Abstract
Adults do not learn languages as easily as children do. It has been hypothesized that the late-developing prefrontal cortex that supports executive functions competes with procedural learning mechanisms that are important for language learning. To address this hypothesis, we tested whether a temporary neural disruption of the left Dorsolateral Prefrontal Cortex (DLPFC) can improve implicit, procedural learning of word-forms in adults. Young adults were presented with repeating audio-visual sequences of syllables for immediate serial recall in a Hebb repetition learning task that simulates word-form learning. Inhibitory theta-burst Transcranial Magnetic Stimulation was applied to the left DLPFC or to the control site before the Hebb task. The DLPFC-disrupted group showed enhanced learning of the novel phonological sequences relative to the control group. Moreover, learning was negatively correlated with executive functions that rely on the DLPFC in the control group, but not in the DLPFC-disrupted group. The results support the hypothesis that a mature prefrontal cortex competes with implicit learning of word-forms. The findings provide new insight into the competition between brain mechanisms that contribute to language learning in the adult brain.
Highlights
It is well known that children surpass adults in their language learning ability, in particular for certain aspects of language that involve grammar and phonology[1,2], but it has remained unclear why this is the case[3,4]
We addressed the hypothesis that the left dorsolateral prefrontal cortex competes with procedural learning mechanisms that are important for language learning, potentially explaining the inferior language learning skills in adults relative to children
We found that the group with disrupted Dorsolateral Prefrontal Cortex (DLPFC) showed improved Hebb repetition learning compared to the control group, but only for sequences that did not overlap with the filler sequences
Summary
It is well known that children surpass adults in their language learning ability, in particular for certain aspects of language that involve grammar and phonology[1,2], but it has remained unclear why this is the case[3,4]. Motor learning studies have shown that enhancing cognitive effort during learning results in impaired procedural learning[27] with greater activity in the prefrontal cortex[28] These studies support the idea that the suppressed reliance on the prefrontal lobe, and in particular conscious executive functions that support declarative memory, improves implicit procedural learning of a novel skill. The long-term memorization of phonological sequences, or word-forms, is argued to rely on procedural memory mechanisms[8] Experimental evidence for this comes from research with the Hebb repetition paradigm[31,32,33]. When adults are forced to attend to smaller two-syllable-structures within the sequence (thereby simulating children’s smaller working memory capacity), the HRE for non-overlapping, but not overlapping, sequences increases to the same level as children[45] These findings suggest that children outperform adults in procedural language learning tasks that benefit from limited capacity in working memory
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