Decoding and disrupting left midfusiform gyrus activity during word reading

Significance A central issue in the neurobiology of reading is a debate regarding the visual representation of words, particularly in the left midfusiform gyrus (lmFG). Direct neural recordings, electrical brain stimulation, and pre-/postsurgical neuropsychological testing provided strong evidence that the lmFG supports an orthographically specific “visual word form” system that becomes specialized for the representation of orthographic knowledge. Machine learning elucidated the dynamic role lmFG plays with an early processing stage organized by orthographic similarity and a later stage supporting individuation of single words. The results suggest that there is a dynamic shift from gist-level to individuated orthographic representation in the lmFG in service of visual word recognition. The nature of the visual representation for words has been fiercely debated for over 150 y. We used direct brain stimulation, pre- and postsurgical behavioral measures, and intracranial electroencephalography to provide support for, and elaborate upon, the visual word form hypothesis. This hypothesis states that activity in the left midfusiform gyrus (lmFG) reflects visually organized information about words and word parts. In patients with electrodes placed directly in their lmFG, we found that disrupting lmFG activity through stimulation, and later surgical resection in one of the patients, led to impaired perception of whole words and letters. Furthermore, using machine-learning methods to analyze the electrophysiological data from these electrodes, we found that information contained in early lmFG activity was consistent with an orthographic similarity space. Finally, the lmFG contributed to at least two distinguishable stages of word processing, an early stage that reflects gist-level visual representation sensitive to orthographic statistics, and a later stage that reflects more precise representation sufficient for the individuation of orthographic word forms. These results provide strong support for the visual word form hypothesis and demonstrate that across time the lmFG is involved in multiple stages of orthographic representation.