantic. Correspondingly, there are two groups of studies dedicated to novel word learning: some of them deal with phonological familiarization to pseudowords, while other studies investigate mechanisms of word meaning acquisition via passive associative learning that adds semantics to pseudowords. Still there is no consistent understanding whether semantic association is strictly required to integrate new pseudowords into lexical domain, or the brain can treat pseudowords as ‘empty’ lexical entries. The current study aimed to clarify the influence of semantics on pseudoword processing. We used a novel lexical trial-and-error learning paradigm to establish associations between pseudowords and actions. We inquired when and where processes associated with semantics take place in the human brain. MEG neuroimaging technique was used as it provides high spatial and temporal resolution. Participants were presented with eight pseudowords; during learning blocks, four of them were assigned to specific body part movements through commencing actions by one of participants’ left or right extremities and receiving a feedback. The other pseudowords did not require actions and were used as controls. Magnetoencephalogram was recorded during passive listening to the pseudowords before and after learning blocks. The cortical sources of the magnetic evoked responses were reconstructed using distributed source modeling. All of the 24 participants reached successful performance on the task. Phase-locked neural response selectively increased for pseudowords that acquired association compared with control pseudowords. Using data-driven approach, we localized significant differential activation into the left hemisphere, including insula, Broca's complex, intraparietal sulcus and anterior STS-MTG. Differential activation started 150 ms after the uniqueness point. These areas can be viewed as both low-tier (STS-MTG), and higher-tier (intraparietal sulcus, temporal pole) structures involved in speech processing. We report rather widespread brain activity induced by processing of newly-learnt words. Relatively large size of areas involved may be explained by the fact that the experiment lasted less than two hours and no consolidation could have occurred within such a short time. The results show that acquisition of word semantics induced enhanced activation within brain areas involved in phonological word processing. Notably, we observed a widespread involvement of articulatory zones, i.e. Broca’s complex, which stays in accord with Lieberman’s motor theory of speech perception. It is likely that words that associatitively acquired meaning gained high priority to be recognized and remembered. Thus, brain circuits responsible for phonological decoding became attuned to new sets of phonemes. Importantly, this mechanisms, known as receptive field tuning, was revealed within studies dedicated to operant conditioning. We observed essentially similar effects within classical speech areas, thus a mechanism analogous to receptive field tuning may be a principal mechanism of word learning.