Orthographic training finetunes a neuronal letter position code in primate visual cortex

Reading is an acquired skill that enhances brain activity in human visual cortex and is thought to repurpose preexisting ventral visual circuitry for the fast parallel processing of letter strings. However, the neuronal code underlying position-invariant written word recognition remains elusive. Here, we examined this issue in a macaque monkey model before and after orthographic training. Based on prior simulations of reading acquisition in a convolutional neural network model of the ventral visual pathway, we hypothesized the existence of neurons sensitive to both specific letters and their ordinal position within the word, which should be enhanced by orthographic training. By wirelessly recorded neural activity from the inferior temporal (IT) cortex of macaque monkeys trained on orthographic tasks over five successive days, we indeed discovered IT neurons tuned to letters and sensitive to either ordinal or retinotopic positions. Ordinal units existed prior to training, but their responses were enhanced after training, matching with behavioral improvements in word recognition. These findings support the neuronal recycling hypothesis and demonstrate, at the single-cell level, how reading refines pre-existing neural circuits to facilitate fluent word recognition.