Early language dissociation in bilingual minds: Non-invasive magnetoencephalography evidence through a machine learning approach

Abstract

Is language selection decodable from neural activity in balanced bilinguals? Previous research employing various neuroimaging methods has not yielded a conclusive answer to this issue. However, direct brain stimulation studies in bilinguals have detected different brain regions related to language production in separate languages. In the present MEG study, we addressed this question in a group of proficient Spanish-Basque bilinguals (N=45), who performed two tasks. In a picture naming task, they were asked to name a picture either in Basque or in Spanish if the ink turned from black to green after one second (10% of trials). Similarly, in a word reading task, participants read printed words out loud only when the ink turned to green. Sensor-level evoked activity was similar for the two languages on both tasks. However, decoding analyses classified the language used in both tasks, starting ~100 ms after stimulus onset. Crucially, searchlight analyses revealed that the right occipital-temporal sensors contributed most to language decoding in the picture naming task, while the left occipital-temporal sensors contributed most to decoding in word reading. Temporal generalization analyses for the picture naming task showed a sustained effect, while for the word reading task two stages of processing (at ~100 ms and starting ~300 ms) were evident. Cross-task decoding analysis highlighted robust generalization effects from the picture naming to the word reading task in the later time interval. The present findings provide support for the possibility of exploring the trial-by-trail variability in MEG activity for dissociating languages in the bilingual brain. For the first time, we provide evidence for a differential role of the two hemispheres in language selection for picture naming and word reading. We also provide novel evidence on the shared language representations between naming and word reading.