The effect of print size on reading speed in dyslexia

TLDR

The study predicts that dyslexic readers require larger critical print sizes to reach maximum reading speed, based on a letter‑position coding deficit hypothesis. Reading speed was measured across twelve print sizes (20/12–20/200) in dyslexic children (grades 2–4, 7–10 y) and matched non‑dyslexic children (grades 1–3, 6–8 y). Results show that dyslexic and non‑dyslexic readers share the same two‑limbed rate‑by‑print‑size curve, but dyslexic readers have larger critical print sizes and shallower slopes below that size, supporting the letter‑position coding deficit hypothesis, while non‑dyslexic readers exhibit a developmental decrease in critical print size and a developmental‑lag model fails to explain the group differences.

Abstract

This article details a study which predicted that across a wide range of print sizes dyslexic reading would follow the same curve shape as skilled reading, with constant reading rates across large print sizes and a sharp decline in reading rates below a critical print size. It also predicted that dyslexic readers would require larger critical print sizes to attain their maximum reading speeds, following the letter position coding deficit hypothesis. Reading speed was measured across twelve print sizes ranging from Snellen equivalents of 20/12 to 20/200 letter sizes for a group of dyslexic readers in Grades 2 to 4 (aged 7 to 10 years), and for non‐dyslexic readers in Grades 1 to 3 (aged 6 to 8 years). The groups were equated for word reading ability. Results confirmed that reading rate‐by‐print size curves followed the same two‐limbed shape for dyslexic and non‐dyslexic readers. Dyslexic reading curves showed higher critical print sizes and shallower reading rate‐by‐print size slopes below the critical print size, consistent with the hypothesis of a letter‐position coding deficit. Non‐dyslexic reading curves also showed a decrease of critical print size with age. A developmental lag model of dyslexic reading does not account for the results, since the regression of critical print size on maximum reading rate differed between groups.