Development of coarse- and orientation-tuning for native letters in children aged 3-9 years old: Oddball fast...

To examine the processes contributing to reading, we investigated neurophysiological letter coding in typically developing children (3.6-9.3 years) using an oddball fast periodic visual stimulation and a 32-channel electroencephalogram. Non-native (Georgian) letters appeared every fifth item in a stream of native (Russian) letters, presented at a frequency of 6 Hz. This contrast (foreign condition, n = 46) elicited significant discrimination responses at the frequency of non-native letters presentation - 1.2 Hz - and its harmonics, even in children who did not yet acquire reading skills. We also investigated native letter orientation sensitivity using 180-degree-rotated Russian letters in a stream of upright Russian letters (inverted condition, n = 42, overlap with foreign condition n = 37). Discrimination responses increased with age in both conditions but followed different trajectories. In the foreign condition, it significantly increased in the subgroup of pre-reading children and plateaued in older children. In the inverted condition, discrimination response emerged only in reading children and showed significant correlation with age in this subgroup. The results support two systems of tuning for letters: a coarse tuning system that differentiates native from foreign letters and develops in pre-reading age through environmental exposure, and an orientation tuning system that develops with reading acquisition.