Automatic morpheme identification in reading development: MEG evidence from Fast Periodic Visual Stimulation


Society for the Neurobiology of Language (SNL) Annual Meeting, virtual edition, October 21-24, 2020


October, 2020


Valentina N. Pescuma, Maria Ktori, Elisabeth Beyersmann, Paul Sowman, Anne Castles & Davide Crepaldi

Morphemes are the smallest linguistic units that carry meaning (e.g., a complex word such as artist is composed of a stem, art-, and a suffix, -ist). Behavioural evidence suggests that morphological structure is accessed when processing complex words (Amenta & Crepaldi, 2012) and that reading development benefits from the morphological structure of words, especially from the presence of stems (e.g., Grainger & Beyersmann, 2017). Relying on previous EEG evidence for selective word representations in the brain (e.g., Lochy, Van Belle & Rossion, 2015), the aim of this study was to investigate selective neural responses to morphemes embedded in pseudowords, in both adults and children. Using Fast Periodic Visual Stimulation (FPVS) with an oddball paradigm (base frequency: 6 Hz, oddball frequency: 1.2 Hz) and MEG recording, we presented skilled adult readers (N=28, native English speakers) with pseudoword combinations of 12 stems (e.g., soft), 12 suffixes (e.g., ity), 12 non-stems (e.g., trum) and 12 non-suffixes (e.g., ust). The participants were administered four experimental conditions: Condition 1 with stem+suffix oddballs (e.g., softity) and nonstem+suffix base stimuli (e.g., trumess); Condition 2 with stem+nonsuffix oddballs (e.g., softert) and nonstem+nonsuffix base stimuli (e.g., trumust); Condition 3 with stem+suffix oddballs (e.g., softity) and stem+nonsuffix base stimuli (e.g., stopust); Condition 4 with nonstem+suffix oddballs (e.g., terpity) and nonstem+nonsuffix base stimuli (e.g., trumust). In order to investigate the developmental trajectory of morphological sensitivity, we asked whether a discrimination response to morphemes would emerge in developing readers as well. Fifth and sixth graders (N=17, native English speakers) were therefore presented with a simplified version of the same paradigm, using pseudoword combinations of 6 stems, 6 suffixes, 6 nonstems and 6 nonsuffixes, and only two of the four experimental conditions administered to adults (Conditions 1 and 3). All participants engaged with the unrelated task of monitoring the colour change of the fixation cross. Preliminary sensor-level analysis revealed, both in developing and skilled readers,  a successful discrimination response at the oddball frequency (1.2 Hz) and its harmonics (2.4, 3.6, 4.8 Hz), in left occipito-temporal regions of interest, for the two experimental conditions in which the oddball stimuli were fully decomposable into stems and suffixes (e.g., softity). These results provide evidence for automatic morpheme identification, even at relatively early stages of reading development, and are in line with all major accounts of morphological decomposition. Critically, these initial findings also suggest that morpheme identification can be modulated by the context in which the morphemes appear. Additional ongoing analyses aim at providing more refined spatial and source-level information, in order to shed light on the neural underpinnings of morpheme identification in visual word processing.