Effects of Orthographic and Phonological Grain Size on Reading Acquisition, Spoken Word Processing, and Hierarchical Organization of the Human Auditory Cortex

Three studies investigated the effect of grain size on reading instruction, cross-linguistic differences during spoken word processing, and organization of the auditory cortex. Grain size is the number of phones or graphemes in a word or word segment. The first study asked if grain size emphasis during reading instruction resulted in differences in grain size sensitivity and processing speed. Previous investigations considered sensitivity to letters or words, whereas this study systematically tested multiple grain sizes. The main findings revealed better sensitivity to rimes and faster processing speed given large grain instruction. The results suggest that large grain instruction may be beneficial for adults learning a new alphabetic orthography that emphasizes rime structure. The second study asked if small versus large orthographic grain size experience changes the phonological network. Unlike previous neuroimaging studies that considered developmental changes within languages, this study directly compared developmental changes given English versus Chinese orthographic experience during spoken word processing and revealed greater developmental increases in phonological network engagement given small versus large grain orthographic experience. The results suggest that orthographic grain size experience leads to cross-linguistic divergence in the phonological network for spoken word processing. The final study asked if the auditory cortex is organized with selectivity to phonological grain size by testing multiple grain sizes. Unlike previous investigations that looked at lower level acoustic features or only single grain sizes, this study compared activation for multiple grain sizes. The main findings included greater small grain sensitivity closer to the primary auditory cortex, greater large grain sensitivity farther from the primary auditory cortex, and greater sensitivity to syllable boundaries in a region overlapping with small grain size. These studies extend previous work and demonstrate that orthographic and phonological grain size plays a critical role in reading instruction for adults learning a new orthography, phonological network activation during spoken word processing, and organization of the auditory cortex for speech processing. Future research should determine if grain size organization changes with language development and deficits in reading skill. Organization should become more distinct over development and should be less distinct in those with reading disability.