Neural underpinning of Japanese particle processing in non-native speakers

Abstract

Grammar acquisition by non-native learners (L2) is typically less successful and may produce fundamentally different grammatical systems than that by native speakers (L1). The neural representation of grammatical processing between L1 and L2 speakers remains controversial. We hypothesized that working memory is the primary source of L1/L2 differences, by considering working memory within the predictive coding account, which models grammatical processes as higher-level neuronal representations of cortical hierarchies, generating predictions (forward model) of lower-level representations. A functional MRI study was conducted with L1 Japanese speakers and highly proficient Japanese learners requiring oral production of grammatically correct Japanese particles. We assumed selecting proper particles requires forward model-dependent processes of working memory as their functions are highly context-dependent. As a control, participants read out a visually designated mora indicated by underlining. Particle selection by L1/L2 groups commonly activated the bilateral inferior frontal gyrus/insula, pre-supplementary motor area, left caudate, middle temporal gyrus, and right cerebellum, which constituted the core linguistic production system. In contrast, the left inferior frontal sulcus, known as the neural substrate of verbal working memory, showed more prominent activation in L2 than in L1. Thus, the working memory process causes L1/L2 differences even in highly proficient L2 learners.

Figure 1

Sequence of events in the task. Each trial comprised two phases: preparation and production. In the preparation phase, the participant observed and listened to the predicate portion (Camera is). In the production phase, the subject (This) my father was presented. Two conditions were prepared in this phase: when an empty line was shown, the participant was required to utter a particle (Grammar condition). In this particular case, no (the possessive case) was the correct answer. When a Japanese letter was presented with an empty line, the participant was required to read out the letter (Letter condition). Activities during the task were modelled with boxcar functions for each phase, except for the rest condition. The English phrases in the figure is for explanatory purposes, and were not presented during the experiment.

Figure 2

Behavioral results. Non-native learners demonstrated significantly (A) longer reaction times and (B) higher error rates than native speakers. Data are presented as box plots and violin plot datasets for each group.

Figure 3

Regions associated with correct grammar processing. Brain activation was associated with grammar processing (grammar > letter) in (A) non-native learners, (B) native speakers, (C) the conjunction of non-native learners and native speakers, (D) non-native learners greater than native speakers, and (E) native speakers greater than non-native learners. The level of activation was set at a threshold p-value of < 0.05 and the FWE was corrected for multiple comparisons over the whole brain, with the height threshold set at a p-value of < 0.001 (uncorrected).